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Hair Structure and Hair Removal

A Summary Report of Scientific Literature on “Structure of Hair” and “Chemistry and Kinetics of Depilatories”

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Hair Structure and Hair Removal

Table of Contents

[A] Executive Summary 3

[B] Structure of Hair

1. Morphological Structure of Hair 4 [1.1] Introduction 4 [1.2] Hair Follicle 5 [1.3] Hair Fibre (Shaft) 10 2. Structure of Hair across Various Body Sites 19 3. Chemical Composition of Hair 21

[C] Chemistry and Kinetics of Depilatories

1. Reducing Human Hair 25 [1.1] Bonding in Keratin 25 [1.2] Depilation by Reduction of Hair 26 2. Kinetics of Reduction 29 [2.1] Determination of Equilibrium Constant 30 [2.2] Mechanism 30 [2.3] Factors Affecting the Kinetics of Reduction 31 [2.4] Effect of Mercaptan Structure on Reaction Rate 35 [2.5] Kinetics/Reactivity in Microemulsion Media 36

[D] References 38

Hair Structure and Hair Removal

[A] Executive Summary:

The present review report involves the study of scientific literature to elaborate on the “structure of hair” and "the chemistry and kinetics of depilation”.

In the first section, the report summarizes the morphological structure of different hair components, the structure of hair across various body sites and the chemical composition of hair.

In the second section, the study of bonding in hair keratin protein, reduction of hair by depilatories and kinetics of reduction of hair is incorporated.

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[B] Structure of Hair :

[1] Morphological Structure of Hair:

[1.1] Introduction:

Hair is a primary characteristic of mammals, and exerts a wide range of functions including thermoregulation, physical protection, sensory activity, and social interactions. Hair (the stratified epithelium) is an appendage of skin that proliferates from large cavities or sacs called follicles [1]. The length of the hair extends from its root or bulb embedded in follicles through the dermis, epidermis, stratum corneum, skin, then continues into a shaft and terminates at the tip end [1] [Figure: 1].

(Figure 1: Diagrammatic Representation of Structure of Hair)

Hair Structure and Hair Removal

[1.2] Hair Follicle [2]1:

The mature (anagen) hair follicle can be divided into two parts:

[1.1.1] a ‘permanent’ upper part, which does not cycle visibly, and

[1.1.2] a lower part, which is continuously remodelled in each hair cycle.

[1.2.1] Upper Part of Hair Follicle: The upper part of the hair follicle consists of the (a) Infundibulum, and (b) Isthmus.

(a) Infundibulum: It is the most proximal part of the hair follicle relative to the epidermis, extending from the sebaceous gland duct to the epidermal surface. It includes the hair canal and the distal Outer root sheath. It is the opening of the hair canal to the skin surface. At the proximal end, the infundibulum joins the isthmus region of the outer root sheath, where the arrector pili muscle is inserted [Figure: 2A].

• Sebaceous Gland: It is Acinar gland composed of lipid‐filled sebocytes, localized close to the insertion of the arrector pili muscle. It secretes sebum to the epidermal surface via a holocrine mechanism. Sebum helps making hair and skin water‐proof. It forms the pilosebaceous unit together with the hair follicle and the arrector pili muscle. • Outer Root Sheath: The outermost layer of the hair follicle which merges proximally with the basal layer of the interfollicular epidermis and distally with the hair bulb.

(b) Isthmus: It is middle part of the hair follicle extending from the sebaceous duct to the ‘Bulge region’. The upper isthmus is joined with the infundibulum; while The lower isthmus also harbours epithelial and melanocytic hair follicle stem cells in the ‘Bulge region’, which is the end of upper part of hair follicle.

• Bulge region: It is a convex protrusion of the outer root sheath in the most distal permanent portion of the hair follicle. It is located just below the sebaceous gland

1 The information provided in this section is taken from: [2] Marlon R. Schneider, Ruth Schmidt‐Ullrich, and Ralf Paus. Current Biology 19, R132–R142, February 10, 2009.

Hair Structure and Hair Removal

and at the insertion site of the muscle arrector pili. It contains the hair follicle stem cells. • The Arrector Pili Muscle: The arrector pili muscle is an appendage to the hair that attaches itself to the underside of the hair at the dermal papilla and the hair shaft, midway between the bulb and the sebaceous gland. This tiny muscle fiber is responsible for lifting the hair, thereby trapping a layer of air on the skin’s surface. As a result, the arrector pili is partially responsible for heat regulation. Both fear and cold stimuli cause the arrector pili muscle fiber to contract, lifting the hair straight upward.

[1.2.2] Lower Part of Hair Follicle: The lower, cycling part represents the actual hair shaft factory, the ‘Anagen Bulb’ [Figure: 2A] [3]. The anagen bulb contains the matrix keratinocytes and the hair follicle pigmentary unit.

Bulge and Anagen Bulb, are separated by a long stretch of suprabulbar hair‐follicle epithelium [Figure: 2A, 2C].

Hair Structure and Hair Removal

(Figure 2. Histomorphology of the hair follicle: (A) Sagittal section through a human scalp hair follicle (anagen VI) showing the permanent (infundibulum, isthmus) and anagen associated (suprabulbar and bulbar area) components of the hair follicle. (B) High magnification image of the isthmus. The dashed square indicates the approximate location of the bulge. (C) High magnification image of the bulb. insulation (BM: basal membrane; APM: arrector pili muscle; CTS: connective tissue sheath; DP: dermal papilla; M: matrix; HS: hair shaft, IRS: inner root sheath; ORS: outer root sheath; SG: sebaceous gland))

(Figure 2D: Light micrograph of the different layers of the hair follicle. In the bulb region, a proliferating epithelial matrix surrounds the mesenchymal dermal papilla. The hair shaft of cortex, medulla and cuticle layers enclosed by the inner root sheath move outwards within the outer root sheath which is continuous with the epidermis)

• Papilla: At the base of the follicle is a large structure that is called the papilla. The papilla is made up mainly of connective tissue and a capillary loop. Cell division in the papilla is either rare or non‐existent. While infundibulum, isthmus, bulge and hair bulb are all part of the hair follicle epithelium, i.e. of ectodermal origin, the dermal papilla is mesoderm‐derived. The dermal papilla [Figure: 2C, 2E], which consists of a small cluster of densely packed fibroblasts, dictates hair bulb size, hair shaft diameter and length, and anagen duration [4, 5, 6, 7].

Hair Structure and Hair Removal

The inner root sheath (IRS) is a layer extending from the base of the bulb to the isthmus, which surrounds and protects the development of the hair fiber. The IRS can be subdivided into several layers. A single cell thick IRS cuticle layer, adjacent to the hair fiber, closely interdigitates with the hair fiber cuticle layer. The next IRS layer is called the Huxley layer that may consist of up to four cell layers. Outside of this there a single cell layer called the IRS Henle layer. The Henle layer runs adjacent to the outer root sheath (ORS) layer. The outer root sheath forms a non‐keratinizing region at the periphery of the follicle and is continuous with the epidermis. It extends all the way to the tip of the bulb.

When looking at a cross‐section of the hair follicle, its epithelium forms a cylinder with at least eight different concentric layers, each one expressing a distinct pattern of keratins [3]. Starting from the periphery, these layers include the outer root sheath, the companion layer, the inner root sheath,and finally the hair shaft [Figure: 2E]. In its bulge region, the outer root sheath contains the epithelial hair follicle stem cells [6]. The central part of the

(Figure 2E: Schematic drawing illustrating the concentric layers of the outer root sheath (ORS), inner root sheath (IRS) and shaft in the bulb. The inner root sheath is composed of four layers: Companion layer (CL), Henle’s layer, Huxley’s layer, and the inner root sheath cuticle. The companion layer cells are tightly bound to Henle’s layer, but not to the outer root sheath, thus allowing the companion layer to function as a slippage plane between the stationary outer root sheath and the upwards moving inner root sheath. Further inwards, the inner root sheath cuticle is composed of scales that interlock with the scales of the hair shaft cuticle, anchoring the shaft in the follicle and enabling both layers to jointly move during hair follicle growth. (CTS: connective tissue sheath; DP: dermal papilla; IRS: inner root sheath; ORS: outer root sheath))

Hair Structure and Hair Removal

hair follicle epithelium holds the hair shaft [Figure: 2E]. The entire hair follicle epithelium is surrounded by a mesoderm‐derived connective tissue sheath [Figure: 2C, 2E], a loose accumulation of collagen and stromal cells resting upon a basement membrane. The hair shaft is enwrapped by the cuticle, and further components are the cortex and the medulla [Figure: 2E].

Hair Structure and Hair Removal

[1.3] Hair Fibre (Shaft):

Morphologically, three distinct varieties of cells or units are produced within the hair follicle which results in the formation of three basic structural layers of any human hair fibre [1]. The three layers are:

(i) (Outer) Cuticle (ii) Cortex (iii) (Central) Medulla, which may be absent or discontinuous along the hair shaft.

[1.3.1] Cuticle:

The outermost or external layer of the fibre consist of flattened overlapping scales known as cuticle, which is responsible for the much of the chemical resistance and stability of the hair [1][Figure: 3].

(Figure: 3 ‐ Stereogram of the hair fiber structure, illustrating substructures of the cuticle and the cortex. [1])

Hair Structure and Hair Removal

In developed hair, the cuticle cells are square sheets approximately 0.2‐0.5 µm thick, and 45‐60 µm in length [1] with an overall thickness of approximately 5‐10 scales [8, 9]. The cuticle cells are attached at the proximal end (root end), and they point toward the distal end (tip end) of the hair fiber, like shingles on a roof [1].

The free length of each cuticle cell visible at the surface depends mainly on the overall diameter of the hair. In vellus hair, for example, three‐quarters of the surface cuticle cell is visible, with a relatively large distance between the free margin of successive scales of terminal hair, in which scale margins appear closer. The cell junctions between adjacent cuticle cells and the cuticle and underlying cortex are usually flat, and folds are infrequently but regularly seen. This may contribute to the mechanical strength of the cuticle [10].

The mature cuticle cell comprise of number of distinct layers namely: epicuticle (thin layer), A‐layer, exocuticle and endocuticle [1, 11]. All these layers are surrounded by cells called Cellular Membrane Complex (CMC) [1, 11] [Figure: 4].

(Figure: 4 ‐ Cross section of a hair cuticle [17])

• Epicuticle [1, 11]: Each cell of the cuticle contains a thin external membrane, the epicuticle (~3 nm), which is a protein coat covered by a strong lipid structure. The strongly bound structural lipid is also called F‐layer. Epicuticle contains about 25% lipids and 75% proteins, with 12% of cystine, which represents a high content of sulphur. The 25% of fatty acid is predominantly 18‐methyleicosanoic acid [12]. A model of the epicuticle wherein the fatty acid layer (lipid layer) is connected to the

Hair Structure and Hair Removal

underlying fibrous protein layer through thioester linkages involving the cysteine residues of the protein [13, 14] was proposed which is an important development in the field of keratin fibre science. The protein matrix is directed to the hair fiber surface and it is abundant in cysteil groups, near the surface, which are acidulated by fat acids (lipids) [15]. • A‐Layer [11]: It is a resistant structure containing cystine (>30%). The cross links of proteins, in this layer, not only give physical resistance but also makes them relatively resistant to chemical attack. These properties protect the fiber against both mechanical and chemical attacks. • Exocuticle [11]: It is also known as Layer B. It corresponds to 55% of the cuticle area and it is rich in cystine (~15%), and it is physically rigid (but less intensively than Layer A). • Endocuticle [11]: This is a layer with low grade of cystine (~3%). It is an inferior part of the membrane, which is also considered as epicuticle. It is much softer than the superior layers and there are evidences that it swells with water. • Cellular Membrane Complex (CMC) [11]: These are cells with constant thickness (30 nm) which surround/isolate all the cells in the cuticle. The 18‐methyl eicosanoic acid (18‐MEA) is one of the very important lipid components of the CMC. The CMC consists of a central core (δ layer) bound on both sides by 2 lipid‐endowed β‐layers. As yet, the composition of the δ layer is not fully determined, but it contains little protein and seems to be polysaccharidic in nature [17].

[1.3.2] Cortex :

2The cortex occupies most of the hair area (75%). In the same way as the cuticle, it has cells filled by cross links of cystine and hard cells separated by the cell membrane complex (CMC). Each one of the cortex cells has a spindle shape, with a 50‐100 μm length and a 3 μm diameter. Each cell distal surface is rough, irregular, and they tie crossly to each other (18, 19, 20].

The cortical cells are closely packed and oriented along the axis of the hair. Each cell

2 The paragraph is taken from: Velasco, M. V. R.; Dias, T. C. S.; Freitas, A. Z.; Vieira Junior, N. D.; Pinto, C. A. S. O.; Kaneko, T. M. ; Baby, A. R. Brazilian Journal of Pharmaceutical Sciences 45:153 (2009)

Hair Structure and Hair Removal

contains a nuclear remnant (nuclear “ghost”), which is stellate in transverse section.

• Macrofibrils [10]: The major structures within cortical cells are the closely packed macrofibrils. Each macrofibril is a solid cylindric unit 0.1 to 0.4 μm in diameter and of variable length but often as long as the whole cell. Between the macrofibrils is a variable amount of intermacrofibrillar matrix and melanin granules; this matrix is analogous in structure to the cuticular endocuticle and contains the remnants of cytoplasmic organelles [Figure: 5]. In some cells, macrofibrils are so densely packed that individual units are difficult to see even by electron microscopy (paracortical cells), while others are less‐densely aggregated (orthocortical cells).

(Figure: 5 ‐ Transmission electron micrograph of hair macrofibrils [17])

Human hair cortex is generally considered to be of the paracortical cell type throughout the cortical thickness. Caucasian (curly) hair is mainly paracortical and Negro (woolly) hair is segmented into two zones‐the outer side of the crimp curl being orthocortical and the inner paracortical [9].

• Micorfibrils (Intermediate Filaments): Macrofibrils are composed of rodlike microfibrils approximately 7 nm in diameter, arranged‐ in whorls (pseudohexagones) and embedded in a structureless intermicrofibrilar matrix [10]. The microfibril in keratin fibre is classified into a class of intermediate filaments (IF) based on their sequences and structural homologies with the other IF proteins such as vimentine, desumin, glial filaments and neurofilaments. Hence, microfibril are known as a‐ keratin intermediate filament (KIF) or Intermediate Filament (IF).

Hair Structure and Hair Removal

All IF proteins consist of a central rod domain, and N‐ and C‐terminal domains [1, 21‐ 26]. The rod domain has four helical segments in which two α‐helix chains are associated to form a double‐stranded coiled‐coil chain. Besides, the structure of N‐ and C‐terminal domains is considered to be disordered. The coiled‐coil structure has been first proposed by Crick [27] and Pauling and Corey [28], and the detailed structure has been studied with X‐ray and n.m.r, analyses [29, 30, 31]. Fraser and MacRae proposed that closely packed coiled‐coil α‐helices are accommodated in a fibril called protofibril and four protoflbrils are packed in a single microfilament (microfibril) [32].

3The assembly is said to have a coiled coil structure because each a helix itself follows a helical path. The conformation of α‐keratin’s coiled coil is a consequence of its primary structure: The central ~310‐residue segment of each polypeptide chain has a 7‐residue pseudorepeat, a‐b‐c‐d‐e‐f‐g, with nonpolar residues predominating at positions a and d [Figure: 6A]. Since an α‐helix has 3.6 residues per turn, α‐ keratin’s a and d residues line up along one side of each α‐helix. The hydrophobic strip along one helix associates with the hydrophobic strip on another helix. Because the 3.5‐residue repeat in α‐keratin is slightly smaller than the 3.6 residues per turn of a standard α‐helix, the two keratin helices are inclined about 18o relative to one another, resulting in the coiled coil arrangement. This conformation allows the contacting side chains of each helix to interdigitate [Figure: 6B]. The higher order structure of α‐keratin is not well understood. The N‐ and C‐terminal domains of each polypeptide facilitate the assembly of coiled coils (dimers) into protofilaments, two of which constitute a protofibril [Figure: 6]. Four protofibrils constitute a microfibril, which associates with other microfibrils to form a macrofibril [33] [Figure: 7].

3α‐Keratin is rich in Cys residues, which form disulfide bonds that crosslink adjacent polypeptide chains. The α‐keratins are classified as “hard” or “soft” according to whether they have a high or low sulfur content. Hard keratins, such as those of hair, horn, and nail, are less pliable than soft keratins, such as those of skin and callus,

3 The paragraph is taken from: [33] Voet, D.; Voet, J.G.; Pratt, C.W. Proteins: Three‐Dimensional Structure in: Fundamentals of Biochemistry: Life at the Molecular Level, 2nd Edition, p.138‐139 (2005).

Hair Structure and Hair Removal

because the disulfide bonds resist deformation.

(Figure: 6A: View down the coil axis showing the alignment of nonpolar residues along one side of each α‐helix. The helices have the pseudorepeating sequence a‐b‐c‐d‐e‐f‐g in which residues a and d are predominately nonpolar [33])

(Figure: 6B: Side view of the polypeptide backbones in skeletal (left) and space filling (right) forms. Note that the side chains (red spheres in the space‐filling model) contact each other [33])

(Figure: 7: (A) Two keratin polypeptides form a dimeric coiled coil. (B) Protofilaments are formed from two staggered rows of head‐to‐tail associated coiled coils. (C) Protofilaments dimerize to form a protofibril, four of which form a microfibril [33])

Hair Structure and Hair Removal

Still, studies are going on for further elaboration on structure and aggregation of intermediate filament (IF). Jones et al. proposed that native intermediate filaments of hard α‐keratin have uniquely defined structures with, on average, about 32 chains in cross‐section [34]. Fraser and Parry have studied the macrofibril assembly and structural principles governing IF aggregation and their interaction with neighboring IF in a sheet [35]. Fraser and Parry have further explored three‐dimensional structure of trichocyte (hard α‐) keratin intermediate filaments [36].

[1.3.3] Medulla:

4In human hair, the medulla—if present—generally comprises only a small percentage of this mass. The medulla may be either completely absent, continuous along the fiber axis, or discontinuous, and in some instances a double medulla may be observed. Medullary cells are loosely packed, and during dehydration (formation), they leave a series of vacuoles along the fiber axis. Medullary cells are spherical and hollow inside and are bound together by a cell membrane complex type material [Figure: 8A, 8B]. Because the medulla is believed to contribute negligibly to the chemical and mechanical properties of human hair fibers [37] and is difficult to isolate [38, 39], it has received comparatively little scientific attention.

The medulla has high lipid content compared to rest of the fibre and it is deficient in cystine but rich in citruline [40]. Morphologically, the medulla has a porous structure formed by sponge‐like keratin and some vacuoles filled with the air resulting from differentiation process [41, 42] [Figure: 8A, 8B]. A layer of CMC separates medulla from the cortex.

4 The paragraph is taken from: [1] Robbins C. R. Chemical and physical behavior of human hair. Fourth Edition, ed., Springer‐Verlag, Newyork, p. 50 (2002).

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(A) (B)

(Figure: 8‐ (A) Scanning electron micrograph illustrating the porous medulla of a hair fiber cross section; (B) Scanning electron microscope illustrating hollow‐sphere‐like structures of Medulla [1])

Hair Structure and Hair Removal

• Structure of Hair Fibre (Summary): In summary, a hair fibre is composed of the cortex and the cuticle. Each of the two components is formed of various other morphological components [Figure: 9]. The cortex contains cortical cells and the cell membrane complex. The cortical cell is further composed of macrofibrils and intermacrofibrillar material. The macrofibrils consist of microfibrils and an intermicrofibrillar matrix. The microfibrils consist of protofibrils. This ensemble is wrapped up in the cuticle, as an external sheath which has also its own architecture, being formed of four layers: the epicuticle, the A‐layer, the exocuticle and the endocuticle. The medulla in coarser hair fibres consists of hollow cells and cell membrane complex type material.

(Figure: 9‐ The morphology of a hair fibre [43])

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[2] Structure of Hair across Various Body Sites [44, 45, 46]:

The structure of hair varies somewhat depending on the type of hair: languo, terminal, intermediate, or vellus.

Lanugo hair is the soft, fine hair that covers much of the fetus and is shed just before birth. They are unmedullated, and normally unpigmented hairs. The surface of lanugo hair is smooth with almost indiscriminate scales.

Vellus hair is the very fine, short, nonpigmented hair with a small cross‐sectional area that is found on the seemingly hairless parts of the body such as eyelids, forehead, and bald scalp. Vellus hairs shafts are not pigmented and are arbitrarily defined as having a cross‐sectional diameter of 30 μm or less. Both vellus and terminal hairs go through all stages of the follicular life cycle, but the length of anagen is much shorter in vellus hairs.

The prominence of various hair shaft features (amount of pigment, hair shaft diameter, and extent of medulla formation) increases from vellus hairs to terminal hairs. The hair diameter is determined by the size of the papilla and the hair. The cross‐sectional diameter of the bulb from terminal hairs ranges from 200 to 300 μm. Typical diameters of the associated terminal hair shafts are somewhat smaller, ranging from 40 to 120 μm.

Intermediate hair is intermediate in length and diameter and is found on arms and legs of adults. Terminal hair is the coarse, long, pigmented hair with large cross‐sectional area found in the hairy areas of the body such as the scalp, beard, eyebrows, eyelashes, armpits, and pubic area.

The differences in these hair types are due to the differences in hair follicles. Intermediate hair follicles, located on the arms and legs, do not change after puberty and are not influenced by hormones. Ambisexual hair follicles, located in the pubic area, axilla, and temple of the scalp, are influenced by hormones and change during puberty from fine, vellus hair to coarse, terminal hair. In addition, there are male hair follicles unique to males and found in the beard area, ears, nose, chest, and abdomen. These follicles respond to high androgen concentrations and change from vellus hair to terminal hair at puberty.

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[Table: 1] Differences between Terminal and Vellus Hairs.

Terminal hairs Vellus hairs

Hair follicle connected to a sebaceous gland Hair follicle not connected to a sebaceous gland Medulla may be present or absent No distinct medulla Long hairs (longer than 1.0 mm) Short hairs (~1.0 mm) Thick hairs (40 to 120 μm) Thick hairs (30 μm or less) Generally one hair per pilosebaceous unit More than one hair per pilosebaceous unit Usually pigmented Unpigmented Longer life cycle (6 to 8 years in anagen) Shorter life cycle (in telogen ~90% of time)

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[3] Chemical Composition of Hair [10]:

• Proteins [10]:

Most of the extractable keratinous protein is contained within cortical cells, but significant and important tractions are present within the cuticle; medullary proteins are probably of little physicochemical significance in human hair [47]. It is characteristically insoluble and resistant to proteolytic enzymes. Because of solubility problems, it is difficult to compare quantitative results from different laboratories. Solubility may vary from 10% to 70%. Solubilization involves breaking disulfide bonds by either reduction or oxidation. The proteins produced by reduction are keratins; those from oxidation are keratoses. S‐ carboxymethyl keratins (SCMK proteins) produced by reduction separate into a low sulfur group (SCMKA proteins, molecular weight 45,000) and a high‐sulfur group (SCMKB proteins, molecular weight 20,000); these compose approximately 60% and 30% respectively of the total protein of hair [48].

After oxidation, the keratoses can be separated into alpha‐keratose and y‐keratose. In relation to sulfur content and amino acid content, alpha and y‐keratoses are similar to SCMKA and SCMKB proteins respectively. As well as these two protein groups, 2% to 3% of the protein consists of low‐sulfur heterogeneous protein that is rich in glycine and tyrosine. Much recent work on the detail of keratins produced normally has suggested differences between individuals, possibly of forensic significance. Also, further refinement of the techniques may at last lead to a logical classification of hereditary hair shaft defects [49, 50].

To summarize, human hair contains a large number of proteins extracted by a variety of methods, but it is quite likely that these are the technical products of much fewer proteins present in vivo. It has been shown by electron histochemical methods that in the hair cortex the high‐sulfur proteins are predominantly in the matrix and the low‐sulfur proteins are in the filamentous protein. During the past 25 years, many investigators have analyzed the constituent amino acids of whole hair specimens; such analysis is often quoted for genetically diseased hair. The results are of limited use, since they provide only average values for the amino acid contents of average proteinaceous substances of the hair; also, some amino acids undergo hydrolytic decomposition. Hydrochloric acid is most commonly used for keratin fiber analysis. With this method the following amino acids have been

Hair Structure and Hair Removal

[Table: 2] Amino acid composition of whole human hair, and of hair cortex and cuticle (in μmol g‐1) [43].

Amino Acid Whole Hair Cortex Curicle Cysteic acid 32 27 59 Aspartic acid and Asparagine 399 416 300 Threonine 554 580 412 Serine 967 850 1628 Glutamic acid and Glutamine 916 930 848 Proline 588 532 900 Glycine 437 368 836 Alanine 347 370 500 Valine 405 374 644 Half‐cystine 1435 1350 1880 Methionine 13 9 39 Isoleucine 174 172 186 Leucine 457 466 404 Tyrosine 158 162 134 Phenylalanine 124 126 115 Ornithine — — — Lysine 196 172 331 Histidine 62 65 63 Arginine 466 496 289

shown to undergo partial decomposition: cystine, threonine, tyrosine, phenylalanine, arginine, and tryptophan. The amino acids isolated from normal hair are shown in Table 1, together with average relative amounts found by quantitative analysis. The figures are gross and in health and disease cannot be compared unless a large number of varying factors are taken into consideration; these include genetic variation, weathering, diet, cosmetic treatment, and the extraction and analytical methods used. In general, male scalp hair contains more cystine than female hair, while dark hair is said to contain more cystine than light shades. The tip of scalp hair contains significantly less cystine and cysteine than the root end; the converse applies for cysteic acid. Human hair cuticle is said to contain more cystine, cysteic acid, proline, threonine, isoleucine, methionine, leucine, tyrosine, phenylalanine, and arginine than whole hair. In general, cuticular cells contain a higher proportion of amino acids not usually found in alphahelical polypeptides than whole hair. The chemical composition of the A layer and

Hair Structure and Hair Removal

exocuticle is considerably different from that of the endocuticle in that they are highly cross‐ linked by cystine, giving a tough and resilient layer. The endocuticle contains very little cystine. Since the cortex makes up the main part of the hair fiber (by both weight and volume), whole‐fiber analytical studies relate closely to cortical chemistry. The greatest error will be in those amino acids present in smallest quantities. Medullary protein is notoriously insoluble and difficult to isolate. Consequently, complete analytic studies have not so far been possible; much of the known information has come from analysis of porcupine quill proteins. Medulla has a very low cystine and sulfur content and contains relatively large quantities of acidic and basic amino acids and hydroxyamino acids. The proteins are further analysed chemically and classified as [43]:

Low‐sulfur proteins (LS‐proteins): ca. 50 wt% of the total protein, are considered to be the intermediate filamentproteins. They are partly crystalline, and presumed to form the a‐helical components. High‐sulfur proteins (HS‐proteins): ca. 25 wt% of the total protein, are considered to constitute the intermediate filament associated proteins (IFAPs), the amorphous part of the fibre. High glycine and tyrosine proteins (HGT‐proteins): ca. 10 wt%, are also part of the amorphous IFAPs. Others, low‐sulfur and high‐sulfur proteins: ca. 15 wt% of the total proteins, are amorphous and considered to stem from the exo‐ and endocuticle, the cell membrane complex and residues of the cells.

• Water [10]: Water content is important in relation to its physical and cosmetic properties.’ The density of dry hair is 1.09 on the basis of geometric weight measurements and 1.37 from pychometric measurements. Consequently, the porosity is about 20% and hence is hygroscopic. When hair is impregnated with water, its weight increases by 12‐18%. The process of absorption is very rapid; 75% of the maximum possible amount of water is absorbed within 4 minutes. The water binding of amino and guanidino groups is responsible for the large percentage of water absorption capacity of keratin, particularly at low humidities (~25%) water molecules are bonded to hydrophilic sites by hydrogen bonds. With

Hair Structure and Hair Removal

increasing humidity more water is absorbed, producing a decrease in the energy binding of water already associated with the protein. At greater than 80%’ relative humidity, water absorption becomes more and more important.

• Hair Lipids [10]: Much of our knowledge of hair lipids comes from “fat solvent” studies. Depending on the solvent used, different results are obtainable; ethanol removes more lipid from hair than do solvents such as benzene, ether, and chloroform. The values obtained represent mainly sebum, and the chromographic fractions obtained consist primarily of free fatty acids and neutral fats‐esters, glyceryl, wax, hydrocarbons, and alcohols. The lipids of human hair are often thought of as of minor importance. It has been shown that hair lipid increases after puberty in both sexes. This declines with age in women but not to the same extent in men. Negroid hair produces more lipid than Caucasian hair. Squaline content in children is approximately one quarter that of adults, while cholesterol exists in similar proportions. In relation to age or sex, there is no difference with regard to fatty alcohol content of human hair lipid.

• Trace Elements [10]: It is not known to which chemical group in hair structuretrace elements are attached; however, the principal metal content of human hair probably exists as an integral part of fiber structure‐that is, as salt linkages or coordinated complexes with the side chains of pigment or proteins [51]. Trace elements may be incorporated into hair from several sources, both exogenous and endogenous. Of endogenous sources, the matrix, connective tissue papilla, the sebaceous, eccrine, and apocrine glands, and the surface epidermis are important. The environment also contributes greatly, particularly by pollution (e.g., from industry and hair cosmetics): scalp hair has been used in many studies as a sensitive index of environmental pollution. Until recently, the most frequently investigated elements in hair have been As, Cd, Cr, Cu, Hg, Pb, and Zn.

Hair Structure and Hair Removal

[C] Chemistry and Kinetics of Depilatories :

[1] Reducing Human Hair:

[1.1] Bonding in Keratin [43, 52, 53]:

The side groups in polypeptide chains of keratin, which account for a considerable portion (50 wt%) of the protein molecular mass, interact with each other, thereby stabilise the peptide by forming links between the chains and rings within a chain, as demonstrated by the following schematic representation of five such links between segments of two hypothetical peptide chains [43].

(Figure: 10‐ Interactions of side groups of polypeptide chains: From the top to downwards: interactions between phenyl rings, hydrogen bonds between an asparagine residue and a serine residue, a salt bridge between an arginine residue and a glutamic acid residue, a disulfide bridge between two cysteine residues, and an isodipeptide bridge between a glutamic acid and a lysine residue. [43])

In the α‐keratin arrangement, cohesion or structural stability of the hair fibre is provided by a variety of bonding mechanisms. The hydrogen bonding, disulfide bridge and salt bonding are important among them:

The Hydrogen Bond: The hydrogen bond is formed between the C=O and –NH groups

Hair Structure and Hair Removal and is responsible for the ability of the hair to be stretched elasticity and return back to its original shape.

The Salt Bridge: The salt bond is also an ionic (electrolytically controlled) bond formed by the electron transfer from the side chain of a basic amino group (an amino acid with an

COO‐ group) to the side chain of an acidic amino acid, i.e. NH3+. This occurs in a position paralleled to the axis line of the rotation of the helix of the hair.

The Cystine Bond/Disulfide Bridge: The covalent cystine linkages or disulphide (‐S‐S‐) cross‐links are the strongest type of bonds or associations present, and contributes significantly to the physical and chemical properties of hair keratin [53]. This major covalent bond between the polypeptide chains is provided by the diamino acid cystine, which on condensation can result in an inter‐ or intra‐chain bond with each of the two amino acid residues forming a component of a polypeptide chain. The presence of the cystine in α ‐keratin fibers is primarily responsible for the high stability of these fibers to environmental degradation by heat, cold, light, water, biological attack, and mechanical distortion [54].

[1.2] Depilation by Reduction of Hair : • Reduction of Hair with Thioglycolic Acid: As mentioned above, hair keratin has an unusually high proportion of cysteine in its structure and sulfur‐sulfur bonds are highly significant. So the key to chemical hair removal is breaking the sulfur‐sulfur bonds using a suitable reducing agent such as thioglycolic acid.

These reducing agents transform every disulfide cross‐link of cystine into two sulfhydril groups (cysteine) via two reversible nucleophilic displacement reactions [55]:

K - S - S - R + K - S K - S - S - K + R - S

R - S - S - R + K - S K - S - S - R + R - S

Hair Structure and Hair Removal

The only active species are the nucleophile–mercaptide ions which arise from the dissociation of mercaptans in water [55]:

OH R - S - H R - S + H O 2

The reduction of hair with thioglycolic acid can be shown as follows: 2SH‐CH2‐COOH + R‐S‐S‐R ‐‐‐‐‐> 2R‐SH + COOH‐ CH2‐S‐S‐CH2‐COOH (thioglycolic acid) (cystine) (dithiodiglycolic acid)

• Side Reactions During the Reduction of Keratin Fibers with Mercaptans [1]: The reaction of mercaptans with keratin fibers is a relatively specific reaction in mild acid. However, in alkaline media, peptide bond hydrolysis and the formation of lanthionyl residues can also occur [56]. Zahn et al. [57] have suggested that mercaptides such as thioglycolate or cysteinate can accelerate the rate of formation of lanthionyl residues in wool fiber.

HOOC COOH S

NH2 NH2

Lanthionine

Hydrolysis of peptide and amide linkages is also a possible complication in an alkaline medium. Hydrolysis of the amide groups of the residues of aspartic and glutamic acids will increase the ratio of acidic to basic groups in the fibers, conceivably altering the isoelectric and/or isoionic points of the hair.

H H CO C NH CO C NH

CH2 + OH CH2 + NH3 C O C O H2N O

Amide of Aspartic Acid Residue

Hair Structure and Hair Removal

Peptide bonds are the major repeating structural unit of polypeptides and proteins, and they form the structural backbone of human hair. Hydrolysis of peptide bonds can also occur at high pH, and both reactions (hydrolysis of amide and peptide bonds) are far more prevalent in the action of depilatories formulated near pH 12.

O R O R O

C CH NH C CH NH C + OH

O R O R O

OCCH NH C CH NH C

Hair Structure and Hair Removal

[2] Kinetics of Reduction:

The literature survey revealed many articles regarding kinetics of reduction of keratin fibre. However, most of them described the kinetics for reducing agents contained in hair‐waving compositions. The articles can be considered relevant for kinetics of reduction for depilatory products as most depilatories are of the same basic chemistry as thiol permanent waves and hair straighteners, but they are more reactive compositions due to the higher pH (from 11 to 12.5) of compositions and therefore they produce a more rapid and more complete reduction and a greater alkaline degradation of the hair [1].

Experiments relating to equilibrium reactions of disulfides with mercaptans commonly use reaction times up to 24 h or longer. Although this may seem unrealistic to those in product development, extremely valuable information with practical implications has been gained from these studies [1].

The cleavage of the disulfide bond in keratin fibers (I) by mercaptans (II) is a reversible equilibrium reaction summarized by Equation A, where the K substituent represents keratin [57].

Equation A :

KA K - S - S - K + R - S - H R - S - S - R + 2 K - S - H ( )()() (I) II III IV

This reaction actually proceeds through two steps, each a nucleophilic displacement reaction by mercaptide ion on the symmetrical disulfide (I in Equation B), and then on the mixed disulfide (V in Equation C) [57].

Equation B :

KB K - S - S - K + R - S - H K - S - S - R + K - S - H (I)

Hair Structure and Hair Removal

Equation C :

KC K - S - S - R + R - S - H R - S - S - R + K - S - H (V)

In considering these disulfide scission reactions, the equilibrium constant of the reaction shown in Equation A tells to what extent the total process will go to completion [55].

(R - S - S - R) (K - S - H)2 Equilibrium Constant = KA = 2 (K - S - S - K) (R - S - H)

[2.1] Determination of Equilibrium Constant [1]: Among the ways to determine or approximate the equilibrium constant of this type of reaction, two ways predominate: (1) Analysis of ingredient concentrations at equilibrium, and (2) analysis of redox potentials [59–61]. In either case, one may use cystine as a model for hair, since the literature [59–61] shows that the redox potential of “cystine‐type” disulfides is virtually independent of the charge group about the disulfide bond. However, reduction potentials of mercaptans do vary with pH [60]. Therefore, equilibrium constants for these reactions will also vary with pH. Patterson et al. [62] have shown that when wool fiber is reacted with 0.2M thioglycolic acid solution for 20 h, the extent of reduction increases with increasing pH above 6. Assuming equilibrium, this suggests that the difference in redox potential between thioglycolic acid and cysteine in keratin fibers increases with increasing pH above 6, and the equilibrium constant for this reaction increases similarly.

[2.2] Mechanism: Much research has been performed to describe the kinetics and mechanism by which reducing agents function. Currently, there are two mechanisms cited in the literature [55]: Pseudo‐first‐order kinetics and moving boundary kinetics.

To obtain the pseudo‐first‐order model, it is assumed [63] that the diffusion of the reducing

Hair Structure and Hair Removal

agent through the hair is fast compared to the rate of reduction and that the reducing agent is present in large excess. The rate of bond breaking is:

d(S – S)/dt = k C0 (S – S)

where C0 is the concentration of reducing agent, and (S‐S) is the instantaneous concentration of intact disulfide bonds. The back reaction is negligible because of the presence of a large excess of reducing agent.

To derive the moving boundary model, it is assumed [63] that diffusion into unreacted hair is much slower than the reaction rate, and that the reaction greatly increases permeability of the hair toward the reducing agent. Under these conditions a moving boundary of reducing agent will be formed in the hair. On the inside of the boundary the concentration of reducing agent is assumed to be zero, and on the outside of the boundary the

concentration is C0. In this case the distance X(t), that the boundary has moved into the hair at time, t, can be approximated by:

X(t) = At ½ Where, A is a constant.

[2.3] Factors Affecting the Kinetics of Reduction:

(i) Kinetics/Equilibrium Constants and Chemical Structure of Reducing Agent [1]: Equilibrium constants at pH 7 or lower, for the reduction of cystine by simple mercaptans such as cysteine and thioglycolic acid or even more complex mercaptans such as glutathione are all approximately 1 [59, 60]. Fruton and Clark [59] have shown that the redox potentials of other cysteine‐type mercaptans are very similar at pH 7.15. However, Cleland [60] has shown that dithiothreitol and its isomer, dithioerythritol, have much lower redox potentials than cysteine at neutral pH.

Cleland suggests that the equilibrium constant KB in Equation B (of dithiothreitol and

Hair Structure and Hair Removal

cystine) should be close to 1. However, the cyclization of dithiothreitol to a stable six membered ring disulfide during the reaction described in Equation C, provides an 4 equilibrium constant of the order of 10 = KC; therefore, KB x KC = KA is of the order of 104. Wickett and Barman [63–65] have expanded this area of research through a series of studies that involve reduction of hair fibers under stress using analogs of dithiothreitol, dihydrolipoic acid, and 1,3‐dithiopropanol. They have demonstrated that monothio analogs of dihydrolipoic acid reduce hair at a slower rate than the corresponding dithio compounds. This correlates with the higher equilibrium constant of reaction of dihydrolipoic acid and cystine. The dithio compounds can cyclize to form stable five‐membered ring disulfide structures during reduction, but the monothio compounds cannot. This confirms that cyclization to stable ring structures during the reduction step can be an important driving force in this reaction. Wickett and Barman have further demonstrated that these five‐ and six membered ring‐forming reducing agents penetrate into hair via a moving boundary. This suggests nearly complete reduction as the thiol penetrates into the hair. Wickett and Barman have also demonstrated that thioglycolic acid below pH 9 does not exhibit moving boundary kinetics, but above pH 10 it does. They have also studied structure‐activity relationships of a variety of analogs of these three cyclizing dithiols illustrating the effects of hydroxyl groups and alkyl chain groupings on the rate of this reaction.

(ii) pH/Time: Because the rate‐controlling step in this reaction can be diffusion of the reducing agent into the fibers or the chemical reaction itself, it is important to consider the rate in terms of these two potentially rate‐limiting factors. The pH region most commonly employed for the reduction of hair fibers by mercaptans is above neutral (generally 9 to 9.5). For example, glyceryl monothioglycolate (GMT) was introduced in Europe in the 1960s and into the United States in the 1970s for use in commercial acid waves (i.e., where the waving solution has a pH near 7) [66]. It would appear that the reaction of GMT with hair is a reaction‐controlled rate process because the pH of the system is near 7. The processing time for a GMT permanent is about twice as long as for a

Hair Structure and Hair Removal

conventional thioglycolate wave, and it requires a covering cap and the heat of a dryer to enhance the rate of reduction. Cysteamine hydrochloride is another active thiol used in professional products at a lower pH about 8.3. Manuszak et al. [58] have compared the reduction of hair by cysteamine and thioglycolic acid. At a pH where similar concentrations of mercaptide ion were present, thioglycolic acid was much more effective in reducing the fibers. One explanation is that cysteamine forms an internal five‐membered ring structure via internal hydrogen bonding from the protonated amine group to the mercaptide group, reducing its availability for reaction. Recently, Kuzuhara et al. have studied the diffusion behavior of thioglycolic acid (TG), thiolactic acid (TL), and l‐cysteine (CYS) by Microspectrophotometry and Raman Spectroscopy and showed that penetration and rate of reaction of thioglycolic acid (TG) and thiolactic acid (TL) was increased by increasing the treatment time and pH. However, l‐cysteine (CYS) could not penetrate into the cortex region of the virgin human hair. The apparent diffusion coefficient of TG into human hair was 10‐9 cm2/s and 10‐10 cm2/s at pH 7.0 and pH 9.0 respectively [67]. It was concluded that free amino groups of l‐cysteine electrostatically interacted with the anionic ions of the fiber surface, which resulted in decreased the reaction rate (the disconnection of — SS— groups) of l‐cysteine at pH 9.0. [68, 69]. Wickett and Mermelstein showed that the reaction of hair with thioglycolic acid (TGA) under depilating conditions was enhanced by increasing pH, by adding guanidine salts, and by oxidative pretreatment. Also, Reduction with TGA was slowed by the addition of n‐propranol, triacetin, glycerin, and propylene glycol [70]. Wickett and Barman [65] have demonstrated that thioglycolic acid below pH 9 does not exhibit moving boundary kinetics, but above pH 10 it does exhibit increase in rate of reaction and moving boundary kinetics.

(iii) Temperature [1]: The activation energy for the reduction of either human hair or wool fiber at alkaline pH is of the order of 12 to 28 kcal/deg‐mol [63, 66, 71]. Wickett [63] explains that when the mechanism is diffusion‐rate‐controlled, the activation energy is higher (28.0 kcal/deg‐mol) because the boundary movement depends on both reaction and

Hair Structure and Hair Removal

diffusion. However, when the rate depends only on the chemical reaction, the activation energy is lower (19.7 kcal/deg‐mol). Therefore, reaction rates for both of these systems are only moderately affected by increases in temperature.

(iv) Hair Swelling and Hair Condition: Above the isoelectric point, the swelling of hair increases substantially with increasing pH [72]. Herrmann [66] has shown a corresponding increase in the rate of diffusion of mercaptans into hair fibers with increasing pH. Hydrogen‐bond‐breaking agents (hair‐swelling agents), namely urea and other amides, have been added to depilatory formulations for the purpose of enhancing the rate of reduction [66, 73]. Heilingotter [74] has shown that the addition of urea to thioglycolic acid solution increases the rate of swelling of the fibers. Depilatory systems are generally high‐pH mercaptan systems (pH 11 to 12) where moving boundary kinetics exist under all conditions [63], and a common depilatory ingredient is calcium thioglycolate [Figure :11]. Note the axial folds created by the extreme swelling and then rapid dehydration on drying. These folds are created because of the differential shrinkage in the different cuticle layers due to the leaching out of solubilized proteinaceous matter. Undoubtedly, the condition of the hair also plays a role in the rate of reduction, especially under conditions where diffusion is rate‐limiting. Permanent‐ waving [76] and bleaching [77] produce alterations to hair that result in increased swelling in solvents. One might also anticipate more rapid rates of reduction for fibers that have been previously bleached or permanent‐waved than for chemically unaltered fibers.

(Figure : 11‐Scanning Electron Micrograph‐Hair fiber after treatment with calcium thioglycolate (depilatory))

Hair Structure and Hair Removal

[2.4] Effect of Mercaptan Structure on Reaction Rate:

• Electrostatic Effects:

Herrmann [66] has described a minimum at acid pH for the diffusion of a cationic containing thiol (thioglycolhydrazide) into human hair.

He has also examined the influence of pH on the rate of diffusion of thio acids (thioglycolic and thiolactic acids) into human hair. For this latter type of mercaptan, the minimum in diffusion rate occurs near neutral pH. These thio acids are of anionic character in alkaline media, and they diffuse faster in alkaline than in acidic media. Therefore, hair swelling must play a more important role than electrostatics for the diffusion of these simple mercaptans into human hair.

• Nucleophilicity of the Mercapto Group:

The nucleophilicity of the mercaptan grouping depends on the nature of the groups directly attached or in close proximity to the mercaptan functional group. In general, nucleophilicity increases with increasing basicity of the mercaptan function [77]. Over the range of conditions where diffusion is rate‐limiting, changes to the nucleophilicity of the mercapto group will have little effect on the rate of reduction. However, where the chemical reaction is rate‐controlling, the nucleophilicity of the mercapto group will be of considerable importance. Theoretically, in a diffusion‐controlled reduction, one could increase the rate of reduction by sacrificing nucleophilicity (decrease the basicity of the mercaptide ion) to increase diffusibility.

Haefele and Broge [78] have reported the mercapto acidities for a large number of mercaptans (pK RSH 4.3 to 10.2). Hydrogen sulfide, the simplest mercaptan, has a pK RSH of 7.0 [79]. As one might predict, the substitution of electron‐withdrawing groups (carbonyl, alkyl ester, alkyl amide) for a hydrogen atom increases the mercapto acidity. Electron‐donating groups (carboxy, alkyl) decrease mercaptan acidity.

Under conditions of lower pH, where this reduction process is reactioncontrolled rather than diffusion‐controlled, equation B or C can be rate‐limiting. If equation B is rate‐limiting, the reaction is simply second‐order—first‐order with respect to mercaptan and first‐order with respect to keratin disulfide—and analysis is not as complicated as when Equation C is rate‐limiting. In kinetic studies for a complex material like human hair or wool fiber, an excess of thiol is most commonly employed, and one generally assumes the reaction in Equation B to be ratecontrolling.

The reaction is then described by pseudo‐first‐order kinetics (first‐order with respect to keratin disulfide).

• Steric Effects:

The rate of diffusion of mercaptans into human hair is undoubtedly influenced by steric considerations. For example, molecular size (effective minimum molecular diameter) of the mercaptan molecule should affect the rate of diffusion into hair. Therefore, the rate of reduction of human hair by ethyl mercaptan in neutral to alkaline media, where diffusion is ratedetermining, should be faster than that of higher homologs. (The possible effects of variation in the structure of

Hair Structure and Hair Removal cystinyl residues in hair on the rate of reduction was considered in the previous section on cystinyl residues of differing reactivities.)

• Counterion Effects:

Ammonia or alkanolamines such as monoethanol amine are the primary neutralizing bases for reducing solutions of thioglycolate permanent waves. Ammonia is said to facilitate diffusion of thioglycolate through hair as compared to sodium hydroxide [80].

Heilingotter [81, 82] has compared a large number of neutralizing bases including ammonia, monoethanol amine, sodium hydroxide, isopropanol amine, ethylene diamine, diethanol amine, and triethanol amine with regard to the ability of the corresponding salts of thioglycolic acid to decrease the 20% index (at a pH close to 9.2).This criterion was used to assess the ability of these different thioglycolates to function as permanent‐wave reducing agents. He found that ammonia and monoethanol amine provide the maximum effects. Furthermore, the reducing power of triethanolamine thioglycolate is so weak as to render it ineffective as a permanent‐waving agent.

Heilingotter suggested that of the two most effective reducing systems, ammonium thioglycolate provides the more satisfactory waving characteristics.

Rieger [83] suggests that this “catalytic activity” of nitrogen containing bases is due to their ability to swell the hair, thus allowing faster diffusion of mercaptan into the interior of the hair.

Other salts of thioglycolic acid have been described as potential permanent‐waving agents, including potassium [83], lithium [84], and magnesium [85]. Magnesium thioglycolate has been described as an odorless permanent wave, although this system has never achieved commercial success.

[2.5] Kinetics/Reactivity in Microemulsion Media :

5Several experiments have been made for the reduction in microemulsion media. The studies carried out with thioglycolic acid as mercaptan (pK=10.4 ), have shown that the replacement of regular‐type solutions i.e. microstructured type such as microemulsions media could also induce modification of the cystine reduction rate [86‐91]. By using microemulsions incorporating a constant thioglycolic acid concentration (3 wt%) as reaction media and for a given treatment time, it was found that: (i) cystine reactivity was related to the microemulsion structure used as reaction media (ii) typically inverse W/O micro‐ emulsions induced the highest levels of cystine reactivity and (iii ) keratin cystine reduction was generally lower than in aqueous media [86‐91]. The authors interpreted the lower cystine reactivity in microemulsion media with respect to aqueous medium as due

5 The paragraph is taken from: [55] Savelli, M.‐P.; Solans, C.; Rodenas, E.; Pons, R.; Clausse, M.; Erra, P. Colloids Surf. A 143:103 (1998)

Hair Structure and Hair Removal

to the partition of thioglycolic acid among the different microemulsion pseudophases: aqueous, organic and interfacial. Since this reducing agent is soluble in both water and n‐ pentanol [91], it was assumed that for a given thioglycolic acid concentration, the concentration of effective thiol ions generated in the core of the water droplets of W/O microemulsions was lower than that in aqueous solution. Savelli et al. [55] studied reduction kinetics of hair cystine in water‐in‐oil‐type microemulsion media and compared with that in aqueous media. Cystine reactivity was evaluated by the cysteine formation as a function of treatment time. The kinetic results could be fitted to a pseudo‐first‐order model for both media, microemulsions and aqueous solutions and it was assumed that the reduction of keratin cystine with thioglycolic

acid is diffusion controlled in both media. The fact that the initial reaction rate, v0, is higher in aqueous media than in the W/O microemulsions studied agrees with the lower thioglycolate ions concentration available in, microemulsion media.

Hair Structure and Hair Removal

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