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Home , Carbohydrate chemistry, Oligosaccharide

Essential of

Wood Chemistry

Carbohydrates

• Photosynthesis is probably the most fundamental of all life processes and provides a means of converting “inorganic carbon”, in the form of carbon dioxide, into and then into other organic compounds.

• The early photosynthetic bacteria first appeared about 3,000 million years ago, and were joined much later by blue-green algae (2,000 million years ago) and the first vascular land plants (400 million years ago). Carbohydrates

• Together these organisms now produce approximately 14 × 1010 tones of organic matter every year according to the process shown in

hυ CO ＋ H O Carbohydrates ＋ O 2 2 Photosynthetic 2 organism

Carbohydrates

• Definitions

– The name carbohydrate was originally derived from the general

formula Cx(H2O)y formally to hydrates of carbon, but this type of sample definition does not cover the broad class of carbohydrates. • The in a plant usually function as a source of energy while , such as , fulfill the need for the storage of reserve food or they contribute mechanical strength to the plant cell. Carbohydrates

A variety of carbohydrates are included as essential building elements in natural compounds performing vital functions in living organisms.

Carbohydrates may be classified into following three large groups

Monosaccharide

Oligosaccharides

Polysaccharides

• is the simple sugars. • – consist of several monosaccharide residues joined together by glycosidic linkages, namely di-, tri-, tetra-saccharide…. The name oligosaccharide is usually restricted to the group of carbohydrates in which the number of monosaccharide units less than 10. • Polysaccharides – Polysaccharides are complex composed of a large number of monosaccharide unit joined together by glycosidic linkages. Some Terms Used in • Aglycone – The nonsugar part of a ; usually an alcohol, phenol or amine. • – Monosaccharide contain aldehyde function. • Aldopentose – A five-carbon with an aldehyde or cyclic hemiacetal functional group. • Aldohexose – A six-carbon sugar with an aldehyde group or cyclic hemiacetal group.

Some Terms Used in Carbohydrate Chemistry • – Monosaccharide contain keto function. • – Five-membered cyclic sugar from furan. • – Six-membered cyclic sugar from pyran. • Septanose Glycosans – Seven-membered cyclic sugar. Anhydro sugars are from sugars by the elimination of water from a pair of • Glycosans hydroxyl groups. Glycosan are strictly – Intramolecular glycoside intramolecular gycosides. Some Terms Used in Carbohydrate Chemistry

 α- or β-Glucoside  Acetal derived from the cyclic hemiacetal from α- or β-glucoside .  α- or β-Glycoside  General term for the acetal derivative of any suger.   Sugar isomers differing only in the configuration of the hemiacetal carbon ; they are designated asαor β. anomers，異位異構 物，異位碳原子上立體  Epimers 構造的差異所形成的 α 和 β型異構物  Sugars that differ only in the configuration at 1 carbon . 表異構物 (epimers)   Change in optical rotation as a fresh solution stands.

Configuration of Monosaccharide

 Isomers

 Compounds which have the same molecular formula but differ in some way in the arrangement of .

 Type of Isomers

 Structural isomers

 Stereoisomer Structural isomers

 Isomers in which the bonding arrangement of atoms differ; they include chain isomers, position isomers, and functional group isomers

 Chain isomers (Sketal isomers)

Butane Isobutane

 Position isomers

1-Propanol 2-Propanol

 Functional group isomers

Propionic acid Methyl acetate Stereoisomer

 Isomers in which the bounding of the atoms in the same but the spatial arrangements of the atoms differ; they include geometric isomers and optical isomers, etc.  Geometric isomers

cis-2-butene trans-s-butane

 Optical isomers

Enantiomers vs. Diastereoisomers

Diastereomers If a compound contains two chiral atoms, it may exist in four stereoisomeric forms. Since the configuration at each chiral carbon may be either R or S, there are four stereochemical possibilities: RR, SS, RS, and SR. The RR and SS stereoisomers are enantiomers. The RS and SR stereoisomers are also enantiomers. The RR stereoisomer is a diastereomer of both the RS and the SR stereoisomers. The SS stereoisomer is a diastereomer of both the RS and SR stereoisomers. Configuration of Aldoses

 For many system which “n” chiral carbon atoms, there are 2n stereoisomers, composed 2n-1 enatiomeric (mirror-image) pairs.

 An aldotetrose is a four-carbon sugar that has two chiral centers. There are 22 = 4 possible stereoisomers, or two D, L pairs of enatiomers called and .

D, L System of Carbohydrate Configuration Nomenclature

 Compounds are assigned to the D-family or the L-family according to the projection of the –OH group at the lowest chiral carbon atom.

 If the –OH group projects to the right in a plane

projection structure, the compound is in the D-family. If this –OH projects to the left, the substances is in the L- family. D- L-Glyceraldehyde

D-Erythrose D-Threose Fischer Projection Formulas for Acyclic Forms of D-aldoses

Mutarotation

On dissolution of sugars

in water, the optical

rotation of the solution

changes continuously

until an equilibrium is

reached. This

phenomenon, termed

Mutarotation. Conformation of the Six-membered Ring Sysem

Chair Boat

Skew boat Half-chair

4C 1C 1 4

Monosubstituted molecules the substituent favors the equatorial position

Stable chair conformation of β-D-glucopyranose and β-D-fructopyranose Strong hydrogen- bonding interactions

More stable Conformation of the Five-membered Ring System

Envelope conformation Twist conformation

Pseudoequatorial carbon-hydrogen bonds

Monosaccharide  Most of the occur as and as units in oligosaccharide and and only comparatively few of them are present free in plants.

 D-Glucose is the most abundant monosaccharide in nature. It occurs in a free state in many plants, especially in fruits and can be prepared from and starch by acidic or enzymic .

D-Glucose α-D-Glucose Haworth perspective formulas Fischer-Tollens projection α: -CH2OH and -OH at the same side Monosaccharide : 半乳糖

 D- and D-galactose, which are aldohexoses, are

important components in D-mannose D-galactose .

 The most comment aldopentose, abundant members of the hemicellulose, are D- and L-. D-xylose L-arabinose D- is a constituent of nucleosides.

D-ribose

Monosaccharide

 No or have been

detected free in plants, but D-erythrose D-erythrose 4-phosphate

4-phosphate is an important

intermediate in many transformation,

and D-glyceraldehyde and D-glyceraldehyde are essential

components in cellular metabolism.

dihydroxyacetone Monosaccharide

 Deoxysugar

 L-rhmnose (6-deoxy-L-mannose) occurs as a constituent in gum polysaccharides and traces of it are present in hemicellulose L-rhmnose (xylan)

 D-, which represents the only abundant in plants, is present both free and in a combined state. Compositae and Gramineae families store of D-fructose such as insulin, as serve material rather than starch.

Monosaccharide Derivatives

In principal, the sugar derivation are formed by:

Reaction of the free carbonyl or the anomeric hydroxyl at C-1.

Reaction of the hydroxyl groups at other positions. Glycosides

 Sugars react as hemiacetals with hydroxyl compounds, such as alcohols and phenols forming glycosides.

 The glycosides are easily hydrolyzed by aqueous acids to free sugars but they are fairly stable toward alkali. Acetals

aldehyde Cyclic acetal

Acetal formation Ethers

 Etherification is often used in the determination of structure and types of linkages between sugars in oligo and polysaccharides.

 Ethers are very stable against both acids and bases.

 Carbohydrates can be converted into ethers by treatment with an alkyl halide in the presence of base (Williamson ether synthesis)

Ag2O

CH3I

Cellulose Etherification

 Cellulose etherification: by treating alkali cellulose with

 Alkyl or aryl halides (or sulfate)

oxides

 Unsaturated compounds activated by electron-attracting group

- - Cell-OH + OH Cell-O + H2O

- - Cell-O + R-Cl Cell-OR + Cl Anhydro sugars

• Anhydro sugars are formed from sugars by the elimination of

water from a pair of hydroxyl groups

• Glucosans are strictly intramolecular glycosides. Its anhydro

linkage is readily by action of acids, sometimes also by bases.

Anhydro sugars

Epoxides

Ethers are derived only from alcoholic hydroxyls and the hydroxyl group in the anomeric center does not participate.

Epoxides are formed when the sugar contains both a good leaving group and a

suitable located ionized hydroxyl group. (SN2

reaction). Anhydro sugars

 The ring size of epoxides can vary from three- to six- membered rings.

 Oxiranes (three membered derivatives) intermediates are probably formed during alkaline hydrolysis of polysaccharides such as cellulose and starch.

Formation of methyl 3,4-anhydro-β-D-galactopyranosides

Esters

 Hydroxyl groups of sugars can form esters both with organic and inorganic acids.

 The phosphate esters, such as D-glucose 6- phosphate, are important natural products and key intermediates in the and bioconversion of various carbohydrates. Esters

1: D-glucose 1-phosphate

2: 2-O-acetyl-β-D-xylopyranosides

3: 3-O-acetyl-β-D-xylopyranosides 4: β-D-galactopyranose 4-sulfate 5: cis-Inositol inositol: 肌醇

Oligo- and Polysaccharides

• More than 500 oligosaccharides are known today,

most of them occurring as free natural substances.

• Oligosaccharides are also obtained by partial acidic

or enzymic hydrolysis of polysaccharides.

• can be considered to be glycosides in

which the aglycon part is another monosaccharide. Oligosaccharides

 Disaccharides are called reducing or nonreducing, depending on whether one or both reducing groups are involved in the formation of the glycosidic linkage.

Disaccharides

纖維二醣 麥芽醣 蔗醣

Maltose

Sucrose Polysaccharides

Polysaccharides are the most abundant constituents of living

matter.

The chain molecules can be either linear or branched, a fact

that markedly affects the physical properties of the

polysaccharides.

Reaction of Carbohydrates

 Oxidation

 By mild oxidants, aldoses are oxidized to aldonic acid or to corresponding aldonic acid end groups, whereas ketoses are resistant.

 Stronger acids, convert aldoses to dicarboxylic acids, aldaric acid termed aldaric acid. 醛糖二酸

 Aldonic and aldaric acids occur in acidic solution mainly in the form of lactones, which are intramolecular esters. D-glucose D-gluconic acid

D-glucaric acid

Preparation of D-glucuronic acid from D-glucose Examples of Neutral Oxidation Products of Aldoses and Ketoses

• The neutral oxidation products of carbohydrates include dialdose, aldosuloses, and glycodiuloses. • They are important intermediates in the synthesis of carbohydrates and are prepared by chemical or enzymic oxidation of hydroxyl groups in the free aldoses or ketoses or their protected derivatives.

D-galactose can be D-glucose is oxidized by L- is oxidized by selective oxidized to meso- pyranose-2-oxidase to pyranose-2-oxidase to galacto-hexodialdose by arabino-hexos-2-ulose threo-2,5-hexodiulose galactose oxidase

Periodate Oxidation of 1,4-β-D-

• 1°-OH groups are oxidized to formaldehyde, 2°-OH to higher aldehyde, and 3°-OH to ketones. • α-Hydroxyaldehydes are oxidized to formic acid and an aldehyde. • Useful for structural studies. Reduction  Aldoses and ketoses can be reduced to alditols by various agents for which purpose sodium borohydride is very useful.

 Only one product is formed from aldoses, whereas ketoses give rise to two diastereoisomers because of a new asymmetric center.

* *

Addition and Condensation Reaction of Carbonyl Groups • Addition reaction of carbonyl groups severed as valuable tools

for structural studies of carbohydrates.

• Hydroxylamine, hydrazine, and phenylhydrazine react with 鹽酸羥胺 聯胺 苯基聯胺 carbonyl groups to yield oximes and hydrazones.

phenylosazone 苯脎 Kiliani Reaction

 Cyanide react reversibly with sugars to yield cyanohydrins.

 Because of the formation of a hydroxyl group in place of the aldehyde group, a new asymmetric center is generated.

 Catalytic hydrogenation of the cyanohydrins gives the corresponding aldoses, and the kiliani reaction thus opens the possibility for chain lengthening of aldoses.

Kiliani Reaction Formation of epimeric α-hydroxysulfonic acids from D- xylose in the presence of hydrogen sulfite ions

• The equilibrium of this reaction depends on the configuration of the sugar • Mannose and xylose form more stable bisulfite addition products than glucose, and ketoses (fructose) show almost negligible affinity toward hydrogen sulfite ions

The Influence of Acid

 Mechanism of the acid-catalyzed hydrolysis of methyl β- D-glucopyranoside to D-glucose Reactions of Sugars in the Presence of Concentrated Mineral Acids

R = H : furfural

R = CH2OH: hydroxymethylfurfural

prolonged heating time hydroxymethylfurfural decompose to form α- and β-angelica lactones

α- and β-angelica lactones

The Influence of Alkali Enolization to an 1,2- enediol  In alkaline solutions

aldoses and ketoses

undergo rearrangements.

 Lobry de Bruyn-Alberda D-glucose D-mannose van Ekenstein transformation.

D-fructose D-allulose

Allulose 阿洛異糖 Endwise Alkaline Degradation (Peeling reaction)

isomerization enediol formation alkoxy elimination

tautomerization benzilic acid rearrangement Termination of the Peeling Reaction

β-hydroxy elimination

In kraft pulping, the cellulose molecules are subjected to this endwise peeling, which results in a loss of about fifty glucose units from a single molecule.