FORSCHUNG 45 CHIMIA 49 (1995) Nr. 3 (Miirz) Chim;a 49 (1995) 45-68 quire specific molecules, pigments or dyes © Neue Schweizerische Chemische Gesellschaft (biochromes) or systems containing them, /SSN 0009-4293 to absorb the light energy. Photoprocesses and colors are essential for life on earth, and without these biochromes and the photophysical and photochemical interac- tions, life as we know it would not have The Function of Natural been possible [1][2]. a Colorants: The Biochromes ) 1.2. Notation The terms colorants, dyes, and pig- ments ought to be used in the following way [3]: Colorants are either dyes or pig- Hans-Dieter Martin* ments, the latter being practically insolu- ble in the media in which they are applied. Indiscriminate use of these terms is fre- Abstract. The colors of nature belong undoubtedly to the beautiful part of our quently to be found in literature, but in environment. Colors always fascinated humans and left them wonderstruck. But the many biological systems it is not possible trivial question as to the practical application of natural colorants led soon and at all to make this differentiation. The consequently to coloring and dyeing of objects and humans. Aesthetical, ritual and coloring compounds of organisms have similar aspects prevailed. This function of dyes and pigments is widespread in natl)re. been referred to as biochromes, and this The importance of such visual-effective dyes is obvious: they support communication seems to be a suitable expression for a between organisms with the aid of conspicuous optical signals and they conceal biological colorant, since it circumvents revealing ones, wl,1eninconspicuosness can mean survival. But the purpose of natural the difficulty to distinguish between dye pigments and biochromes is broader. Emergence and development of life on earth is and pigment. Notwithstanding these ob- inseparably associated with the radiating energy source, the sun. This led to the jections all terms will be used when dis- evolution of biochromes with various functions: collection and harvesting of light, cussing color in living systems [4][5]. transduction into chemical energy, transport of electrons or gases, photoreceptor processing of information and color discrimination, to mention only a few. Without 1.3. Function these pigments - with their photobiological, biochemical, and physiological reactions The functions of biochromes can be - life, as we know it, would not have been possible. Color is essential. Several times either biochemical and metabolic or bio- during the evolution protecting devices and mechanisms had to be invented against the physical and physiological, the visual-ef- destructive and damaging influence of reactive oxygen, light and UV radiation. To this fect functions belonging to the latter end nature employed accessory pigments as carotenoids and flavonoids. It is notable category .The connection between the two and interesting to recognize that human epidemiology and animal studies have groups is made by the light-perceptor func- indicated that cancer risk may be modified by changes in dietary habits or dietary tions [6]. These three main functions are components. Recent studies indicate that phytochemicals, among them the carotenoids presented in Fig. 1 and flavonoids, can inhibit tumor genesis at one or several stages. 1. Introduction 1.1. Life, Light, and Color Light from the sun has been associated ® Light Source with life since the very origin of life itself. This is born out by the fact that all organ- isms, from bacteria to man, exhibit some "IMetabaliC FunctionI form of photosensitivity to solar radiation. y According to Grotthus-Draper's Jaw of absorption photobiological phenomena re- ~ Response ~ *Correspondence: Prof. Dr. H.-D. Martin Institut fur Organische Chemie und Organism with Organism with Visual and/or Makromolekulare Chemie Lehrstuhl I other Photoreceptor Pigment Heinrich-Heine- Universitat Integumental Pigment Universitatsstrasse I D-40225 Dusseldorf IVisual-Effect Function I IPhotoreceptor Function I ") This article is an extended version of a lecture given at the' 12th International Color Symposium', Bad Homburg v.d.H., Germany, Fig. I. Visual-effect, photoreceptor and metabolic functions. IP: integumental pigment. R: photore- September 18-22, 1994. ceptor, M: metabolism. FORSCHUNG 46 CHIMIA 49 (1995) Nr. 3 (Miirz) 2. The Physical and Chemical Basis of tals form stacks of layers. The twisting of behind the retina in their eye that gives rise Natural Color them produces diffraction and interfer- to brilliant metallic reflection (tapetum ence. The iridescent color of scarab beetle lucidum in the choroid). The iridescent 2.1. Structural Colors Lomaptera jamesi and Plusiotis gloriosa scales of butterflies, e.g. Morpho, contain When a physical structure is capable of belongs to this category [7]. a variety of layered structures, and the producing color this may be called struc- under-scales are pigmented with melanin. tural color (the name 'schemochrome' has 2.1.2. Thin Film Inte1erence It is this structure of the scales with its also been suggested [4]). The majority of iridescent colors in vanes that is responsible for interference biological systems originates from this color, as shown schematically in Fig. 2 2.1 .1. Diffraction Gratings phenomenon.There is still a color change [6][7] (color prints Fig. 39-43). These colors are rather rare in nature. with the viewing angle, although not as They areon]y observable using direct light strong as with diffraction.The iridescence 2.1.3. Light Scattering and disappear when light becomes dif- coined the terms 'metallic or enameld' Rayleigh scattering is observed when fuse. The change of color with the angle of color. Outstanding examples are the beau- the responsible particles are smaller than viewing is strong, and the reflection is tiful colors caused by multiple film inter- the wavelength of light, so that good blue iridescent. Examples are the surface of ferences of butterfly wings and humming- scattering can be seen from particles as beetles Serica sericea and the indigo snake bird or peacock feathers as well as mother- large as 300 nm on down to sizes of I nm, Drymarchon corais (co]or print Fig. 38). of-pearl. Some nocturnal animals (cat, bush this is also called Tyndall blue. Most non- Cholesteric mesophases of! iquid crys- baby) have a reflecting layer-structure iridescent blue colors in animals are Tyn- dall blues: blue eye color in humans, bril- liant blue and purple areas in the faces, buttocks and genital areas of monkeys, Thickness Distance Stack of Angle of Wavelength blue color of bird feathers located on the of plate (P) between plates (P) plates incidence reflected and barbules, blue color of fishes and repti les colour (nm) usually based on guanine particles (color print Fig. 44). When the size of the scattering parti- 60° 405 violet cles becomes larger and eventually ex- 0.05 0.15 ceeds that of the wavelength of light, no 90° 454 blue-violet G D Rayleigh but Mie scattering is observed. White or whitish scattering arises in this 60° 464 blue way: snow, fog, mist, clouds, white hair, 0.10 0.10 white butterflies and moths, white and 90° 506 blue-green G D silver fish, white feathers. It should be mentioned here, that many green and pur- ple colors originate from blue and green Tyndall , 520 0.15 0.05 . 60° an additional yellow or red pigment, re- , . yellow-green G D 90° 559 specti vely [2] [4][7]. Fig. 2. Origin of butterfly wing interference. Structure of plates arises from the vanes in the iridescent 2.2. Chemical Color scales (modified after Fox [4]). 2.2.] . Localization of Biochromes The biochromes that will be discussed Table. Some Functional Localizations of Biochromes in the following exert their function in specialized organs, tissues, fluids, cells, pe ie. L alll.an n FunCli n organelles, and plastids. Some of these that are important for our discussion, are Ph I ) mhl Purple Ba ten a hmmalllphol1' listed in the Table. hlnr )SOIll , Ph to )'nthe:ls Green Bac~ nn Thylal-Olds Phot s nthesl. Pr chloron Energy Collection and Transduction: a) Thylalmlds Phot . ynthe " nnoba ten Lamellae, tubes, vesicles, called chromatophores, continuous with mem- Ph t • ynthcSls anobacteria, Red 19ae Phyc('blll'llme. brane (phototrophic purple bacteria); b) Higher Plant!.. Iga hl()mpla.\ts Phot . )nth 'sis Chlorosomes attached to but not continu- lembmn Light-Mediated IIal bactcnum ous with cytoplasmic membrane (pho- PI' t n Pump totrophic green bacteria); c) cytoplasmic membrane only (Heliobacteria); d) inter- IIgma. Paratla 'ellar PhOIOIU 'j, Euglena. hlam 'domona1> nal thylakoid membrane structure (Pro- Boo. lembran' chloron); e) lamellar multilayered thy]a- Pineal F \: C I 'f hange. Rh)'lhms mphlbl3n . Reptiles. Bird: koid membrane system especially preva- l~)t.'s 1 Ion Animnb lent towards the periphery of the cell (cy- Mllochondna Re pirati n ells anobacteria); f) phycobi]isomes (cyano- bacteria and red algae); g) chloroplasts hromoplasls. lIeunl 'S Isunl-ElTe t Higher Plants (higher plants and most algae); h) mem- OImal hromat ph r i uaJ- _ff t branes (Halobacterium). FORSCHUNG 47 CHI MIA 49 (1995) Nr, 3 (Marz) Sensory Transduction: a) Stigma and paraflagellar swelling H Me S-¥roteln eyespot and plasma mem- (Euglena); b) Me Me H a brane (Chlamydomonas); c) plasma mem- M.o~eI I brane (Halobacterium); e) extra-ocular re- MeO ';:: H ceptors like median eye (arthropods), front o 7-10 organ (amphibians), parietal eye (reptiles) Quinone and pineal organ (birds and other);f) in- ae H a a (Ubiquinone) tegumental photoreceptors (bivalves, as- I COOMe cidians, insects); g) nerve cells (Aplysia, o~ echinoids, crinoids); h) visual organs and Porphyrin 8iIin eyes (eye cups, compound eyes, pinhole (Chlorophyll a ) (Phytochrome Pr(Z.syn» eyes, lens eyes). MeO Metabolic Functions: OMe a) Chloroplasts (higher plants); b) mi- Carotenoid tochondria (respiration); c) blood, mus- (Spirilloxanthln) cle, root nodules (oxygen transport).