Abstract— The variation with temperature, pH and light of the ESR signal of hydrated melanin powders from Japanese black hair has been studied. An explanation of the results is proposed on the basis of quinhydrone type complexes and of acid‐base equilibria of melanin and its semiquinone radicals. During exposure to light of wavelengths 254–600 nm, both stable and unstable radicals have been observed. The action spectrum for the formation of stable melanin radicals has been determined.
Synopsis The ESR spectrum of hair presents a symmetrical absorption band with the same characteristics as pure melanin (g= 2.0030; deltaH= 5 G). Between 250 degrees K and 490 degrees K, the intensity of the signal increases as for pure melanin. This variation may be explained by a change in the moisture content of hair as function of the temperature. Below 250 degrees K, in a frozen medium, melanin-water interactions remain constant and the number of radicals is no longer a function of temperature. Under exposure to light of hair, melanin-derived radicals were formed. Stable and short-lived radicals were observed at lambda > 345 nm; only unstable radicals were formed at lambda > 450 nm. At shorter wavelengths (lambda > 280 nm), a new asymmetric signal (g= 2.0065H= 20 G) appeared on UV exposure of black, grey or white hair; this signal was attributed to radicals formed in the direct excitation of keratin amino acids.
R e~u le 5 mars 1980 LOUISE-MARIE COULANGEON, GILLES PERBET, PIERRE BOULE et JACQUES LEMAIRE. Can. J. Chem. 58,2230Chem. 58, (1980. En solution aqueuse, 1'0-phknyl-phenol prksente une double fluorescence qui traduit un Cquilibre de deprotonation dans I'Ctat excite singulet. Cette propriCtC permet d'interprkter I'influence de I'oxygene sur la production photochimique primaire des radicaux phknoxy. On montre que, seul I'etat excite singulet de la forme phenolate donne lieu a la photo-kjection d'electrons. L'oxygene inhibe la recombinaison des electrons eject& avec les radicaux phknoxy. D'autres capteurs d'ilectrons comme les Cd2+ ou NO3-sont tgalement susceptibles d'accroitre le rendement quantique de disparition de 1'0-phenyl-phenol en solution dtgazee. L'effet de I'oxygene au niveau du processus primaire permet d'interprkter les resultats acquis en photo-oxydation d e derives phCnoliques donnant naissance a des radicaux phenoxy inoxydables.LOUISE-MARIE COULANGEON, GILLES PERBET, RERRE BOULE, and JACQUES LEMAIRE. Can. J. Chem. 58,2230Chem. 58, (1980. The dual fluorescence observed in aqueous solution of o-phenyl-phenol is attributed to a deprotonation equilibrium in the excited singlet state. The effect of oxygen on the primary photochemical process of formation of phenoxy radicals can be explained in a parallel study of the variations of fluorescence and photo-oxidation quantum yields of this phenolic compound at different pH. It is shown that photoionisation occurs only from the excited singlet state of the phenolate form. Oxygen inhibits recombination of the solvated electron and phenoxy radical. Increase of quantum yield for the disappearance of irradiated o-phenyl-phenol in degased solution also is observed in the presence of electron scavengers like Cd2+ or NO3-ions. Even when the phenoxy radicals are not able to react with oxygen, quantum yield for the disappearance of the phenolic compounds is higher in an aerated than in a degassed solution; the oxygen effect on the primary photochemical step may explain this difference.
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