E.S.R. study of free radicals formed in skin proteins by u.v. irradiation

Synopsis In order to better understand the mechanisms of action, of sunlight on skin, a study of the free radicals formed by irradiation of its protein constituents, has been undertaken. Samples of stratum corneum keratin and of skin collagen have been u.v. irradiated, in a vacuum, at - 196 degrees C, then transferred to an electron spin resonance spectrometer (E.S.R.). The E.S.R. signals have then been recorded at - 160 degrees C and at increasing temperatures in order to follow the transformations of primary free radicals into secondary species more stable at room temperature. For keratin, u.v. rays produce radicals of the type -NH-CHR at low temperature but it is not possible to identify them more precisely. These free species react with combined glycine of neighbouring protein chains to form -NH-CH-CO-(I) radicals, the E.S.R. spectrum of which can be observed from -60 degrees C onwards. In the presence of air, rather unstable peroxide radicals are formed. For irradiated collagen, the E.S.R. signals recorded at increasing temperatures show in addition to (I) some evidence of alanine free radicals (-NH-CH-CH(3) at - 160 degrees C and -NH-C(CH(3))-CO- from -20 degrees C onwards). These results allow a better understanding of the mechanisms which lead to premature ageing of the skin through sunlight exposure and to the transformation of collagen into even more cross-linked proteins. In addition, this study of the free radicals formed in stratum corneum keratin, allows a more refined discussion of the protective effect of skin melanin pigments.