Abstract
To understand the action of sunlight on human skin, free radicals formed by irradiation of skin protein constituents were studied. Samples of stratum corneum keratin and skin collagen were UV irradiated in a vacuum at -196.degree. C and transferred to an ESR spectrometer. The ESR signals were recorded at -160.degree. C then at increasing temperatures to follow the transformations of primary free radicals into secondary species more stable at room temperature. UV irradiated keratin produces radicals of the -NH-.ovrhdot.CHR [1] at low temperatures. More positive identification is impossible. These free species react with combined glycine of neighboring protein chains to form -NH-.ovrhdot.CH-CO-(I) radicals; their ESR spectrum is from -60.degree. onwards. In the presence of air rather unstable peroxide radicals are formed. For irradiated collagen the ESR signals recorded at increasing temperatures, show some evidence of alanine free radicals (-NH-.ovrhdot.CH-CH3 at -160.degree. C and -NH-.ovrhdot.C(CH3)-CO- from -20.degree. C onwards) in addition to I. These results clarify the mechanisms which lead to premature aging of the skin and extensive cross-linkage in collagen proteins. The free radicals formed in stratum corneum keratin allows refined discussion of the protective effect of skin melanin pigments.