PHOTOCHEMISTRY OF TYPE-I ACID-SOLUBLE CALF SKIN COLLAGEN - DEPENDENCEON EXCITATION WAVELENGTH

Although previous studies have demonstrated that the predominant photo chemistry of type I collagen under 254 nm irradiation may be attribute d either to direct absorption by tyrosine/phenylalanine or to peptide bonds, direct collagen photochemistry via solar UV wavelengths is much more likely to involve several age- and tissue-related photolabile co llagen fluorophores that absorb in the latter region. In this study, w e compare and contrast results obtained from irradiation of a commerci al preparation of acid-soluble calf skin type I collagen in solution w ith UVC (primarily 254 nm), UVA (335-400 nm) and broad-band solar-simu lating radiation (SSR; 290-400 nm). Excitation spectroscopy and analys is of photochemically induced disappearance of fluorescence (fluoresce nce fading) indicates that this preparation has at least four photolab ile fluorescent chromophores. In addition to tyrosine and L-3,4-dihydr oxyphenylalanine, our sample contains two other fluorophores. Chromoph ore I, with emission maximum at 360 nm, appears to be derived from int eracting aromatic moieties in close mutual proximity. Chromophore II, with broad emission at 430-435 nm, may be composed of one or more age- related molecules. Collagen fluorescence fading kinetics are sensitive to excitation wavelength and to conformation. Under UVC, chromophore I fluorescence disappears with second-order kinetics, indicating a rea ction between two proximal like molecules. Adherence to second-order k inetics is abrogated by prier denaturation of the collagen sample. A n ew broad, weak fluorescence band at 400-420 nm, attributable to dityro sine, forms under UVC, but not under solar radiation. This band is pho tolabile to UVA and UVB wavelengths. Amino acid analysis indicates sig nificant destruction of aromatic amino acids under UVC, but not under UVA or SSR. When properly understood, collagen fluorescence fading phe nomena may act as a sensitive molecular probe of structure, conformati on and reactivity.