Fluorescence of collagen — properties of tyrosine residues and another fluorescent element in calf skin collagen

Recent studies have revealed two types of ultraviolet fluorescence in collagen [I--3]. The first type with fluorescence parameters of 275 nm/3 15 nm is due to tyrosine residues; the nature of the compound responsible for the “long-wave” UV fluorescence (345 nm/440 nm) remains obscure [ 1, 21. The relationship between collagen cross-linking and longwave UV fluorescence have been considered by LaBeIIa and Thornhill [ 1 J . These authors showed the accumulation with age both of long-wave fluorescence and cross-linking density. The only direct evidence associating cross-linking and such fluorescence in collagen is available from the irradiation studies of Fujimori [2]. Hoermann and Balekjian [3] studying the luminescence properties of individual a-chains were able to show that collagen appears to consitute a special class of proteins with regard to its luminescence properties. According to their experiments, the element responsible for emission at 39.5 nm is not associated with the cross-link formation. Furthermore, dityrosine, which is one of the fluorescent cross-link sites in resilin and elastin, is absent in collagen [4] or present in trace amounts only [S] . Because the fluorescent element (or elements) is apparently important both in collagen structure and physiology, an attempt was made to localize these elements within the structure and to get a somewhat deeper insight into the tyrosine fluorescence in collagan.