The derivative 4-fluorotryptophan was confirmed to have negligible fluorescence at 25°C and 285 nm (tryptophan/4-fluorotryptophan quantum-yield ratio grater than 100:1). However, photolysis experiments on tryptophan and 4-fluorotryptophan, in which loss of starting material was measured by reverse-phase h.p.l.c., demonstrated that 4-fluorotryptophan was significantly more photochemically active than the parent tryptophan, with the 4-fluorotryptophan photolysis quantum yield being 7 times larger than that of tryptophan at 25°C and 285 nm. In addition, at 77 K and 275 nm 4-fluorotryptophan displayed strong fluorescence and phosphorescence, with emission quantum yields comparable with those of tryptophan at 77 K and 275 nm.
Insoluble and crosslinked proteins and increased pigmentation in the eye lens are features of aging and cataracts. Determining the amino acids which are involved in insolubilization, crosslinking and visible light scattering will shed light on the mechanisms by which cataracts form. Calf lens gamma-II crystallin was irradiated at 295 nm, digested and separated into tryptic peptides. Additional tryptic peptides were found in the digest of irradiated gamma-II which were not present in the dark control digest. These peptides were identified by amino acid sequencing and shown to correspond to expected tryptic fragments of the protein, indicating more facile digestion in the UV-irradiated protein than in dark controls. Amino acid analysis of the irradiated protein and peptides showed losses of histidine, methionine and cysteine residues as compared to control samples. Tryptophan, which is not detected by amino acid analysis, was also found to be reactive since losses in its fluorescence intensity were observed after irradiation. Some of the photochemically active amino acids had lower than expected responses in amino acid sequencing experiments. This suggested specific sites of photochemical activity in the various peptides. The evidence for peptide crosslinks is also discussed.
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