LIGHT‐INDUCED CHANGE IN RHODOPSIN EMISSION: PHOSPHORESCENCE and FLUORESCENCE

Abstract— Phosphorescence measurements of rhodopsin in bovine rod disk membranes were made to study changes in protein conformation on bleaching by probing the environment of tryptophan and tyrosine residues of the protein. Bleaching decreased the tyrosine phosphorescence by about 25% and significantly affected the amplitude of triplet decay when rhodopsin was excited at 280 nm, where both tyrosine and tryptophan absorb. Computer analysis using one or two exponential model functions showed the presence of two components in the decay curve at 410 nm—one with a lifetime of 2.2 s, the other with a lifetime of 4.8 s>—which are typical of tyrosine and tryptophan respectively. When the rod outer segment sample was bleached, there was a significant decrease in the amplitude of the tyrosine component. However, the lifetime values of the two components did not change. Analyses of the fluorescence spectra of dark and bleached membranes at different excitation wavelengths and the phosphorescence change on bleaching suggest energy transfer between tyrosine and tryptophan singlet states, which may result from a conformational change of the opsin moiety on bleaching.