Revisiting time-resolved protein phosphorescence

Analysis of time-resolved phosphorescence data from proteins presents certain problems. Care must be taken in establishing that the analysis is not confounded in the early part of the decay by other emitting species. These species may include tyrosine, impurities found in the solvent, or impurities bound to the protein. In this paper, analysis of the phosphorescence of simple mixtures of tryptophan, tyrosine, and tryptophan + tyrosine in glycerol-water solvent has demonstrated the necessity of accounting for tyrosine emission in the analysis of protein phosphorescence. The tyrosine emission is especially strong at cold temperatures and becomes negligible above approximately 185 K in this solvent. Two fitting procedures have been developed to describe the bimodal emission that results from a single-tryptophan protein that contains a significant number of tyrosine residues. The methods utilize either a maximum entropy method-derived lifetime distribution or the stretched exponential function. In both cases some prior information regarding the expected decay characteristics of the tryptophan residue is applied to guide the separation of the tryptophan component from the tyrosine component. This prior information is obtained by comparing the tail of the protein decay to decays of free-tryptophan in solvent at a variety of temperatures until a match is found having close overlap on a log-intensity decay plot.