Correlation of conformational heterogeneity of the tryptophyl side chain and time-resolved fluorescence intensity decay kinetics

摘要

The time-resolved fluorescence properties of a tryptophan residue should be useful for probing protein structure, function, and dynamics. To date, however, the non-single exponential fluorescence intensity decay kinetics for numerous peptides and proteins having a single tryptophan residue have not been adequately explained. Many possibilities have been considered and include: (1) contributions from the $+1$/L$-a$/ and $+1$/L$-b$/ states of indole; (2) excited-state hydrogen exchange; and (3) environmental heterogeneity from $chi$+1$/ and $chi$+2$/ rotamers. In addition, it has been suggested that generally many factors contribute to the decay and a distribution of probabilities may be more appropriate. Two recent results support multiple species due to conformational heterogeneity as the major contributor to complex kinetics. First, a rotationally constrained tryptophan analogue has fluorescence intensity decay kinetics that can be described by the sum of two exponentials with amplitudes comparable to the relative populations of the two rotational isomers. Second, the multiple exponentials observed for tyrosine-containing model compounds and peptides correlate with the $chi$+1$/ rotamer populations independently determined by $+1$/H NMR. We now report similar correlations between rotamer populations and fluorescence intensity decay kinetics for a tryptophan analogue of oxytocin. It appears for this compound that either $chi$+2$/ rotations do not appreciably alter the indole environment, $chi$+2$/ rotations are rapid enough to average the observed dependence, or only one of two possible $chi$+2$/ populations is associated with each $chi$+1$/ rotamer.