CONFORMATION HETEROGENEITY IN PROTEINS AS AN ORIGIN OF HETEROGENEOUS FLUORESCENCE DECAYS, ILLUSTRATED BY NATIVE AND DENATURED RIBONUCLEASE T-1

We examined the frequency-domain intensity decays of the intrinsic tryptophan fluorescence (Trp-59) from ribonuclease T1 (EC 3.1.27.3) (RNAase T1). At pH 5.5 in native state (below 30.degree.C), the intensity decay of the single tryptophan residue is a single-exponential process. Conditions which result in protein unfolding were found to induce more complex intensity decays. At temperatures above 40.degree.C, or in the presence of guanidine hydrochloride, the intensity decays became obviously double exponential. In general, the main effect of temperature or guanidine was to induce a second subnanosecond component in the intensity decay. The increased complexity of the decays could not be explained by a unimodal distribution of decay times. These results indicate that conformational dispersion of protein structure can be one origin of the multi-exponential decays which are generally observed for protein fluorescence.