Homogeneous spectrally- and time-resolved fluorescence emission from single-tryptophan mutants of IIA(Glc) protein

We report an experimental study of protein relaxation dynamics on the picos econd and nanosecond time scales. The protein equilibrium state is perturbe d by the redistribution of electric charge density over the side chain of a tryptophan residue. Electronic excitation of the residue induces the charg e shift and triggers a relaxation process, the dynamics of which is reflect ed in tryptophan fluorescence emission. In the case of homogeneous emission , the relaxation dynamics can be extracted from a time-dependent red shift in the emission spectrum. In the case of heterogeneous emission, the spectr al shift may not represent relaxation dynamics. A criterion For distinguish ing between homogeneous and heterogeneous fluorescence emission is suggeste d here. Emission from the mutants E21W and F3W of IIA(Glc) is found to be f ree from permanent or long-lived heterogeneity, In E21W, the only tryptopha n residue is in a rigid globule, whereas in F3W it is on a flexible tail. T he relaxation dynamics reported by the tryptophan residue in F3W is much fa ster than that in E21W. Addition of glycerol to the solvent slows down the relaxation dynamics for both tryptophan residues.