Homogeneous spectrally- and time-resolved fluorescence emission from single-tryptophan mutants of IIAGlc protein

We report an experimental study of protein relaxation dynamics on the picosecond and nanosecond time scales. The protein equilibrium state is perturbed by the redistribution of electric charge density over the side chain of a tryptophan residue. Electronic excitation of the residue induces the charge shift and triggers a relaxation process, the dynamics of which is reflected 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 spectral shift may not represent relaxation dynamics. A criterion For distinguishing between homogeneous and heterogeneous fluorescence emission is suggested here. Emission from the mutants E21W and F3W of IIA(Glc) is found to be free from permanent or long-lived heterogeneity, In E21W, the only tryptophan residue is in a rigid globule, whereas in F3W it is on a flexible tail. The relaxation dynamics reported by the tryptophan residue in F3W is much faster than that in E21W. Addition of glycerol to the solvent slows down the relaxation dynamics for both tryptophan residues.