Evidence for reversible excited-state process in tryptophan zwitterion

摘要

This report gives evidence that the biexponential fluorescence decay of tryptophan zwitterion in H{sub}2O solution is due to the occurrence of a reversible two-state excited-state process whereby the corresponding ground-state species are excited. The rate constants (Scheme 1) are within the intervals: 0 < k{sub}01 < 0.57 (ns){sup}(-1), 0.76 (ns){sup}(-1) < k{sub}21 < 1.33 (ns){sup}(-1), 0 < k{sub}02 < 0.58 (ns){sup}(-1), 0.77 (ns){sup}(-1) < k{sub}12 < 1.35 (ns){sup}(-1). These limits were calculated using the values for S{sub}1 = k{sub}01 + k{sub}21 (1.33±0.01 (ns){sup}(-1)), S{sub}2 = k{sub}02 + k{sub}12 (1.35±0.01 (ns){sup}(-1)), and P = k{sub}21k{sub}12 (1.03±0.01 (ns){sup}(-2). The emission spectra of the two excited-state species can be uniquely determined and are different from those associated with the decay times. These results were obtained by repetitive global compartmental analyses of the fluorescence decay surface of tryptophan zwitterion measured over the entire emission spectrum as a function of quencher concentration. This new and powerful analysis method is applicable to all biexponential protein fluorescence decays.