Quenching interactions and nonexponential decay: tryptophan 138 of bacteriophage T4 lysozyme.

Site-directed mutagenesis has been used to prepare variants of bacteriophage T4 lysozyme that contain only one tryptophan residue at position 138 and to change the residues in the immediate environment of this buried residue. Replacement of glutamine-105 by alanine results in a 2.7-fold increase in fluoresence quantum yield and converts the fluorescence decay from a highly nonexponential form to a single-exponential decay. This is atributed to electron transfer quenching of tryptophan-138 fluorescence by glutamine-105. Replacemeent of alanine-146 by threonine results in a 1.6-fold decrease in fluorescence intensity, indicating enhanced quenching by glutamine-105; replacement of glutamine-105 by alanine in this species results in a 5-fold in crease in fluorescence intensity. The interpretation of the nonexponential decay of the glutamine-105-containing species is discussed in terms of reversibility of the quenching process.