Tryptophan fluorescence lifetimes in lysozyme

Tryptophan fluorescence lifetimes at pH 2 and pH 8 have been obtained for lysozyme and for lysozyme derivatives in which tryptophan-62 or tryptophan-108 or both are nonfluorescent. The lifetimes range from about 0.5 ns to 2.8 ns for the various emitting tryptophans. The tryptophan lifetimes appear to increase with exposure of tryptophan to solvent, but intramolecular contacts, probably with cystine residues, can considerably shorten the lifetime. Intertryptophanyl interactions can also affect fluorescence lifetimes. The trytophan-108 lifetime in lysozyme is shorter than in the derivative in which tryptophan-62 is oxidized; this is ascribed to energy transfer from tryptophan-108 to tryptophan-62. From the lifetime results the relative intensities emitted by specific tryptophans can be estimated, and these values also support the existence of intertryptophanyl energy transfer. The emission intensity from tryptophan-62 is greater in the presence of tryptophan-108, and the emission intensity of tryptophan-108 appears to be greater in the absence of tryptophan-62. Conformational effects accompanying chemical modification of tryptophan cannot be completely ruled out, however. The tryptophan-62 lifetime at pH 8 in lysozyme is shorter than in the derivatives, which might indicate a subtle conformational effect. Studies with tri-(N-acetyl-glucosamine)-protein complexes indicate that both the tryptophan lifetimes and the number of emitting tryptophans may be changing upon complexation. The results illustrate the usefulness and the limitations of lifetime measurements in understanding protein fluorescence.