Red-edge excitation fluorescence measurements of several two-tryptophan-containing proteins

The dependence of the fluorescence emission maximum of the tryptophan residues in several two-tryptophan-containing proteins (horse liver alcohol dehydrogenase, yeast 3-phosphoglycerate kinase, Staphylococcus aureus metalloprotease and bee venom phospholipase A2) on the excitation wavelengths has been studied. Using fluorescence-resolved spectroscopy, we have dissected the contributions of particular tryptophan residues located in different parts of the protein molecule. The results demonstrate that dipolar structural relaxation can occur in the environment of tryptophan residues buried within protein molecules. The observed spectral shifts upon red-edge excitation of these residues can depend on temperature or ligand binding, as demonstrated in case of metalloprotease and alcohol dehydrogenase. No spectral shifts upon red-edge excitation have been observed for tryptophan residues totally exposed to the rapidly relaxing aqueous solvent.