The dependence of the fluorescence emission maximum of the tryptophan residues in several twotryptophan-containing proteins (horse liver alcohol dehydrogenase, yeast 3-phosphoglycerate kinase, Staphylococcus aureus metalloprotease and bee venom phospholipase A,) 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 ofmetalloprotease and alcohol dehydrogenase. No spectral shifts upon red-edge excitation have been observed for tryptophan residues totally exposed to the rapidly relaxing aqueous solvent.The intrinsic fluorescence emission of tryptophan residues in proteins provides a sensitive probe of the dynamic properties of their microenvironment. A variety of fluorescence methods, including time-and phase-resolved spectroscopy, have been used in such studies. Some information about protein dynamics can be obtained on the basis of steady-state fluorescence measurements. The steady-state collisional quenching of tryptophan residues buried inside a protein molecule reveals some information about the protein dynamics, since a contact between a fluorophore and a quencher is required in quenching reactions. Other dynamic properties of proteins, which can also be deduced from steady-state measurements, are associated with the phenomenon called the rededge excitation effect [I]. In this phenomenon the tryptophan fluorescence emission spectra can shift to the longer wavelengths as the excitation wavelength is increased toward the red side of the tryptophan absorption.Recently Demchenko [2] has shown that, in several singletryptophan-containing proteins, the red-edge effect is particularly visible in proteins which show fluorescence emission maximum at about 325 -340 nm. In the case of short-wavelength-emitting proteins like azurin, parvalbumin and ribonuclease T I and proteins emitting at a long wavelength (about 350 nm), like melittin in water, no spectral shifts upon the rededge excitation have been observed [2]. This might be due to the fact that in azurin, parvalbumin and ribonuclease TI the single tryptophan residues are located in a non-polar environment or due to the absence of dipole relaxation processes in the tryptophan residue's surroundings.On the other hand, in proteins like melittin (at low ionic strength) the single tryptophan residue is totally exposed to the rapidly relaxing aqueous solvent [2]. Such an explanation remains in agreement with a number of observations indicating that excitation-dependent shifts of fluorescence emission can be easily detected for polar fluorophores in a polar viscous solvent [l].The fluorescence emission of most proteins is heterogeneous s...