The Fluorescence Decays of Tryptophan in Solution at Neutral pH and in Horse Liver Alcohol Dehydrogenase

Abstract

It is now well established that the fluorescence decay kinetics of many single tryptophan-containing polypeptides and proteins are complex, but can be fitted by a multiexponential decay law. A mixture of different species in the ground state can give rise to a bi-or multiexponential decay law. Ground-state microheterogeneity may involve different protein conformations or more subtle changes such as pH-dependent protonation of an ionizable group or the movement of a charged group a few angstroms closer to the fluorophore. In cases where the ground state is homogeneous, excited-state reactions giving rise to one or more products may lead to deviations from monoexponential decay behaviour. Depending upon the mechanism, the fluorescence may follow a mono-, bi-, multi-, or non-exponential decay law. Data interpretation is aided, however, by many observations associating particular kinds of excited-state reactions, such as proton transfer or solvent relaxation, with characteristic behaviour of the decay parameters (αi,τi) as a function of wavelength. Here we examine the fluorescence decay of tryptophan, indole and several of their derivatives as model systems.