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.
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