The origin of multi-exponential fluorescence decay property of tryptophan (Trp) in protein has been in controversy, and dielectric relaxation is thought to be one of the most plausible candidates of that origin. In this study, we studied melittin-calmodulin interaction on the concept of dielectric relaxation by spectrally and time-resolved fluorescence spectroscopy. Trp residue in melittin demonstrated drastic change in its dielectric relaxation rate and scale by binding with calmodulin. Expected change of relaxation rate suggested that dielectric relaxation accounts for multi-exponential property of fluorescence decay. We also examined the time variation of radiative and non-radiative decay rates. That result demonstrated the distinct difference profiles of non-radiative decay rate of Trp in melittin and the complex.
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