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protonated TRP

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The electronically excited states of indole and substituted indoles have attracted considerable attention in the context of understanding the complex photophysical behaviour of tryptophane (Trp) side chains in proteins. The fluorescence of Trp is highly sensitive to environment, making it an ideal choice for reporting protein conformation changes and interactions with other molecules. The properties used, are changes in the fluorescence intensity, wavelength maximum (lmax), band shape, anisotropy and fluorescence lifetimes.

            The power of this probe has been considerably amplified since Trp can often be substituted for other amino acids by site-directed mutagenesis, with minimal effect on structure and activity.

            We have shown that the energy gap between the pp*/ps lowest excited singlet states of indole is strongly modulated upon the local electric field acting on the indole ring. The pp*/ps energy gap of 0.4 eV, in absence of the field, can effectively be eliminated by a moderate electric field of about 5´10-3 a.u. Since the lifetime of the emitting pp* singlet state is governed by the pp*/ps crossing (as demonstrated in many experiments in clusters), this proposition provides an attractive mechanistic picture for understanding the variation of the tryptophan fluorescence lifetime in proteins.

            

Variation of the pp*/ps* energy as a function of an electric field applied along the x,y,z axis