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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
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