F. Gai

Solvation and excited-state proton transfer of 7-azaindole in alcohols

Abstract Fluorescence lifetime measurements of purified 7-azaindole in methanol and butanol as a function of temperature are performed. The results indicate that in alcohols there exists a population of 7-azaindole that is solvated in such a way that excited-state tautomerization is no longer a viable mode of nonradiative decay, that is, in a way that has been observed for water but to a lesser extent. Solvation by alcohols or by water is thus more similar than has been previously proposed. An excellent correlation is demonstrated between the time constant for double-proton transfer and pKauto, where Kauto is the autoprotolysis constant of the solvent. This correlation, along with earlier results, suggests that the actual proton-transfer event, and not the solvation step forming the necessary cyclic hydrogen-bonded complex, is the rate-limiting step in dilute solutions of 7-azaindole.

Photoionization and dynamic solvation of the excited states of 7-azaindole

The excited-state photophysics of the biological probe, 7-azaindole, are examined in water and methanol. Electrons in a presolvated state absorbing in the infrared appear within the excitation pulse width of 130 fs. 330±100 fs is required for the presolvated electron to achieve the spectrum characteristic of the completely solvated electron. An excited-state transient absorbance decays in ∼350 fs for 7-azaindole and its methylated analog, N 1 -methyl-7-azaindole (IM7AI), in the region 400-450 nm in water and methanol. The instantaneous appearance of the electron in the infrared is attributed to the decay of the 1 L b excited-state that overlaps the 1 L a excited state of 7-azaindole

Using 7-azaindole to probe condensed phase dynamics

In order to study experimentally the ultrafast (<1 picosecond to several hundreds of picoseconds) molecular dynamics of protein-protein interactions, an optical probe is required. Tryptophan has been the most widely used intrinsic optical probe of protein structure and dynamics. There are, however, two major problems attendant to the use of tryptophan, especially in fluorescence measurements. First, since tryptophan is a naturally-occurring amino acid there are often several tryptophans in a protein molecule whose emission must be distinguished. Second, the fluorescence decay of tryptophan itself in aqueous solution is nonexponential. We have consequently investigated alternatives to tryptophan. Our work has led us to the amino acid analog, 7-azatryptophan, and its chromophoric moiety, 7-azaindole.