J . Phys. Chem. 1994,98, 8801-8805
8801
Acid Catalysis of Excited-State Double-Proton Transfer in 7-Azaindole Chen-Pin Chang,' Hwang Wen-Chi, and Kuo Meng-Shin Department of Chemistry, Fu Jen Catholic University. Shin Chuang, Taiwan, R.O.C.
Pi-Tai Chou'lt and John H. Clements Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208 Received: March 3, 1994; In Final Form: June 22, 1994'
The acid-catalyzed excited-state double-proton transfer (ESDPT) in 7-azaindole has been studied. In carboxylic acids and phosphoric acids, the formation of a 1:l cyclic hydrogen bonded acid/7-azaindole complex was observed with a remarkably large association constant of >1.0 X lo4 M-l. In contrast to alcohol-catalyzed ESDPT, in which the slow ESDPT dynamics involve a large amplitude of solvent reorganization, the rate of acid catalyzed ESDPT is much greater than the decay rate (- 1.O X lo9 s-I) of the normal emission, resulting in a unique tautomer emission. The highly efficient acid-catalyzed ESDPT in 7-azaindole points to the biological application of 7-azaindole as a suitable acid derivative probe in a hydrophobic environment such as in the cell membrane.
Introduction The spectroscopy and dynamics of 7-azaindole (7AI) have received considerable attention since the original observation of excited-state double-proton transfer (ESDPT) by Taylor et al.' It is well accepted that the ESDPT of 7AI occurs in hydrocarbon and alcohol solvents through the formation of hydrogen-bonded dimers and solute/alcohol complexes, respectively.l-I3 As a result, two fluorescence bands are observed in alcohol and in concentrated hydrocarbon solutions at room temperature. In hydrocarbon 490 nm) results solvents the long-wavelength emission (Amx from the double-proton transfer of symmetric, hydrogen bonded dimers'-3 while monomers and other configurations not favorable for proton transfer exhibit short-wavelength "normal" fluorescence (Amx 340 nm in very dilute hydrocarbon solution). Recently, interest in the photophysics of 7AI has been revitalized by its application in two major fields. In one area, 7AI has been used as a suitable probe of excited-state solvation dynamics. Studies of 7AI in various alcohol solutions have served to increase our understanding of the influence of solvation dynamics to the solution phase chemical reaction (e.g., the protontransfer reaction). In alcohol solvents, the initial interpretation of the dynamics of ESDPT involved a two-step model in which the actual proton-transfer event could occur only after a ratedetermining solvent/solute reorganization step.1° This mechanism has been recently supported by Moog and Maroncelli based on the complete picosecond time-resolved emission spectra." In another area, the application of using a derivative of 7A1, 7-azatryptophan, as a novel optical probe for protein structure and dynamics had recently been reported by Petrich and coworkers.l4J5 7-Azatryptophan was proposed to be an ideal noninvasive in situ probe due to its analogous chemical structure with tryptophan and distinguishable (single-exponential decay) emission property. As a prototype molecule to simulate 7-azatryptophan, spectroscopicand dynamic studies of 7AI in aqueous solution have recently been reinvestigated.l6lg Owing to its great potential in both solvation dynamics and biological applications, we have studied the acid-catalyzed ESDFT of 7AI in nonpolar solvents. Our results show remarkable efficiency of the 1:1 acid/7AI complex formation and ultrafast rate of acid-catalyzed ESDPT. Consequently, the biological
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Current address: Department of Chemistry, National Chung-Cheng University, Chia-Yi, Taiwan, R.O.C. Abstract published in Aduunce ACS Abstructs, August 1, 1994.
0022-3654/94/2098-8801$04.50/0
application of using derivatives of 7AI as a probe to mimic the acid derivatives in hydrophobic environments, such as in the cell membrane, is prospective.
Experimental Section Materials. 7AI (Merck) was twice recrystallized from spectrograde ethanol and once from cyclohexane. The purity was checked by the fluorescence excitation spectrum of 7AI in dilute (
