The mechanism of water-catalyzed excited-state proton transfer (ESPT) for 7-azaindole (7AI ) has long been investigated with controversial conclusions.1,2 Recently, owing to the superiority of sensing biowaters in proteins by a 7AI  analogue, 2,7-diazatryptophan,3 it is timely to reinvestigate water-catalyzed ESPT in 7AI  and its analogues to unify the mechanism. Herein, a series of 7-AI  derivatives and their methylated isomers were synthesized and their pKa, pKa* and fluorescence spectroscopy were carefully measured. For electron-donating substituents (including -H) at the C(3) position (see figure), the pKa* of N(7)-CH3N(1)-H+ tautomer is much greater than 7.0, whereas it is lower than 7.0 upon anchoring electron-withdrawing groups. For the former case, including 7AI , in neutral water or normal physiological condition, we conclude that following ESPT, protonation happens immediately in T* (tautomer), generating an excited cationic species that subsequently undergoes a fast deactivation to yield the normal species in ground state. Therefore, despite ESPT takes place for all 7AI  analogues in neutral water, tautomer emission is not observed in 7AI  but clearly resolved in e.g., 3-cyano-7-azaindole and 2,7-diazatryptophan. For 7AI  in water, we also demonstrate for the first time the tautomer emission at 510 nm in a pH interval of 13-14.5.   References:(1) Petrich, J. W.; et al. J. Phys. Chem. 1993 , 97, 1770(2) Maroncelli, M.; et al. J. Phys. Chem. 1992 , 96, 8430(3) Chou, P.-T.; et al. Nature Commun. 2013 , 4, 2611

Revisiting water catalyzed excited-state proton transfer reaction in 7-azaindole and its analogs

248th National Meeting (2014)

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