Azaindole, double proton transfer

Chou PT, Yu WS, Wei CY et al (2001) Water-catalyzed excited-state double proton transfer in 3-cyano-7-azaindole the resolution of the proton-transfer mechanism for 7-azaindoles in pure water. J Am Chem Soc 123 3599-3600... 

Since coiled chains of proteins are known to uncurl because of ionic repulsions when ionization occurs, Reid (1957) suggested that excited state dissociation acts as a trigger in rapid biological processes. The 7-azaindole dimer, which undergoes photo-induced double proton transfer (see Section 4), has similarities to the adenine-thymine and guanine-cytosine base pairs of DNA. Its excited state proton transfers have been proposed as possible mechanisms of mutagenesis (Ingram and El-Bayoumi, 1974). 

Environmental effects, especially changes in solvent polarity, on the photophysical properties of dyes have been described. Likewise, solvation dynamics have been measured for dyes that undergo a substantial increase in dipole moment following excitation.Light-induced intramolecular proton transfer is an important route for non-radiative deactivation of an excited state and has been studied extensively in recent times. Double proton transfer has been reported for [2,2 -bipyridyl]-3,3 -diol ° and for 7-azaindole. ... 

Scheme 1 Excited state double-proton transfer (ESDPT) in 7-azaindole (7-AI) dimers.

Excited-state double-proton transfer in the 7-azaindole dimer in the gas phase 06BCJ373. 

Mukherjee, Bandyopadhyay and Chakraborty " investigated the photophysics of 7-azaindole homodimer (Fig. 52) in Ar and N2 matrices and the souble-proton transfer reaction taken place upon UV excitation. The IR spectra revealed that a doubly hydrogen bonded dimer is predominantly produced upon annealing the seeded matrices at 28 K. The authors found that, unlike in hydrocarbon solutions or in a supersonic jet expansion, the matrix-isolated doubly hydrogen-bonded dimer emits exclusively in ultraviolet from the locally excited state, and the main photophysical process (excited state double proton transfer ESDTP) was slowed down dramatically. They interpreted these observations in terms of matrix effect on the double proton transfer potential, specifically the increase of the effective ESDPT barriers in the rigid inert gas matrices compared to those in the gas phase or hydrocarbon solution. 

One of the most important issues for multi-proton transfer reactions is their mechanism do they occur stepwise or concertedly Schowen highlighted this issue with four examples mutarotation of glucose with 2-pyridone tauto-merization of porphyrin molecules " alcohol-mediated double proton transfer in 7-azaindole and isomerization in 7-azaindole dimer. However, in the 7-azaindole dimer, the mechanism of the excited-state double proton transfer has not been fully resolved, even though careful experimental and quantum mechanical studies have been performed. " The excited-state reaction... 

Research of a more fundamental nature - not directly geared toward finding useful applications - has been reported on two other groups of molecules. 7-Azaindole is another biologically relevant molecule since it is closely related to indole, the core of the amino acid tryptophan. Tryptophan is an important reporter molecule in protein spectroscopy, and replacement of the indole group by an azaindole makes it even more suitable for its simpler decay characteristics and red-shifted spectrum [90]. It was also extensively investigated by Kasha and coworkers [91], and has been the subject of much theoretical work[92].The tendency of 7-azaindole to form dimers in particular solvents has also led to the study of double proton transfer reactions in the excited state [93, 94]. Some of these issues are complicated by the possible presence of anion fluorescence [95] (Figure 1.13). 

Catalan, )., del Valle, J., and Kasha, M. (1999) Resolution of concerted versus sequential mechanisms in photo-induced double-proton transfer reaction in 7-azaindole H-bonded dimer. Proc. Natl. Acad. Sci. U.S.A., 96, 8338-8343. 

Figure 3.19 Resolution of the concerted versus stepwise mechanism of excited-state double proton transfer in doubly hydrogen-bonded dimer of 7-azaindole [90, 91]. Schematic presentations of (a) the concerted, one-step, synchronous double proton transfer in 7-azaindole dimer and (b) the stepwise proton transfer following formation...

Hetherington, W. M., Ill, Micheels, R. H., and Eisenthal, K. B., Picosecond dynamics of double proton transfer in 7-azaindole dimers, Chem. Phys. Lett., 66, 230,1979. 

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