On the 7-azaindole in acetonitrile anhydrous solutions as an inappropriate photophysical model for DNA base pairs

Abstract

Multiple Hydrogen-bonding is a fundamental issue to explain base-pairing in DNA structures, which was firstly described by Watson and Crick^1^ using stable keto tautomer forms. In their analysis, they considered the possibility of mutations via proton transfer reactions within a base pair. Such reactions can be induced by electronic excitation, for example, adenine-cytosine mispairing may be caused by double-proton phototautomerism.^2^ The 7-azaindole (7AI) doubly-hydrogen bonded dimer was firstly proposed as a suitable model for explaining the DNA base mispairing owing to excited state two-proton phototautomerization in 1969 by Taylor et al.^3^ The concerted nature of this excited state biprotonic transfer has been strongly supported by available evidence (see references 4-6 and references therein). Recently, Kwon and Zewail^7^ (K&Z) have reported femtosec time-resolved evidence on the stepwise mechanism in polar solvents, using very concentrated solutions of 7AI (0.1 M) in anhydrous acetonitrile, diethylether and dichloromethane on excitation at 320 nm. However, based on a careful spectroscopic analysis of the absorption and emission spectra of anhydrous 0.1 M 7-azaindole solutions in acetonitrile and butyronitrile, we demonstrate in this letter that the 7AI molecule does not form the doubly hydrogen bonded dimer at room temperature (rt) in acetonitrile, but it does generate another aggregate which emits fluorescence at ca. 500 nm. Consequently, the assertion of Kwon and Zewail^7,8^ that the rate of proton transfer in 7-azaindole dimers is significantly dependent on the solvent polarity and its stepwise mechanism for the process is not rightly stated as no C~2h~ dimer is formed in the medium used to record their femtosecond time resolved and fluorescence spectroscopic evidence (viz. 7-azaindole 0.1 M solution in acetonitrile at rt).