Abstract
AbstractMultiple H-bonded base-pairing as a fundamental element of DNA structure was first described by Watson and Crick using stable keto tautomer forms. In their analysis, they considered the possibility of mutations via tautomeric proton transfer shifts. Among other phenomena, such shifts can be caused by electronic excitation; for example, anomalous adenine-cytosine pairing may be a result of two-proton phototautomerism. One suitable model base pair for two-proton translocating tautomerization is the C2h dimer of 7-azaindole (7AI) proposed by Taylor et al. in 1969. The most salient contribu&x00AC;tion of their work was that the double proton transfer in such a dimer occurs in a concerted manner. After the strong controversy raised in 1995 by the proposal of Douhal et al. of a stepwise mechanism for the process was overcome, its concerted nature has been strongly supported by available evidence (see references and references therein). Very recently, however, Kwon and Zewail claimed to have obtained new supportive evidence that the proton phototransfer in 7AI dimer in polar solvents is in fact a stepwise rather than concerted process. In this communication, we conducted a systematic spectroscopic study of the dimerization of 7AI at a 0.1 M concentration in acetonitrile that may allow us to demonstrate whether the 0.1 M 7AI solutions in acetonitrile at room temperature used by Kwon and Zewail contained enough 7AI doubly hydrogen bonded dimer to enable its photoselection and hence its photophysical characterization with femtosecond / fluorescence spectroscopy.
Highlights
To the author's knowledge, 7AI dimerization has been the subject of two studies in polar solvents that warrant discussion here
When the first absorption band for the monomer and dimmer of 7AI are structured and the dimer is red-shifted from the monomer[1,10,13] ( Figures 6 and 7 of Supporting Information), it is easy to expose dimerization from absorption and emission spectra[10, 13, 14]
In this work we studied dimerization of 7AI at a 0.1 M concentration in acetonitrile by using a procedure involving lowering the temperature in a very gradual manner in order to maintain the solution in near-equilibrium conditions throughout the process
Summary
To the author's knowledge, 7AI dimerization has been the subject of two studies in polar solvents that warrant discussion here. The former study excluded the possibility of 0.1 M 7AI in ether occurring in dimeric form, as conceded by Kwon and Zewail[10].
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