Abstract
AbstractMultiple 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).
Highlights
We concluded[9,10] that if there is not 7AI dimer under the conditions set by Kwon and Zewail (K&Z), the excited state double proton transfer must be still regarded as concerted
These authors questioned our results with unfair severity in a much longer reply-letter in PNAS[8] and raised three essential comments, challenging us to reply them, from which the following questions may be inferred: i) if there is no 7AI dimer in anhydrous acetonitrile, why are they able to extract a dimer formation constant?; ii) why are the excitation spectra important under that set of conditions, for instance, at such large concentration?; iii) what are those kinetical changes measured with polarity and with isotopical substitution that their C2h 7AI dimer sample undergoes? This last question is out of context if we demonstrate that for 7AI 0.1 M solution in acetonitrile at room temperature the C2h 7AI dimer is not formed
The only reason to assume that the clear display for a buildup of an emission band around 500 nm in polar solvents is that “it is widely accepted as a signature of excited-state double-proton transfer in 7AI dimers”[8]
Summary
The existence of doubly-hydrogen bonded 7AI dimers under those experimental conditions set by K&Z [7] does not agree with some evidence found by our laboratory using 7AI 10-4 M in (spectrometric quality) acetonitrile and butyronitrile solutions, and lowering the temperature allows us to conclude that the 7AI monomer does not form C2h dimers; a brief summary of them was published in PNAS as a short letter[9], and in Nature Preceeding[10] : i) absorption, emission, and excitation spectra of 7AI 0.1 M at room temperature and lower temperatures demonstrate that the C2h 7AI dimer is not formed in (spectrometric quality) acetonitrile, diethyl ether, and dichloromethane; and ii) on excitation at 320 nm no emission is found that could be assigned to the doubleproton transfer fluorescence in any of the above-mentioned solvents.
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