7-Azaindol tautomers

Recently, we reported observations of the femtosecond dynamics of tautomerization in model base pairs (7-azaindole dimers) containing two hydrogen bonds. Because of the femtosecond resolution of proton motions, we were able to examine the cooperativity of formation of the tautomer (in... 

Figure 9-9. (A) Fluorescence emission spectra of 7-azaindole (7AI) in different solvents acetonitrile ( - ), diethyl ether (A-A), -propanol (0-0), water-saturated diethyl ether (A-A), and deionized water ( - ). The emission of 7AI in -propanol is multiplied by 20-fold. Inset, fluorescence spectra of 7AI in diethyl ether (A-A) and water-saturated diethyl ether (A-A) expanded by a factor of 80 to show the growth of the second band at -520 nm in organic-water solution. All spectra were recorded at 25°C and 2- xM 7AI, by exciting the samples at 280nm. (B) Consensus scheme explaining the emission properties of 7AI in alcohol (R = alkyl) or water solution (R = -H). The polyhydrated species in water are more populated and emit radiation at 400nm. In water-restricted environments (e.g., water-saturated diethyl ether), the concentration of the normal monohydrated species increases, and upon excitation, it is converted into the tautomer form, which is responsible for the longer wavelength emission at -520 nm (Inset to panel A) [90].

Mishra and coworkers have reported spectral oscillation in the absorption spectra of adenine solution in water when air was passed through the solution and exposed to UV-radiation intermittently over a long period of time. The main absorption peak near 260 nm (4.77 eV) was shifted to near 300 nm (4.13 eV) region which was explained in terms of the formation of the N3H tautomer of adenine,the process of which is closely related to the tautomerization of 7-azaindole. This assertian (the formation of the N3H tautomer in the excited state) is also supported fixim a recent theoretical study of adenine tautomers at the MCQDPT2/CASSCF levels of theory. ... 

Fig. 52 Optimized geometries of the 7-azaindole homodimer in So, Si (normal species) and Si (tautomer) states The double proton exchange path leading to tautomeric conversion is denoted by arrows,Reprinted from M, Mukherjee, B. Bandyopadhyay and T. Chakraborty Ultraviolet and infrared spectroscopy of matrix-isolated 7-azaindole dimer Matrix effect on excited state tautomerization, Chem. Phys. Lett., 2012. 546. 74-79. Copyright (2012), with permission from Elsevier,...

Catalan, J. and Kasha, M. (2000) Photophysics of 7-azaindole, its doubly-H-bonded base-pair, and corresponding proton-transfer-tautomer dimeric species, via defining experimental and theoretical results. J. Phys. Chem. A, 104, 18012-18020. 

Azaindole is structurally close to purine and provides a simplified model for possible proton-relay for purine-type nucleobases. The mobile proton can stay either on the pyrrole or on the pyridine nitrogen. Theoretical calculations using the 6-31G(d) basis set predicted that the tautomer with the pyrrole hydrogen is more stable by 13-16 kcal mol than the structure with a pyridine hydrogen [74,... 

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