Infrared spectra of tautomers and rotamers of 9-methylguanine. An experimental and theoretical study

Abstract

. Both amino-0x0 and amino-hydroxy tautomeric forms of 9-methylguanine have been identified in approximately equal abundance in. infrared studies of these molecules isolated in the hydrophobic environment of an argon matrix at 12 K. The amino-hydroxy tautomer occurs in two different rotamers correlated with the rotation of the OH group. The ratio of concentrations of the two rotamers is sensitive to UV irradiation, and this ratio then relaxes to an equilibrium value after irradiation is stopped. This sensitivity allows us to separate the experimental spectra related to the 0x0 tautomer and to each of the rotamers of the hydroxy tautomer. The relative concentrations of the amino-0x0 and amino-hydroxy tautomers (3) and of the two rotamers (K(h1-h2) = 0.31 & 0.10 in an argon matrix at 12 K and about 30 2 15 in the vapor at 470 K) are estimated from the observed relative infrared absorbances. From these relative concentrations the differences between the free energies of the tautomers (AG47O (0-h) = 0 & 0.5 kJ mol-') and of the two rotamers (AG(h1-h2) = 0.12 ? 0.03 kJ mol-' in the argon matrix at 12 K and AG470 between +2 and -13 kJ mol-' in the vapor at 470 K) have been estimated. The electronic absorption spectrum of 9-methylguanine isolated in the argon matrix at 12 K and the effect of brief ultraviolet irradiation on it have also been studied. In an effort to interpret the experimental results, ab initio calculations of the infrared spectra have been made for 9-methylguanine at the 3-21G//3-21G level. Comparison with the experimental spectra is of some help with the assignment of the infrared spectra for the different tautomers.Key words: 9-methylguanine, tautomerism, infrared and ultraviolet spectra, matrix isolation, ab initio calculation On a aussi CtudiC le spectre d'absorption Clectronique de la 9-mtthylguanine isolCe dans la matrice d'argon B 12 K ainsi que l'effet d'une brkve irradiation avec de I'ultraviolet. Dans le but dlinterprCter les rCsultats exptrimentaux, on a effectuC des calculs ab initio au niveau 3-21G//3-21G des spectres infrarouges de la 9-mtthylguanine. Une comparaison des rCsultats de ces calculs avec les spectres experimentaux est utile pour attribuer les spectres infrarouges des divers tautomkres. Mots clks : 9-mkthylguanine, tautomine, spectres infrarouge et ultraviolet, isolement dans une matrice, calculs ab initio.[Traduit par la rCdaction]

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“…However, in most cases, various modes with d(N3AH) contributions have not been discerned. This is why, for this region of spectra, some differences may occur in the interpretations [53,[55][56][57].…”

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