Structure of Neutral Molecules and Monoanions of Selected Oxopurines in Aqueous Solutions As Studied by NMR Spectroscopy and Theoretical Calculations

Katarzyna Dybiec , Sergey Molchanov , Adam Gryff-Keller

Abstract

A methodology enabling investigation of a multicomponent tautomeric and acid?base equilibria by 13C NMR spectroscopy supported by theoretical calculations has been proposed. The effectiveness of this method has been illustrated in a study of 2-oxopurine, 6-oxopurine (hypoxanthine), 8-oxopurine, and 2,6-dioxopurine (xanthine) in neutral and alkaline aqueous solutions. For each compound a series of 13C NMR spectra were recorded at pH ranges in which neutral molecules, monoanions and/or dianions occurred in dynamic equilibrium. The carbon chemical shifts for these three forms of the investigated compounds were retrieved from the analysis of pH-dependence of the measured, dynamically averaged values of these parameters. The structures of several stable tautomers of the neutral and monoanionic oxopurine forms were predicted from theoretical calculations and nuclear magnetic shielding constants for 13C nuclei in these tautomers were calculated. At both calculation steps (molecular geometry optimization and calculation of NMR parameters) the PBE1PBE/6-311++G(2d,p) level of theory was used. The populations of the most stable tautomers were determined from the experimental data analysis exploiting the fact that they were population-weighted averages of the chemical shifts of particular tautomers. It has been shown that only the oxo forms of the investigated oxopurines are present in aqueous solutions and that the determined populations in most cases remain in a qualitative agreement with the calculated free energies of the appropriate tautomers. The obtained results are in general agreement with other literature reports on oxopurine tautomerism and confirm importance of the hydration phenomena for the investigated systems. The data analysis has shown that the best compliance between theory and experiment is obtained when the hydration phenomenon is modeled by discrete hydration augmented by PCM (polarizable continuum solvation model).
Author Katarzyna Dybiec
Katarzyna Dybiec,,
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, Sergey Molchanov
Sergey Molchanov,,
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, Adam Gryff-Keller (FC / DOC)
Adam Gryff-Keller,,
- Department Of Organic Chemistry
Journal seriesJournal of Physical Chemistry A, ISSN 1089-5639, (A 35 pkt)
Issue year2011
Vol115
No10
Pages2057-2064
DOIDOI:10.1021/jp110888m
URL http://dx.doi.org/10.1021/jp110888m
Score (nominal)35
Publication indicators WoS Impact Factor: 2011 = 2.946 (2) - 2011=2.941 (5)
Citation count*5 (2015-02-15)
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