Journal archive > 2004 > N 5,September-October

An impact of carboxylate and Na ions on the tautomeric status of 9-methylguanine

S. P. Samijlenko, O. M. Krechkivska, D. A. Kosach, O. O. Sudakov, D. M. Hovorun

Interactions of 9-methylguanine (m9Gua) with carboxylate ion of acetic acid (CH3COO-) and Na+ were studied by 1H NMR spectroscopy and ab initio quantum chemical calculations of the B3LYP/6-31++G(d,p) and B3LYP/6-311++G(d,p) levels of theory. Changes in the m9Gua 1H NMR spectrum in the presence of the equimolar amount of sodium acetate (NaAc), which in anhydrous DMSO dissociates to CH3COO- and Na+, were interpreted as a consequence of a complex formation of m9Gua in the amino-keto -N1H tautomeric form (m9GuaN1H) with carboxylate ions via two H-bonds involving amino and N1H-imino protons.
Quantum chemical calculations of interactions of the m9GuaN1H ground-state tautomer and the m9GuaN3H high energy one with relative energy 20.01 kcal/mol show that the ground state tautomer forms the ground-state complex CH3COO- : m9GuaN1H, by 5.57 kcal/mol more stable than the CH3COO- : m9GuaN3H complex, and coordination of Na+ with the O6 and N7 atoms reduces this energy difference to 2.57 kcal/mol. Such a coordination of Na+ with tautomer m9GuaN3H therewith decreases its relative energy only to 13.31 kcal/mol. Nonadditivity of the two ligands contributions to the 8-times reduction of the relative energy of the high energy tautomer in the CH3COO- : m9GuaN3H : Na+(O6,N7) triple complex was concluded, the role of CH3COO- being dominant. Besides, coordination with Na+ resulted in an iminoproton transfer from the base to CH3COO- in the triple complexes of both tautomers, according to calculations in vacuum. Biological significance of the results is noticed.


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