The influence of N7 guanine modifications on the strength of Watson-Crick base pairing and guanine N1 acidity:: Comparison of gas-phase and condensed-phase trends

Ab initio quantum-chemical calculations have been carried out to investigate the correlation between N-1 - H-1 deprotonation energy and base pairing of modified guanines (neutral 8-oxoguanine (8OG) and monocationic 7,9-dimethylguanineH(+) (DMG)) and dicationic platinated guanines. The calculated intrinsic gas-phase trends are compared with available solution data. In the gas phase, the stability of the base pair increases with acidification of the N-1(H-1) position; however, the relation is not linear. The guanine N-1 gas-phase deprotonation energies are primarily determined by the total charge of the system; however, there is also a nonnegligible contribution caused by polarization effects, which is especially significant for the DMG. The order of gas-phase deprotonation energies differs from solution pK(a,H2O) values. However, when assuming that the polar environment annihilates the ionic-electrostatic contribution, qualitative agreement is seen between the gas-phase and condensed-phase data. The pK(a,H2O) is primarily determined by polarization effects. In contrast, very poor correlation has been found between the intrinsic guanine-cytosine Watson-Crick (GC WC) basepairing energies and the K-GC,K-DMSO condensed-phase data, even when separately weighting the ionic-electrostatic and polarization contributions. The bell-shaped correlation between the solution N1H acidity of the guanine derivative and the association constant K does not reflect the intrinsic gas-phase trends. The lack of correlation between gas-phase and solution data may be, for example, due to some specific interference with the GC WC base pairing caused by counteranions or presence of structures competing with the desired base pairing.