N.m.r. determination of the solution conformation and dynamics of the ȦG mismatch in the d(CGCAAATTGGCG)₂ dodecamer

ȦG base-paired mismatches that occur during replication are among the most difficult to detect by repair enzymes. Such purine-purine mispairs can exist in two conformations, one of which is stabilized by protons [Gao and Patel (1988) J. Am. Chem. Soc. 110, 5178-5182]. We have undertaken a ¹H-n.m.r. and ³¹P-n.m.r. study of the mismatched dodecamer d(CGCAAATTGGCG) ₂ as a function of both temperature and pH to determine the conformational features of the ȦG mismatch. At pH > 7 the mispaired bases are each in the anti conformation and are stacked in the B-like helix. As the pH is decreased, a second conformation becomes populated (apparent pK,, approx. 5.9) with concomitant changes in the chemical shifts of protons of the mispaired bases and their nearest neighbours. Data from two-dimensional nuclear-Overhauser-enhancement spectroscopy show unequivocally that, at low pH, the dominant conformation is one in which the mismatched G residues are in the syn conformation and are hydrogen-bonded to the A residues that remain in the anti conformation. Residues not adjacent to the ȦG sites are almost unaffected by the transition or the mispairing, suggesting considerable local flexibility of the unconstrained duplexes. Despite the bulging of the mispaired bases, the conformation of the A(anti) ̇G(anti) duplex is very similar to the native dodecamer, whereas the AH⁺(anti)̇G(syn) duplex shows a greater variation in the backbone conformation at the mismatched site. According to the chemical shifts, the duplex retains twofold symmetry in solution. The equilibrium between the syn and anti conformations of G9/G21 is strongly dependent on pH, but only weakly dependent on temperature (ΔH approx. 16 kJ ̇mol^-1). The first-order rate constant for the transition is approx. 9 s^-1 at 283 K and approx. 60 s^-1 at 298 K, with an activation enthalpy of approx. 100 kJ mol^-1. The stabilization of the A(anti)̇G(syn) conformation by protons is consistent with models invoking N1 protonation of adenine. Using the derived glycosidic torsion angles we have used restrained molecular dynamics to build models of the neutral and protonated d(CGCAAATTGGCG)₂ oligomers. The results confirm that the A(anti)̇G(anti) and AH⁺(anti)̇ G(syn) conformations are favoured at high pH and low pH respectively, in accord with n.m.r and single-crystal X-ray data.