Pulse sequences for detection of NH2...N hydrogen bonds in sheared G . A mismatches via remote, non-exchangeable protons.

The non-detectability of NH...N hydrogen bonds in nucleic acids due to exchange broadened imino/amino protons has recently been addressed via the use of non-exchangeable protons for detecting internucleotide 2hJ(NN) couplings. In these applications, the appropriate non-exchangeable proton is separated by two bonds from the NH...N bond. In this paper, we extend the scope of this approach to protons which are separated by four bonds from the NH...N moiety. Specifically, we consider the case of the commonly occurring sheared G x A mismatch alignment, in which we use the adenine H2 proton to report on the (A)N6H6(1.2)...N3(G) hydrogen bond, in the presence of undetectable, exchange broadened N6H6(1.2) protons. Two sequences, the 'straight-through' (H6)N6N3H2 and 'out-and-back' H2N6N3 experiments, are presented for observing these correlations in H2O and D2O solution, respectively. The sequences are demonstrated on two uniformly 15N,13C labelled DNA samples: d(G1G2G3T4T5C6A7G8G9)2, containing a G3 x (C6-A7) triad involving a sheared G3 x A7 mismatch, and d(G1G2G3C4A5G6G7T8)4, containing an A5 x (G3 x G6 x G3 x G6) x A5 hexad involving a sheared G3 x A5 mismatch.