Characterization of the hydrogen bond network in guanosine quartets by internucleotide (3h)J(NC ') and (2h)J(NN) scalar couplings

Scalar coupling correlations across hydrogen bonds with carbonyl groups as acceptors have been observed in a variety of proteins, but not in nucleic a cids. Here we present a pulse scheme that allows such an observation and qu antification of trans-hydrogen bond (3h)J(NC') correlations in nucleic acid base pairs, between the imino nitrogen (15)N1 and the carbonyl (13)C6 nucl ei within the guanine quartets of the Oxy-1.5 DNA-quadruplex. Intra- and in ternucleotide N-H ... O=C connectivities can be traced around each guanine quartet, allowing the hydrogen bonding partners to be unambiguously assigne d. Absolute values of the (3h)J(NC') couplings are approximately 0.2 Hz as quantified by a selective long-range H(N)CO experiment and are thus on aver age smaller than the analogous (3h)J(NC') couplings observed in proteins. I n addition, an improved version of the pseudo-heteronuclear H(N)N-COSY [Maj umdar et al. (1999) J. Biomol. NMR, 14, 67-70] is presented which allows si multaneous detection of the N-15-donor and N-15-acceptor resonances connect ed by (2h)J(NN) couplings in hydrogen bonds involving amino groups. Using t his experiment, values ranging between 6 and 8 Hz are determined for the (2 h)J(NN) couplings between (15)N2 and (15)N7 nuclei in the guanine quartet. These values are not strongly influenced by the presence of a significant a mount of chemical exchange broadening due to amino group rotations.