Characterization of the hydrogen bond network in guanosine quartets by internucleotide 3hJNC′ and 2hJNN 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 acids. Here we present a pulse scheme that allows such an observation and quantification of trans-hydrogen bond (3h)J(NC') correlations in nucleic acid base pairs, between the imino nitrogen (15)N1 and the carbonyl (13)C6 nuclei within the guanine quartets of the Oxy-1.5 DNA-quadruplex. Intra- and internucleotide N-H ... O=C connectivities can be traced around each guanine quartet, allowing the hydrogen bonding partners to be unambiguously assigned. 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 average smaller than the analogous (3h)J(NC') couplings observed in proteins. In addition, an improved version of the pseudo-heteronuclear H(N)N-COSY [Majumdar et al. (1999) J. Biomol. NMR, 14, 67-70] is presented which allows simultaneous detection of the N-15-donor and N-15-acceptor resonances connected by (2h)J(NN) couplings in hydrogen bonds involving amino groups. Using this experiment, values ranging between 6 and 8 Hz are determined for the (2h)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 amount of chemical exchange broadening due to amino group rotations.