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 3hJ(NC)' correlations in nucleic acid base pairs, between the imino nitrogen 15N1 and the carbonyl 13C6 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 3hJ(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 3hJ(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 15N-donor and 15N-acceptor resonances connected by 2hJ(NN) couplings in hydrogen bonds involving amino groups. Using this experiment, values ranging between 6 and 8 Hz are determined for the 2hJNN couplings between 15N2 and 15N7 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.