We describe the first direct observation of N-H ... O=C hydrogen bonding in nucleic acids via the four-bond (4h)J(NiNj) coupling within an N1(i)-H1(i)... O6(j)-N1(j) segment of a G.G.G.G tetrad. The experiment, two-dimensional (2D) HN(N)-TOCSY, makes use of band-selective N-15 isotropic mixing to transfer magnetization exclusively via the (4h)J(NN) couplings to yield correlations between N1H(i)(omega(2)) and N1(j)(omega(i)) across the hydrogen-bonded segment. In a complementary experiment, 2D cross-polarization (CP)-H(N)CO-(NN)-TOCSYI employing band-selective heteronuclear N-15-C-13 cross-polarization sequences on either side of the N-15-N-15 TOCSY period, correlations are obtained between N1H(i)(omega(2)) and C6(j)(omega(1)) nuclei across the hydrogen bond. The symmetric A(2)X(2)-type coupling topology of the four-spin nitrogen system in the G.G.G.G tetrad permits accurate measurement of these couplings by a new procedure that fits the experimental data with known analytical isotropic mixing transfer functions. The techniques are demonstrated on a G G G G tetrad formed within a novel dimeric quadruplex fold of a uniformly C-13/N-15-labeled d(G-G-G-T-T-C-A-G-G) DNA sequence. The value of the (4h)J(NN) coupling constant for this system is estimated to be 0.136 +/- 0.021 Hz.