Variations in the hydrogen bond network of the Oxy-1.5 DNA guanine quadruplex have been monitored by trans-H-bond scalar couplings, (h2)J(N2N7), for Na+-, K+-, and NH4+-bound forms over a temperature range from 5 to 55 degrees C. The variations in (h2)J(N2N7) Couplings exhibit an overall trend of Na+ > K+ > NH4+ and correlate with the different cation positions and N2-H2... N7 H-bond lengths in the respective structures. A global weakening of the (h2)J(N2N7) couplings with increasing temperature for the three DNA quadruplex species is accompanied by a global increase of the acceptor N-15(7) chemical shifts. Above 35 degrees C, spectral heterogeneity indicates thermal denaturation for the Na+-bound form, whereas spectral homogeneity persists up to 55 degrees C for the K+- and NH4+-coordinated forms. The average relative change of the (h2)J(N2N7) couplings amounts to similar to 0.8 x 10(-3)/K and is thus considerably smaller than respective values reported for nucleic acid duplexes. The significantly higher thermal stability of H-bond geometries in the DNA quadruplexes can be rationalized by their cation coordination of the G-quartets and the extensive H-bond network between the four strands. A detailed analysis of individual (h2)J(N2N7) couplings reveals that the 5' strand end, comprising base pairs G1 -G9* and G4*-G1, is the most thermolabile region of the DNA quadruplex in all three cation-bound forms.