H-1-NMR CHEMICAL-SHIFT STUDY ON THE STACKING INTERACTION BETWEEN PURINE AND 6-METHYLPURINE

Mutually induced proton chemical shift changes were measured for purine-6-methylpurine solutions in deuterium oxide at 35-degrees-C. The chemical shift vs. concentration profiles were analysed using a three-state decomposition model based on simultaneous, competitive dimer equilibria. The association parameters for purine-6-methylpurine hetero-association provide some additional information about the effect of methylation on the stacking interaction between nucleic acid bases. The equilibrium constant (2.54 +/- 0.17 and 2.57 +/- 0.29 l mol-1, calculated from the experimental data on purine and 6-methylpurine, respectively) suggests the dominant role of the methylated counterpart in the association. The upfield dimer shifts indicate that the methyl group perturbs the plane-to-plane or stacking-like arrangement in the associated state.