Modeling 2hJiso(N,N) in nucleic acid base pairs:: Ab initio characterization of the 2hJ(N, N) tensor in the methyleneimine dimer as a function of hydrogen bond geometry

The observation of (2h)J(iso)(N, N) coupling has prompted considerable interest in this phenomenon from experimentalists and theoreticians due to the potential these couplings hold for the determination of secondary and tertiary structure in biologically important molecules. Here, we present an ab initio (MCSCF) study of the complete (2h)J(N, N) tensor for a model methyleneimine dimer system as a function of (i) the N-N separation, r(NN), and (ii) the hydrogen bond angle, theta. This simple system models the (2h)J(N, N) tensor of nucleic acid base pairs. Results indicate that although the Fermi-contact mechanism dominates (2h)J(iso)(N, N), the coupling tensor is anisotropic due to contributions from the Fermi-contact spin-dipolar cross term. The variation in (2)hJ(iso)(N, N) as a function of r(NN) is fit to an exponential decay. The influence of theta on the coupling constant is less pronounced but must be considered if experimental coupling constants are to be used for quantitative structure determination. Our results for this simple model system demonstrate that (2h)J(iso)(N, N) is a valuable probe of hydrogen bonding in nucleic acid base pairs.