Internucleotide scalar couplings across hydrogen bonds in Watson-Crickand Hoogsteen base pairs of a DNA triplex

An extensive analysis of trans-hydrogen bond (2h)J(NN) and (1h)J(HN) scalar couplings, the covalent (1)J(HN) couplings, and the imino proton chemical shifts is presented for Hoogsteen-Watson-Crick T . A-T and C+. G-C triplets of an intramolecular DNA tripler. The 2hJNN coupling constants for the Wat son-Crick base pairs have values ranging from 6 to 8 Hz, while the Hoogstee n base paired thymines and protonated cytidines have values of approximatel y 7 and 10 Hz, respectively. Distinct decreases of (2h)J(NN) are observed a t the tripler strand ends. Trans-hydrogen bond J correlations ((1h)J(HN)) b etween the donor H-1 nucleus and the acceptor N-15 nucleus are observed for this tripler by a novel, simple quantitative J-correlation experiment. The se one-bond (1h)J(HN) couplings range between 1 and 3 Hz. A strong correlat ion is found between the chemical shift of the imino proton and the size of (2h)J(NN), With stronger J couplings corresponding to downfield chemical s hifts. A similar, but inverse correlation is found between the proton chemi cal shift and the (absolute) size of the covalent (1)J(HN) constant. Method s of density functional theory were used to investigate the structural requ irements for scalar J coupling and magnetic shielding associated with hydro gen bonding in nucleic acid base pairs. The dependencies of these NMR param eters on hydrogen bond distances were obtained for a representative base pa ir fragment. The results reproduce the trans-hydrogen bond coupling effect and the experimental correlations and suggest that the NMR parameters can b e used to gain important insight into the nature of the hydrogen bond.