Computational and experimental trans-Hbond deuterium isotope shifts suggest that Hbonding and electronic base-stacking interactions, although largely orthogonal, are coupled in B-form DNA duplexes. For an A:T base pair, the Hbond is shorter and stronger in the RAR:YTY than YAY:RTR context. This difference is due to the greater anharmonicity of the N3−H3 vibrational potential of the thymine in RAR:YTY, which arises from electronic interactions between A:T and adjacent bases. As predicted by the calculations, reduction of the base stacking propensity using ethanol abolishes the experimental sequence dependence of 2hΔ13C2.

Solvent-induced effects on nitrogen NMR shielding of 1,2,4,5-tetrazine and two isomeric tetrazoles are calculated using density functional theory combined with the polarizable continuum model and using the continuous set gauge transformation. Direct and indirect solvent effects on shielding are also calculated. It has been shown that the observed solvent-induced shielding variation is more strongly related to the intensity of the solvent reaction field rather than on the change of molecular geometry induced by the solvent.

Density functional theory calculations of isolated Watson-Crick A:U and A:T base pairs predict that adenine 13C2 trans-hydrogen bond deuterium isotope shifts due to isotopic substitution at the pyrimidine H3, (2h)Delta13C2, are sensitive to the hydrogen-bond distance between the N1 of adenine and the N3 of uracil or thymine, which supports the notion that (2h)Delta13C2 is sensitive to hydrogen-bond strength. Calculated (2h)Delta13C2 values at a given N1-N3 distance are the same for isolated A:U and A:T base pairs. Replacing uridine residues in RNA with 5-methyl uridine and substituting deoxythymidines in DNA with deoxyuridines do not statistically shift empirical (2h)Delta13C2 values. Thus, we show experimentally and computationally that the C7 methyl group of thymine has no measurable affect on (2h)Delta13C2 values. Furthermore, (2h)Delta13C2 values of modified and unmodified RNA are more negative than those of modified and unmodified DNA, which supports our hypothesis that RNA hydrogen bonds are stronger than those of DNA. It is also shown here that (2h)Delta13C2 is context dependent and that this dependence is similar for RNA and DNA.

Quantum mechanical calculations are presented that predict that one-bond deuterium isotope effects on the 15 N chemical shift of backbone amides of proteins, 1 D 15 N(D), are sensitive to backbone conformation and hydrogen bonding. A quantitative empirical model for 1 D 15 N(D) including the backbone dihedral angles, U and W, and the hydrogen bonding geometry is presented for glycine and amino acid residues with aliphatic side chains. The effect of hydrogen bonding is rationalized in part as an electric-field effect on the first derivative of the nuclear shielding with respect to N-H bond length. Another contributing factor is the effect of increased anharmonicity of the N-H stretching vibrational state upon hydrogen bonding, which results in an altered N-H/N-D equilibrium bond length ratio. The N-H stretching anharmonicity contribution falls off with the cosine of the N-HÁÁÁO bond angle. For residues with uncharged side chains a very good prediction of isotope effects can be made. Thus, for proteins with known secondary structures, 1 D 15 N(D) can provide insights into hydrogen bonding geometries.