The effect of water on NMR spin–spin couplings in DNA: Improvement of calculated values by application of two solvent models

Influence of water on isotropic one-bond 1J(X,H), 1J(C,X) and two-bond 2J(X,H), (X = C,N) indirect NMR spin–spin coupling constants were investigated theoretically at ab initio levels by application of explicit and PCM solvent models. Sensitivity of the Fermi contact contribution to the total J coupling towards the solvent inclusion calculated using coupled perturbed density functional theory with the B3LYP hybrid functional is dominating, while solvent shifts of the other three J contributions are negligible. Consideration of the solvent leads to a noticeable shift of the 1J(C,H) couplings; the largest shift of 6.1 Hz was found for 1J(C8,H8) coupling in guanine. Further inclusion of solvent decreases the mean absolute deviation between the calculated and experimental J constants from 1.7 to 1.1 Hz for guanine, from 2.4 to 0.6 Hz for cytosine, and from 2.3 to 1.6 Hz for adenine. In the explicit solvent model, the 1J(C8,H8) solvent shift seems to be sensitive to the magnitude of charge transfer from water σ bonding O–H and oxygen lone pair orbitals to the σ* antibonding C8–H8, C8–N7, and C8–N9 orbitals of guanine. Both explicit and PCM solvent models provide similar improvement of calculated couplings when comapred to the experimental data of the 13C, 15N-labeled DNA hairpin molecule d(GCGAAGC).