Ab-initio quantum mechanical calculations of NMR chemical shifts in nucleic acid constituents. I. The Watson-Crick base pairs.

The magnetic shielding constants of the different nuclei of the four nucleic acid bases adenine, uracile, guanine and cytosine are calculated by a non empirical method using a minimal basis set and compared to the available corresponding experimental data. The same calculations carried out for AU and GC pairs give not only the values of the chemical shift variations due to the formation of the pairs but also the relative importance of the three different contributions (geometric, polarization and charge transfer plus exchange) to the total value of delta delta. Their analysis shows the importance of the polarization term. The magnitude of the charge transfer plus exchange term which is obtained for the nuclei belonging to the hydrogen bonding sites indicates that the hydrogen bond length is the major factor in the determination of the magnetic shielding constants of these atoms. On the other hand it appears that the pairing of the bases has a negligible effect on the "geometric" magnetic shielding due to the bases.