Proton NMR study of the B----Z transition of d(CGm5CG)2 and d(CGm5CGCG)2: theory and experiment.

The magnetic shielding produced by the double helix in a B-DNA and a Z-DNA conformation is calculated for each non exchangeable proton of the oligodeoxynucleotides d(CGm5CG)2 and d(CGm5CGCG)2. The differences between the values obtained for the two helical forms are in good agreement with the variations of chemical shift measured when the salt concentration of the solution is changed from 0.1 M to 2 or 4 M. The analysis of the theoretical chemical shift variations shows that the large upfield shift observed for some of the protons of the cytidine residues is due to the sum of the ring current and local magnetic anisotropy effects of the guanines of the two nearest neighbours residues.