ABSTRACT
ABSTRACT We examine the experimental and computational evidence for the effect of conformation on the chemical shielding tensors of 31P in the phosphodiester linkage and 13C in the ribose/deoxyribose moieties of nucleic acids. The phosphodiester group has considerable flexibility and a relatively flat potential surface, resulting in a diversity of conformations in nucleic acids; however, theory shows that the chemical shielding tensors of the low-energy conformations are very similar, which accounts for the very small chemical shift dispersion encountered experimentally. Calculations also show that the orientation of the phosphate chemical shielding tensor is invariant within a couple of degrees over all conformations. In contrast, the ribose/deoxyribose 13C chemical shielding tensors vary dramatically as a function of several conformational variables, and the major task of data analysis is integrating a large number of measurable isotropic chemical shifts and shift tensor elements into a unique conformational solution. We present computational data showing that the full chemical shielding tensors carry additional information not obtainable from isotropic chemical shifts alone.
