Structural studies of A-form sodium deoxyribonucleic acid: phosphorus-31 nuclear magnetic resonance of oriented fibers

Abstract

A highly oriented sample of A-form sodium DNA was investigated by using proton-enhanced 31P NMR. Proton-decoupled spectra taken with different angles between the magnetic field direction and the fiber direction are compared to theoretical spectra which are calculated by assuming the following: the orientation of the phosphate groups in the fiber is given by the A-form DNA coordinates suggested by Arnott et Hukins; the DNA phosphate groups may be considered stationary on the NMR time scale; the relevant features of the spectra are determined solely by chemical shift anisotropy of the phosphorus atoms. The experimental and calculated spectra are in excellent agreement and support the validty of the above assumptions contrary to conclusions drawn in another investigation. In particular, no evidence was found to support the notion of a highly irregular phosphodiester backbone. Comparison of observed and simulated spectra allows the determination of the orientation of the 31P chemical shielding tensor relative to the bonding framework of the phosphodiester group. The orientation agrees with that expected from NMR studies of phosphodiester model compounds and X-ray diffraction of oriented fibers.