Geometry of the phosphodiester backbone in the A form of deoxyribonucleic acid determined by phosphorus-31 nuclear magnetic resonance spectroscopy

31P NMR spectra were obtained from poly(dA-dT) . poly(dA-dT) fibers which gave an X-ray diffraction pattern similar to that of the A form of natural deoxyribonucleic acid (DNA). The analysis of the line shape indicated that the A form of poly(dA-dT) . poly(dA-dT) has a single uniform backbone conformation; the orientation of the phosphodiester group relative to the helical axis was determined to be beta = 70 degrees and gamma = 52 degrees. The 31P NMR spectra of poly(dA-dT) . poly(dA-dT) were in remarkable contrast to the 31P NMR spectra of the A form of natural DNA, which exhibited an unusual line shape. The origins of the abnormalities in the line shape for the A form of natural DNA are discussed in terms of phosphodiester orientations.