Phosphorus-31 Nuclear Magnetic Resonance of Double- and Triple-Helical Nucleic Acids. Phosphorus-31 Chemical Shifts as a Probe of Phosphorus-Oxygen Ester Bond Torsional Angles

The temperature dependence to the ³¹P NMR spectra of poly[d(GC)]·poly[d(GC)], d(GC)₄, phenylalanine tRNA (yeast) and mixtures of poly(A) + oligo(U) is presented. The ³¹P NMR spectra of mixtures of complementary RNA and of the poly d(GC) self-complementary DNA provide torsional information on the phosphate ester conformation in the double, triple, and “Z” helix. The increasing downfield shift with temperature for the single-strand nucleic acids provides a measure of the change in the phosphate ester conformation in the single helix to coil conversion. A separate upfield peak (20–26% of the total phosphates) is observed at lower temperatures in the oligo(U)·poly(A) mixtures which is assigned to the double helix/triple helix. Proton NMR and UV spectra confirm the presence of the multistrand forms. The ³¹P chemical shift for the double helix/triple helix is 0.2–0.5 ppm upfield from the chemical shift for the single helix which in turn is 1.0 ppm upfield from the chemical shift for the random coil conformation.