Phosphorus-31 Fourier transform nuclear magnetic resonance study of mononucleotides and dinucleotides. 2. Coupling constants.
Stereosensitive 3JPH and 4JPH phosphorus-proton coupling constants have been measured directly from the phosphorus-31 nuclear magnetic resonance (NMR) spectra of a variety of adenine, thymine, and uracil 3'-mononucleotides, 5'-mononucleotides, their cyclic analogues, and the corresponding dinucleotides, under various conditions of pH and temperature. For all 5'-mononucleotides, the identical 3JPH coupling to phosphorus of the two H5' protons is found essentially independent of the nature of the base, the presence of a 2'-OH on the sugar ring, the temperature, and the pH; the "gauche-gauche" rotamers about C5'-O5' and C4'-C5' remain overwhelmingly (85%) preferred. The "gauche" arrangement during C3'-O3' is favored in all cases for 3'-mononucleotides. However, while no sizeable pH effect is noted on 3'-monodeoxyribonucleotides, the pH dependence of 3JPH3 in 3'-monoribonucleotides strongly suggests an interaction between the 3'-phosphate and the 2'-OH. Molecular features affecting the magnitude of 4JPH coupling constants are discussed together with pH and temperature effects. The time-averaged preferential structural features of mononucleotides are found in dinucleotides with a higher probability; hence, dimerization induces an increase in the statistical conformational purity of the phosphodiester-sugar backbone, even at extreme pH. Temperature studies point out that the thermal unwinding of stacked dinucleotides occurs mainly via rotation about P3'-O3' and P5'-O5' bond axes.