Phosphorus-31 Fourier transform nuclear magnetic resonance study of mononucleotides and dinucleotides. 2. Coupling constants

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Abstract
Stereosensitive 3JPH [constant for vicinal P.sbd.O.sbd.C.sbd.H coupling] and 4JPH [constnat for 4-bond proton.sbd.P couplings in P.sbd.O.sbd.C.sbd.C.sbd.H systems] P.sbd.proton coupling constants were measured directly from the 31P NMR spectra of a variety of adenine, thymine and uracil 3''-mononucleotides, 5''-mononucleotides, their cyclic analogs and the corresponding dinucleotides, under various conditions of pH and temperature. For all 5''-mononucleotides, the identical 3JPH coupling to P of the 2 H5'' protons is 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.sbd.gauche" rotamers about C5''.sbd.O5'' and C4''.sbd.C5'' remain overwhelmingly (85%) preferred. The gauche arrangement during C3''.sbd.O3'' is favored in all cases for 3''-mononucleotides. 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.sbd.suger backbone, even at extreme pH. Temperature studies point out that the thermal unwinding of stacked dinucleotides occurs mainly via rotation about P3''.sbd.O3'' and P5''.sbd.O5'' bond axes.

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