Temperature dependence of the 31P chemical shifts of nucleic acids. A prode of phosphate ester torsional conformations.

The temperature dependence of the 31P chemical shifts of the ribodinucleoside monophosphates, ApA, GpC, CpC, UpU, and ApU, of the deoxyribonucleic acids, d-ApT, TpT, d-ApA, and d-pApT, and of the homopolyribonucleic acids poly(G), poly(U), poly(A) is shown to provide information on the helix-coli transition in nucleic acids. The base stacked, helical structure with a gauche,gauche phosphate ester torsional conformation is 0.2-0.6 ppm upfield from the random coil conformation. In contrast, the 31P chemical shifts of dimethyl and diethyl phosphate do not change significantly with temperature. These results support our earlier hypothesis that 31P shifts are sensitive probes of torsional conformations with phosphate esters in gauche,gauche conformations having 31P shifts upfield from nongauche conformations.