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.

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The GAUSSIAN 70 program has been modified at Princeton University to permit automatic conversion from 4-310 to 5-21G for lithium and beryllium. Documentation has been submitted to the Quantum Chemistry Program Exchange.(9) W. Abstract: Angle strain and torsional strain energies were calculated using the CNDO/2 LCAO-MO method for various geometries of dimethyl phosphate monoanion and compared with energies for 2',3'-cyclic ribose phosphate and 3',5'-cyclic ribose phosphate. While the calculations fail to identify the source of the strain energy in the 3',5'-cyclic six-membered ring nucleotides, they do indicate that a significant portion of the high heat of hydrolysis of the five-membered ring, 2',3'-cyclic nucleotides is associated with relief of torsional strain and that preferred torsional conformations of acyclic esters are strongly coupled to the RO-P-OR bond angles. This coupling of ester, 0 -P -0 bond angles, and torsional angles is also demonstrated by CNDO calculations on various geometries of trimethyl phosphate. Eclipsing of one of the phosphate ester bonds reduces the bond angle between the esterified oxygen atoms by ca. 5O and eclipsing of both ester bonds further reduces the bond angle by another S o . These predictions of the CNDO calculations are shown to be supported by x-ray crystallographic structures of cyclic and acyclic, monoanionic, and neutral phosphate esters.

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