Proton magnetic resonance studies of the conformational changes of dideoxynucleoside ethyl phosphotriesters

PMR investigations on the diastereomeric phosphate methyl protons of the dinucleoside ethyl phosphotriesters Tp(C 2 H 5 )T, dA, and dIp(C 2 H 5 )dI have been used to study the conformational changes of these dimersin solution. In D 2 O (273°K), the diastereomeric phosphate‐methly groups of Tp(C 2 H 5 )T appear as a triplet. The methyl resonances of dIp(C 2 H 5 )dI and dAp(C 2 H 5 )dA appear as two sets of triplets and their chemical shift differences (δ 1 − δ 2 ), decrease with increasing temperature, finally becoming zero at 292°K and 333°K, respectively. The same phenomenon is observed for dAp(C 2 H 5 )dA in CD 3 OD; in this detacking solvent, the difference (δ 1 − δ 2 ) diminishes to zero at a lower temperature (261°K). At room temperature in D 2 O, the chemical shift of the phosphate methyl of Tp(C 2 H 5 )T appears at lower field than those of dIp(C 2 H 5 )dI or dAp(C 2 H 5 )dA. The differences between the chemical shifts of these groups (δ I − δ T or δ A − δ T ) increase with increasing temperature, and reach maximal values at 301°K and 333°K, respectively. The results suggest that at low temperature the largest fraction of the dimer population exists in a stacked state, with the phosphate‐ethyl groups outside the stack. Increasing temperature causes an oscillation of the bases and a shift in the dimer population away from the stacked state. Finally at high temperature, the planar bases rorate with respect to one another and in the case of dIp(C 2 H 5 )dI and dAp(C 2 H 5 )dA, the ethyl groups experience shielding by the anisotropic ring current of the five‐membered ring of the bases. Thus, the current pmr studies and those reported earlier from our laboratory support an “oscillation‐rotation model” for the unstacking process of the dimers. The relationship of this model and the “two‐state model” is discussed.