Study of the electronic structure of molecules XXII. Additional ab initio computations for the barrier to internal rotation in polynucleotide chains

Abstract

In a previous paper of this series (paper XIX) we reported an ab initia computation for the barrier to the internal rotation in the sugar-phosphate- sugar complex, C10H19O8P; the barrier corresponds to the configuration with o>"=45°, u)' = i|/' = i|i" =<f>' = 0°, and 4>" varies from 0° to 360°. Here we repeat the same study with the C10H18OgP complex (where one proton of the phosphate group has been removed). The new potential curve shows essentially a parallel displacement in total energy relative to the previously computed one. In addition we report two new computations of the barriers obtained by varying <(>' from 0° to 360°, while keeping o>"=45° and cu' = i|/'=t|/" = <f>"=0° (using Olson and Flory's notation). Again the Hartree-Fock model predicts similar features for the rotational barriers of the two complexes C10H 19O8P and C10H18OgP . The calculated barrier heights are 4.0, 3.2, and 7.0 kcal/mole for the <j>" variation in the C10H19O8P complex, and 5.7, 3.2, and 8.2 kcal/mole in the C10H 18O8P complex. The barrier heights for the <(>' variation are 18.8 and 30.3 kcal/ mole for C10H19O 8P, and 34.8 and 35.2 kcal/mole for C,0Hi8O8P . Finally, the total energy for the internal rotation of <j>" is decomposed into a sum of contributions, representing either the electronic rearrangement following the rotation tor a number of fragments composing the C10H 18OgP complex, or the pairwise interaction of such fragments during the rotation. The location of the potential minima and maxima is predicted by the pairwise interaction of the fragments. The energy for the rearrangement within each fragment is large.