Molecular orbital calculations on the conformation of nucleic acids and their constituents: II. Conformational energies of nucleosides with C(3′)- and C(2′)-exo sugars

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Molecular orbital calculations, using the all-valence-electrons PCTLO method, applied previously to the evaluation of conformational energies of purine and pyrimidine nucleosides with C(3′)- and C(2′)-endo sugars as a function of the rotation around the glycosidic linkage, are here extended to nucleosides with C(3′)- and C(2′)-exo sugars. All compounds investigated show preference for the anti conformation. The preferential association of the exo-pucker of the sugar with the α-anomers is accounted for by broader minima and lower potential barriers for that association as compared with the exo-pucker-β-anomer configuration. In the absence of a C(2′)-exo pyrimidine with a C-N glycosidic bond, α-pseudouridine with such a C-C bond was studied. The preference of this compound for the anti conformation is particularly striking.