Conformational stability in dinucleoside phosphate crystals. Semiempirical potential energy calculations for uridylyl-3'-5'-adenosine monophosphate (UpA) and guanylyl-3',5'-cytidine monophosphate (GpC)
Classical potential energy calculations were performed for the dinucleoside phosphates UpA and GpC. Two widely accessible low-energy regions of conformation space were found for the w', w pair. That of lowest energy contains conformations similar to helical RNA, with w' and w in the vicinity of 300° and 280°, respectively. All five experimental observations of crystalline GpC, two of ApU, and the helical fragment of ApApA fall in this range. The second lowest region has w' and w at about 20° and 80°, respectively, which is in the general region of one experimentally observed crystalline conformer of UpA, and the nonhelical region of ApApA. It is concluded that GpC and ApU, which were crystallized as either sodium or calcium salts, are shielded from each other in the crystal by the water of hydration and are therefore free to adopt their predicted in vacuo minimum energy helical conformations. By contrast, crystalline UpA had only 1/2 water per molecule, and was forced into higher energy conformations in order to maximize intermolecular hydrogen bonding.
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- April 4, 2014