MOLECULAR-STRUCTURE OF 2 CRYSTAL FORMS OF CYCLIC TRIADENYLIC ACID AT 1-ANGSTROM RESOLUTION

The three dimensional structures of cyclic deoxytriadenylic acid, c-d( ApApAp), from two different trigonal crystal forms (space groups P3 an d R32) have been determined by x-ray diffraction analysis at 1 Angstro m resolution. Both structures were solved by direct methods and refine d by anisotropic least squares refinement to R-factors of 0.109 and 0. 137 for the P3 and R32 forms, respectively. In both crystal forms, eac h of the two independent c-d(ApApAp) molecules sits on the crystallogr aphic 3-fold axis. All four independent c-d(ApApAp) molecules have sim ilar backbone conformations. The deoxyriboses are in the S-type pucker with pseudorotation angles ranging from 156.7 degrees to 168.6 degree s and the bases have anti glycosyl torsion angles (chi falling in two ranges, one at -104.3 degrees and the other ranging from -141.0 degree s to -143.8 degrees). In the R32 form, a hexahydrated cobalt(II) ion i s found to coordinate through bridging water molecules to N1, N3, and N7 atoms of three adjacent adenines acid oxygen atoms of phosphates. C omparison with other structures of cyclic oligonucleotides indicates t hat the sugar adopts N-type pucker in cyclic dinucleotides and S-type pucker in cyclic trinucleotides, regardless whether the sugar is a rib ose or a deoxyribose.