Dimer and superstructure of the active form of a vitamin D-3; 1 alpha,24(R)dihydroxy-vitamin D-3 monohydrate, C27O3H44 center dot H2O

The crystals of 1 alpha ,24(R)dihydroxy-vitamin D-3 monohydrate, C27O3H44.H 2O are orthorhombic in the space group P2(1)2(1)2(1) with cell dimensions a =25,719, b=42,572, c=9.851 Angstrom and Z=16. The asymmetric unit consists of two subunits with b/8, and each subunit contains a dimer in which two mo lecules are hydrogen-bonded through water molecules into non-crystallograph ical symmetry of C-2. The two-fold axes are the straight lines, x=1/2, z=0. 256 and x=1/2, z=0.623. The two dimers are the same in the rigid ring part, but differ in the conformation of the flexible chains. The dimers further make C-2 symmetry between the rigid ring parts to form a superstructure, an d the two-fold axis of the straight line, y=1/8, z=0.435 goes through a poi nt that is a little apart from the hypercenter (1/2, 1/8, 1/2). The structu re was solved by integrated Patterson and direct methods and refined on F-O (2) under restraints. The final R-1 is 0.228 on F-O for 1623 reflections wi th F-O>3 sigma, resolutions 1.0-3.0 Angstrom, 313 restraints, 490 parameter s and average U-eq=0.120. Not all the atoms of the chains appeared nor the hydrogen atoms. The missing atoms of the dimer were modeled from another pa ir molecule by C-2 symmetry and hydrogen atoms were added. The structure of the dimer was optimized by ab initio molecular orbital of HF/6-31G*.