Dimer and superstructure of the active form of a vitamin D3; 1 alpha,24(R)dihydroxy-vitamin D3 monohydrate,...

The crystals of 1alpha24(R)dihydroxy-vitamin D3 monohydrate, C27O3H44.H2O are orthorhombic in the space group P2(1)2(1)2(1) with cell dimensions a=25.719, b=42.572, c=9.851A and Z=16. The asymmetric unit consists of two subunits with b/8, and each subunit contains a dimer in which two molecules are hydrogen-bonded through water molecules into non-crystallographical symmetry of C2. 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 C2 symmetry between the rigid ring parts to form a superstructure, and the two-fold axis of the straight line, y=1/8, z=0.435 goes through a point that is a little apart from the hypercenter (1/2, 1/8, 1/2). The structure was solved by integrated Patterson and direct methods and refined on Fo2 under restraints. The final R1 is 0.228 on Fo for 1623 reflections with Fo>3sigma, resolutions 1.0-3.0 A, 313 restraints, 490 parameters and average Ueq=0.120. Not all the atoms of the chains appeared nor the hydrogen atoms. The missing atoms of the dimer were modeled from another pair molecule by C2 symmetry and hydrogen atoms were added. The structure of the dimer was optimized by ab initio molecular orbital of HF/6-31G.