MONOCLINIC FORM OF VITAMIN D₂ Ivan Leban, Faculty for Chemistry and Chemical Technology, University of Ljubljana, 61001 Ljubljana, Slovenia; Rudolf A.G. de Graaff, Leiden Institute of Chemistry, Gorlaeus Laboratories, University of Leyden, 2300 RA Leyden, The Netherlands; René de Gelder, Nijmegen SON Research Center, University of Nijmegen, 6525 ED Nijmegen, The Netherlands; Johan Turkenburg, Department of Chemistry, University of York, York Y01 5DD, UK and Leiden Institute of Chemistry, Gorlaeus Laboratories, University of Leiden, 2300 RA Leiden, The Netherlands; Keith S. Wilson and Zbigniew Dauter, European Molecular Biology Laboratory (EMBL) c/o DESY, D-22603 Hamburg, Germany.

The development of the experimental diffraction techniques (synchrotron radiation) and the direct methods together with the graphical modelling and molecular fitting facilities normally used in protein crystallography enable to solve and refine the crystal structure of the monoclinic form of ergocalciferol (vitamin D₂). There are four independent molecules of the steroid skeleton with the ruptured B ring C₂₈H₄₄O in the asymmetric unit of the monoclinic unit cell [a=20.985(8)Å, b=7.277(4)Å, c=36.123(9)Å, b=103.07(5)^o] in space group P 2₁. Molecules exist in the elongated form , the hydroxy OH groups attached to C3 forming an infinite hydrophilic helical spirals along short b-axis with the A rings in turns in two different conformations [a chair (equatorial OH) and b chair (axial OH)]. The outer parts of the hydrophobic side chains at C17 for b conformers are disordered.The packing of the molecules in the monoclinic form is completely different to that in the orthorhombic structure.