EXPERIMENTAL CHARGE DENSITY OF DL-SERINE AND L-SERINE. J. Buschmann, T. Koritsánszky, M. Ramm, P. Luger, Institute for Crystallography, Free University of Berlin, Germany
Full topological analyses /1/ of the electron density of the racemic and the non-racemic form of serine will be presented. This investigation is part of a project where the electronic properties of amino acids are to be determined, using Bader's approach /1/, from the experimental density calculated with the multipole program package XD /2/.
Reflection intensity data were collected up to a limit of (sin [[theta]])/[[lambda]] = 1.18Å-1 on a SIEMENS 4-circle X-ray diffractometer with MoK[[alpha]] radiation at a temperature of -150deg.C which was maintained by a nitrogen gas stream cooling device.
DL-serine: monoclinic, P21/a, Z=4, 10515 reflections measured,
3081 independent observed reflections, conventional
refinement rw, = 0.029;
L-serine: orthorhombic, P212121, Z=4, 7766 reflections measured,
2910 independent observed reflections, conventional
refinement rw, = 0.030.
The L-forms of the molecule in the two different crystal structures are alike, showing the same overall conformation, but only the DL-serine crystal structure possesses a strong intermolecular hydrogen bond. So this is a special situation to look for the transferability of multipole parameters and the dependency of electronic and topological properties on intra- and intermolecular interactions.
/1/ R. F. W. Bader, :Atoms in Molecules, Clarendon Press, Oxford,1994.
/2/ T. Koritsánszky et al.: XD, A Computer Program Package for Refinement and Analysis of Charge Densities from X-Ray Diffraction Data, Free University of Berlin, 1995.