Experimental and theoretical charge density of DL-alanyl-methionine.

X-ray diffraction data up to d = 0.50 A resolution have been collected at 100 K for a DL-alanyl-methionine single crystal using a CCD area detector. Multipolar crystallographic refinement was carried out and the electron density of the molecule has been analyzed. The deformation electron density around the S atom reveals two lone pairs with an sp(3) hybridization and agrees with the results of density functional theory calculations. The topological properties of the covalent bonds and of the hydrogen bonds have been investigated. Two weak polar intramolecular interactions of the type C(5) (pentagonal cyclic structure) have unfavorable geometrical parameters for hydrogen bonds and are devoid of critical points. The two electron lone pairs of the carbonyl oxygen appear asymmetric in the experimental deformation density. This could be attributed to the different strength of the hydrogen bond and intramolecular polar interaction involving the carbonyl oxygen. In the ab-initio-derived deformation maps, the asymmetry of the electron doublets is reproduced only very partially.