CCD charge density study on crystals with large unit cell parameters: The case of hexagonal L-cystine

The electron density of L-cystine has been analyzed using 110 K single-crystal Mo Ka X-ray diffraction data to a resolution of (sin theta/lambda)(max) = 1.123 Angstrom(-1) with a CCD area detector. Due to the large c-parameter (55.9 Angstrom), a discussion is made for choosing the best experimental data collection strategy and data reduction. A multipolar pseudo-atom density model was fitted against the 2309 observed data with I > 3 sigma(I), [R(F) = 0.014, Rw(F) = 0.019, S = 0.73]. The deformation density distribution and topological analysis of charge density clearly reveals disulfide bridge characteristic features and sulfur lone pair electron regions which validate high-level ab initio calculations. The valence shell charge concentration (VSCC) suggests sp(3) hybridization of sulfur atoms.