Accurate charge density of α-glycine by the maximum entropy method

Abstract

Accurate electron densities of α-glycine have been obtained by the maximum entropy method (MEM) applied to low-temperature X-ray diffraction data by Destro et al. (J. Phys. Chem. A, 2000, 104, 1047–1054). Difference Fourier maps have been found to provide a good stopping criterion for the iterations in the MEM, in agreement with our previous findings for trialanine. Properties according to Bader's atoms-in-molecules theory are reported for the MEM electron density. These properties are found to be in agreement with the properties of the MEM electron density as they have been obtained previously for the tripeptide trialanine, thus showing the consistency of the MEM approach when applied to centrosymmetric and acentric organic compounds. The dynamic MEM electron density compares favourably with the static electron density obtained from the multipole model by Destro et al. (2000), with differences being attributed to the specific nature of each method. The independent spherical atom model (ISAM) and the multipole model provide different phases for 17 reflections of which only two are of the type ‘observed’. A MEM calculation with reflection phases from the multipole model leads to an electron density that is only marginally different from the MEM electron density with phases from the ISAM refinement. This suggests, at least for centrosymmetric structures, that the ISAM is sufficiently good to be used as basis for the MEM approach to accurate electron density studies.