Topological Analysis of the Experimental Electron Densities of Amino Acids. 1.<scp>d,l</scp>-Aspartic Acid at 20 K

Abstract

High-resolution X-ray diffraction data collected at 20 K are interpreted in terms of the rigid-pseudoatom formalism to derive the electron density and related properties, such as the electrostatic potential and electric moments, of the crystalline d,l -aspartic acid. The refinement models applied are restricted via rigid-bond type constraints to reduce possible bias in the mean-square displacement amplitudes due to inadequacies in the thermal deconvolution. The density and its Laplacian extracted from the data is analyzed in terms of the topological properties of covalent bonds and nonbonded interactions. The results are compared to those calculated at the Hartree−Fock level of theory and to those obtained experimentally for analogous molecules. The comparison must consider the differences in the locations of the bond critical points of the densities in question, that is, how the bond polarity manifests itself in the distribution of charge obtained by different methods. One of the key questions to the reliability of experimental pseudoatomic densities seems to be whether the treatment of the X-ray data can be standardized so as to reduce model inadequacies, especially those related to the derivation of monopole populations.