A fast and accurate operational model of electron density is crucial in many scientific disciplines including crystallography, molecular biology, pharmaceutical, and structural chemistry. In quantum crystallography, the aspherical refinement of crystal structures is becoming increasingly popular because of its accurate description in terms of physically meaningful properties. The transferable aspherical atom model (TAAM) is quick and precise, though it requires a robust algorithm for atom typing and coverage of the most popular atom types present in small organic molecules. Thus, the University at Buffalo Databank (UBDB) has been renamed to the Multipolar Atom Types from Theory and Statistical clustering (MATTS) data bank, broadened, restructured, and implemented into the software DiSCaMB with 651 atom types obtained from 2316 small-molecule crystal structures containing C, H, N, O, P, S, F, Cl, and Br atoms. MATTS2021 data bank now covers most of the small molecules, peptides, RNA, DNA, and some frequently occurring cations and anions in biological, pharmaceutical, and organic materials, including the majority of known crystal structures composed of the above elements. The multipole model parameters ( P val , κ, κ′, P lm ) obtained for different atom types were greatly influenced by neighboring atom types, hybridization, geometrical strain in the ring system, and charges on the molecule. Contrary to previous findings, the atoms showing variable oxidation states and ions deviate from the linear dependence of monopole-derived charges on the expansion–contraction κ parameter.
One of the kinds of information gained from high resolution (sub-atomic) structures is the observation that electron density parameters are transferable between atoms having similar chemical topology. This stimulated creation of databases of multipolar pseudoatoms (Invariom [1], ELMAM2 [2], MATTS -successor of UBDB [3], etc.) and their applications in (a) structure refinements on standard (atomic) resolution data for small-molecule crystals, and (b) electrostatic properties and non-covalent bonding characterisations for macromolecules.Transferable Aspherical Atom Model (TAAM) of scattering built from a pseudoatom database proved to be advantageous in refining the structure on X-ray diffraction (XRD) data compared to the Independent Atom model (IAM) [4], leading to better fit of the model to the data and improved localization of hydrogen atoms. We have recently showed [5] that also for small-molecule 3D electron diffraction (3D ED) data, a better model-to-data fit and more accurate structures should be expected from TAAM.To improve its usability, we further extended the MATTS bank to cover 98% of atoms found in all the structures deposited in the Cambridge Structural Database [6] composed of chemical elements like C, H, N, O, P, S, F, Cl and/or Br. It is planned that the remaining 1% will be covered by the more general atom types resulting from multidimensional cluster analysis. Some benefits of TAAM over IAM refinements were also reported for macromolecular XRD data of 0.9 Å resolution and better [7]. As most macromolecular crystals diffract to lower resolutions, we recently moved our investigation towards near-atomic resolutions. We quantified the differences between the macromolecular electron density Fourier maps obtained with TAAM and IAM, calculated with a resolution of 1.8 Å. We did the same for electrostatic potential maps, a key property in the context of 3D ED. TAAM refinements affect not only the positions of the atoms, but also the atomic displacement parameters (ADPs) [8]. ADPs appears to be less resolution dependent with TAAM than with IAM. With IAM, ADPs increased for XRD and decreased for 3D ED by about 30%, when the resolution was reduced from 0.6 Å to 0.8 Å [5]. From modified Wilson plots we recently predicted, and then verified by TAAM refinements on macromolecular XRD or 3D ED data, what will happen with ADPs (B-factors) with a further resolution worsening, up to 1.8 Å.All the above helps to understand if there will be any benefits of TAAM refinements on lower than atomic resolutions.
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