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Multipole Populations (multipole + population)
Selected AbstractsThe invariom model and its application: refinement of d,l -serine at different temperatures and resolutionACTA CRYSTALLOGRAPHICA SECTION A, Issue 3 2005B. Dittrich Three X-ray data sets of the same d,l -serine crystal were measured at temperatures of 298, 100 and 20,K. These data were then evaluated using invarioms and the Hansen & Coppens aspherical-atom model. Multipole populations for invarioms, which are pseudoatoms that remain approximately invariant in an intermolecular transfer, were theoretically predicted using different density functional theorem (DFT) basis sets. The invariom parameters were kept fixed and positional and thermal parameters were refined to compare the fitting against the multi-temperature data at different resolutions. The deconvolution of thermal motion and electron density with respect to data resolution was studied by application of the Hirshfeld test. Above a resolution of sin,/,,,,0.55,Å,1, or d,,,0.9,Å, this test was fulfilled. When the Hirshfeld test is fulfilled, a successful modeling of the aspherical electron density with invarioms is achieved, which was proven by Fourier methods. Molecular geometry improves, especially for H atoms, when using the invariom method compared to the independent-atom model, as a comparison with neutron data shows. Based on this example, the general applicability of the invariom concept to organic molecules is proven and the aspherical density modeling of a larger biomacromolecule is within reach. [source] Ultrahigh-resolution crystallography and related electron density and electrostatic properties in proteinsJOURNAL OF SYNCHROTRON RADIATION, Issue 3 2008Claude Lecomte With an increasing number of biological macromolecular crystal structures measured at ultrahigh resolution (1,Å or better), it is necessary to extend to large systems the experimental valence electron density modelling that is applied to small molecules. A database of average multipole populations has been built, describing the electron density of chemical groups in all 20 amino acids found in proteins. It allows calculation of atomic aspherical scattering factors, which are the starting point for refinement of the protein electron density, using the MoPro software. It is shown that the use of non-spherical scattering factors has a major impact on crystallographic statistics and results in a more accurate crystal structure, notably in terms of thermal displacement parameters and bond distances involving H atoms. It is also possible to obtain a realistic valence electron density model, which is used in the calculation of the electrostatic potential and energetic properties of proteins. [source] On the application of an experimental multipolar pseudo-atom library for accurate refinement of small-molecule and protein crystal structuresACTA CRYSTALLOGRAPHICA SECTION A, Issue 2 2007Bartosz Zarychta With an increasing number of biomacromolecular crystal structures being measured to ultra-high resolution, it has become possible to extend to large systems experimental charge-density methods that are usually applied to small molecules. A library has been built of average multipole populations describing the electron density of chemical groups in all 20 amino acids found in proteins. The library uses the Hansen & Coppens multipolar pseudo-atom model to derive molecular electron density and electrostatic potential distributions. The library values are obtained from several small peptide or amino acid crystal structures refined against ultra-high-resolution X-ray diffraction data. The library transfer is applied automatically in the MoPro software suite to peptide and protein structures measured at atomic resolution. The transferred multipolar parameters are kept fixed while the positional and thermal parameters are refined. This enables a proper deconvolution of thermal motion and valence-electron-density redistributions, even when the diffraction data do not extend to subatomic resolution. The use of the experimental library multipolar atom model (ELMAM) also has a major impact on crystallographic structure modelling in the case of small-molecule crystals at atomic resolution. Compared to a spherical-atom model, the library transfer results in a more accurate crystal structure, notably in terms of thermal displacement parameters and bond distances involving H atoms. Upon transfer, crystallographic statistics of fit are improved, particularly free R factors, and residual electron-density maps are cleaner. [source] A comparison between experimental and theoretical aspherical-atom scattering factors for charge-density refinement of large moleculesACTA CRYSTALLOGRAPHICA SECTION A, Issue 3 2004Virginie Pichon-Pesme The differences between two databases describing the polypeptide main chain in terms of charge-density parameters, directly usable in protein structure refinements, are discussed. These databases contain averaged multipole populations of peptide pseudo-atoms obtained from refinement against theoretical simulated data and against high-resolution experimental data on small peptide or amino acid molecules. The main discrepancy becomes apparent when electrostatic properties are calculated. [source] How to easily replace the independent atom model , the example of bergenin, a potential anti-HIV agent of traditional Asian medicineACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2009Birger Dittrich Bergenin, which has been isolated from a variety of tropical plants, has several pharmacological applications in traditional Asian medicine. Its electron-density distribution was obtained from a room-temperature low-resolution X-ray data set measured with point detection making use of multipole populations from the invariom library. Two refinement models were considered. In a first step, positional parameters and ADPs were refined with fixed library multipoles (model E1). This model was suitable to be input into a second refinement of multipoles (model E2), which converged smoothly although based on Cu,K, room-temperature data. Quantitative results of a topological analysis of the electron density from both models were compared with Hartree,Fock and density-functional calculations. With respect to the independent atom model (IAM) more information can be extracted from invariom modelling, including the electrostatic potential and hydrogen-bond energies, which are highly useful, especially for biologically active compounds. The reliability of the applied invariom formalism was assessed by a comparison of bond-topological properties of sucrose, for which high-resolution multipole and invariom densities were available. Since a conventional X-ray diffraction experiment using basic equipment was combined with the easy-to-use invariom formalism, the procedure described here for bergenin illustrates how it can be routinely applied in pharmacological research. [source] | ||||||||||