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Charge Centers (charge + center)
Selected AbstractsCoupled atomic charge selectivity for optimal ligand-charge distributions at protein binding sitesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 16 2006Sathesh Bhat Abstract Charge optimization as a tool for both analyzing and enhancing binding electrostatics has become an attractive approach over the past few years. An interesting feature of this method for molecular design is that it provides not only the optimal charge magnitudes, but also the selectivity of a particular atomic center for its optimal charge. The current approach to compute the charge selectivity at a given atomic center of a ligand in a particular binding process is based on the binding-energy cost incurred upon the perturbation of the optimal charge distribution by a unit charge at the given atomic center, while keeping the other atomic partial charges at their optimal values. A limitation of this method is that it does not take into account the possible concerted changes in the other atomic charges that may incur a lower energetic cost than perturbing a single charge. Here, we describe a novel approach for characterizing charge selectivity in a concerted manner, taking into account the coupling between the ligand charge centers in the binding process. We apply this novel charge selectivity measure to the celecoxib molecule, a nonsteroidal anti-inflammatory agent binding to cyclooxygenase 2 (COX2), which has been recently shown to also exhibit cross-reactivity toward carbonic anhydrase II (CAII), to which it binds with nanomolar affinity. The uncoupled and coupled charge selectivity profiles over the atomic centers of the celecoxib ligand, binding independently to COX2 and CAII, are analyzed comparatively and rationalized with respect to available experimental data. Very different charge selectivity profiles are obtained for the uncoupled versus coupled selectivity calculations. © 2006 Wiley Periodicals, Inc. J Comput Chem, 2006 [source] Electronic and atomic relaxation processes in irradiated cryocrystalsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 2 2005E. V. Savchenko Abstract Relaxation processes and stability of charge centers in preirradiated cryocrystals doped with atomic and molecular species are discussed. The study was performed combining techniques of spectrally resolved thermally stimulated luminescence (TSL), thermally stimulated exoelectron emission (TSEE) and photon-stimulated exoelectron emission (PSEE). Comparison of the yields of electrons and photons in VUV and visible ranges made it possible to discriminate between reactions of charge carriers and neutral reactive species and find interconnection between atomic and electronic processes of relaxation. The radiative mechanism of electronic relaxation stimulation via an atomic subsystem is verified. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Kinetic Study of Thermal Z to E Isomerization Reactions of Azobenzene and 4-Dimethylamino-4,-nitroazobenzene in Ionic Liquids [1-R-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imide with R=Butyl, Pentyl, and Hexyl]CHEMISTRY - A EUROPEAN JOURNAL, Issue 20 2006Keita Baba Abstract Thermal Z to E isomerization reactions of azobenzene and 4-dimethylamino-4,-nitroazobenzene were examined in three ionic liquids of general formula 1-R-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (R=butyl, pentyl, and hexyl). The first-order rate constants and activation energies for the reactions of azobenzene measured in these ionic liquids were consistent with those measured in ordinary organic solvents, which indicated that the slow isomerization through the inversion mechanism with a nonpolar transition state was little influenced by the solvent properties, such as the viscosity and dielectric constant, of ionic liquids. On the other hand, the rate constants and the corresponding frequency factors of the Arrhenius plot were significantly reduced for the isomerization of 4-dimethylamino-4,-nitroazobenzene in ionic liquids compared with those for the isomerization in ordinary organic molecular solvents with similar dielectric properties. Although these ionic liquids are viscous, the apparent viscosity dependence of the rate constant could not be explained either by the Kramers,Grote,Hynes model or by the Agmon,Hopfield model for solution reactions. It is proposed that the positive and the negative charge centers of a highly polar rotational transition state are stabilized by the surrounding anions and cations, respectively, and that the ions must be rearranged so as to form highly ordered solvation shells around the charge centers of the reactant in the transition state. This requirement for the orderly solvation in the transition state results in unusually small frequency factors of 104,107 s,1. [source] | ||||||||||