Absolute rate calculations: atom and proton transfers in hydrogen-bonded systems.

We calculate energy barriers of atom- and proton-transfer reactions in hydrogen-bonded complexes in the gas phase. Our calculations do not involve adjustable parameters and are based on bond-dissociation energies, ionization potentials, electron affinities, bond lengths, and vibration frequencies of the reactive bonds. The calculated barriers are in agreement with experimental data and high-level ab initio calculations. We relate the height of the barrier with the molecular properties of the reactants and complexes. The structure of complexes with strong hydrogen bonds approaches that of the transition state, and substantially reduces the barrier height. We calculate the hydrogen-abstraction rates in H-bonded systems using the transition-state theory with the semiclassical correction for tunneling, and show that they are in excellent agreement with the experimental data. H-bonding leads to an increase in tunneling corrections at room temperature.     

Quantum mechanical valence study of a bond-breaking–bond-forming process in triatomic systems

The recently introduced set of the quadratic, two-electron covalent and ionic valence indices is used to investigate the bond-breaking–bond-forming (BB-BF ) process in an atom exchange reaction between H₂ and X (X = H, F—I) as well as in the O₂—H system. Valence changes accompanying selected charge reorganizations are examined within the three-orbital model and valence diagrams for symmetric transition states (TS s) are given. The UHF valence data for Li₂O and CO₂ and the H—H—X, O—O—H, and O—H—O (ABC) TS s (collinear and angular) are reported and compared to valence data in the separated fragments limits (SFL ), AB and BC. The overall valence, ν(ABC), and the total (ionic plus covalent) diatomic valences, ν_AB and ν_BC, are used as measures of the overall bond-order in a concerted BB–BF reaction, to test the postulate of the bond-energy–bond-order (BEBO ) model. In collinear TS s of H₂X, ν ? ?1, i.e., one bonding electron pari, is found to be roughly preserved, whereas in the angular H₂X and in collinear O—H—O TS s, the effect of increased valence at the saddle-point is observed, relative to that of diatomic fragments (reactiants or products). For the angular O—O—H TS , a similar increase in | ν (ABC)| relative to both O₂ and OH SFL s is detected; smaller changes relative to the O₂ data are found in the collinear TS . This observation is in agreement with earlier predictions from the intersecting-state model. The relative diatomic valences, ν/ν and ν/ν, are shown to conserve the overall relative bond multiplicity around 1 in both collinear and angular TS s of the H₂—X systems. © 1994 John Wiley & Sons, Inc.     

STUDIES OF ADHESION OF METAL PARTICLES TO SILICA PARTICLES BASED ON ZETA POTENTIAL MEASUREMENTS

The zeta-potentials of silica, copper, platinum and gold particles have been measured as a function of pH. The isoelectric points were found to be at pH 3.0, 5.8, 3.0 and 3.5, respectively. In the pH range 3.0 to 5.8 copper and silica particles are oppositely charged and accordingly the coating of silica with copper particles could be demonstrated. In the case of gold and platinum the sign of the charge is such that direct adhesion to silica particles cannot be expected and this was also demonstrated in the case of platinum.     

<Emphasis Type="Italic">N</Emphasis>-arachidonoyl glycine,an abundant endogenous lipid,potently drives directed cellular migration through GPR18, the putative abnormal cannabidiol receptor

Background  

Microglia provide continuous immune surveillance of the CNS and upon activation rapidly change phenotype to express receptors that respond to chemoattractants during CNS damage or infection. These activated microglia undergo directed migration towards affected tissue. Importantly, the molecular species of chemoattractant encountered determines if microglia respond with pro- or anti-inflammatory behaviour, yet the signaling molecules that trigger migration remain poorly understood. The endogenous cannabinoid system regulates microglial migration via CB₂ receptors and an as yet unidentified GPCR termed the 'abnormal cannabidiol' (Abn-CBD) receptor. Abn-CBD is a synthetic isomer of the phytocannabinoid cannabidiol (CBD) and is inactive at CB₁ or CB₂ receptors, but functions as a selective agonist at this G_i/o-coupled GPCR. N-arachidonoyl glycine (NAGly) is an endogenous metabolite of the endocannabinoid anandamide and acts as an efficacious agonist at GPR18. Here, we investigate the relationship between NAGly, Abn-CBD, the unidentified 'Abn-CBD' receptor, GPR18, and BV-2 microglial migration.     

Electron transfer in supercritical carbon dioxide: ultraexothermic charge recombination at the end of the "inverted region"

Charge-recombination rates in contact radical-ion pairs, formed between aromatic hydrocarbons and nitriles in supercritical CO(2) and heptane, decrease with the exothermicity of the reactions until they reach -70 kcal mol(-1), but from there on an increase is observed. The first decrease in rate is typical of the "inverted region" of electron-transfer reactions. The change to an increase in the rate for ultra-exothermic electron transfer indicates a new free-energy relationship. We show that the resulting "double-inverted region" is not due to a change in mechanism. It is an intrinsic property of electron-transfer reactions, and it is due to the increase of the reorganisation energy with the reaction exothermicity.     

Theory of electron transfer reactions in blue-copper proteins

The rates of electron transfer reactions in azurin and plastocyanin are calculated with the Intersecting-State Model and compared with experimental data. The calculated distance, free-energy and temperature dependencies of the intraprotein rates in Ru-modified azurins are in good agreement with the experiment. These calculations do not require the fitting of any parameters. Significant tunneling contributions to the room temperature rate are found in some systems. In some cases the symmetry or the orientation of the donor and acceptor orbitals are not favorable and the ET rates are reduced by factors exceeding 4 orders of magnitude.