Development of asymmetrical flow field-flow fractionation-multi angle laser light scattering analysis for molecular mass characterization of cationic potato amylopectin

The goal of this study is to investigate the applicability of asymmetrical flow field-flow fractionation (AsFlFFF)-multi angle laser light scattering (MALLS), and to develop a method for analysis of cationic potato amylopectin (CPAP) having ultrahigh molecular mass (UHMr). Use of the aqueous carrier having low salt content (3 mM NaN3) resulted in a distortion in AsFlFFF fractograms of CPAP with a general pattern of a sharp rise at the beginning of the elution followed by a long tailing, probably due to combination of attractive and repulsive charge interactions (attractive interaction between CPAP molecules and the channel membrane, and repulsion among cationic CPAP molecules). As the cross flow-rate (Fc) increases, the tailing tends to increase, and the repeatability of the AsFlFFF retention data tends to decrease, which is an indication of the presence of the charge interactions. The tailing gradually decreased, and the repeatability of the AsFIFFF retention data increased, as the salt content of the carrier increased. The distortion of the fractogram finally disappeared at Fc of about 0.2 ml/min and the channel flow-rate (F(out)) of about 1 ml/min with the aqueous carrier having the salt content of 40 mM (3 mM NaN3 +37 mM NaNO3). The weight-average molecular mass (Mw) and the z-average radius of gyration ((rg),) determined by MALLS were 5.2 x 10(7) and 34 x 10(1) nm, respectively. With the flow-rate ratio, Fc/F(out) kept constant, the degree of the charge interactions (and thus the distortion of fractogram) seems to increase with the cross flow-rate (Fc) and with the sample injection mass. AsFIFFF-MALLS was applied for determination of molecular mass distributions (MrDs) and the sizes of CPAPs prepared by various cooking procedures.     

Quantum chemical calculations on the Peterson olefination with alpha-silyl ester enolates

The reaction of stabilized Peterson reagents (alpha-silyl ester enolates) with ketones has been studied theoretically and experimentally. Enolate geometry was studied by trapping experiments and NMR spectroscopy and was found to differ markedly with the nature of the base (LiHMDS vs LDA vs KHMDS). The chelating effect of the lithium counterion was found to be critical for the reaction. For the two ketones studied, the combined weight of experimental and computational data assigns geometrical selectivity to the initial addition transition state, though in general there appears to be a fine balance between three possible choices for the rate-determining step.     

Chromium-salen-mediated alkene epoxidation: a theoretical and experimental study indicates the importance of spin-surface crossing and the presence of a discrete intermediate

The mechanism of alkene epoxidation by chromium(v) oxo salen complexes has been studied by DFT and experimental methods. The reaction is compared to the closely related Mn-catalyzed process in an attempt to understand the dramatic difference in selectivity between the two systems. Overall, the studies show that the reactions have many similarities, but also a few critical differences. In agreement with experiment, the chromium system requires a change from low- to high-spin in the catalytic cycle, whereas the manganese system can proceed either with spin inversion or entirely on the high-spin surface. The low-spin addition of metal oxo species to an alkene leads to an intermediate which forms epoxide either with a barrier on the low-spin surface or without a barrier after spin inversion. Supporting evidence for this intermediate was obtained by using vinylcyclopropane traps. The chromium(v) oxo complexes can adopt a stepped shape or form a more concave surface, analogous to previous results on manganese salen complexes.     

Microsecond timescale MD simulations at the transition state of PmHMGR predict remote allosteric residues

Understanding the mechanisms of enzymatic catalysis requires a detailed understanding of the complex interplay of structure and dynamics of large systems that is a challenge for both experimental and computational approaches. More importantly, the computational demands of QM/MM simulations mean that the dynamics of the reaction can only be considered on a timescale of nanoseconds even though the conformational changes needed to reach the catalytically active state happen on a much slower timescale. Here we demonstrate an alternative approach that uses transition state force fields (TSFFs) derived by the quantum-guided molecular mechanics (Q2MM) method that provides a consistent treatment of the entire system at the classical molecular mechanics level and allows simulations at the microsecond timescale. Application of this approach to the second hydride transfer transition state of HMG-CoA reductase from Pseudomonas mevalonii (PmHMGR) identified three remote residues, R396, E399 and L407, (15–27 Å away from the active site) that have a remote dynamic effect on enzyme activity. The predictions were subsequently validated experimentally via site-directed mutagenesis. These results show that microsecond timescale MD simulations of transition states are possible and can predict rather than just rationalize remote allosteric residues.

Transition state force fields enable MD simulations at the transition state of HMGCoA reductase that sample the transition state ensemble on the μs timescale to identify remote residues that affect the reaction rate.     

Antiproton-neutron interactions at 100 GeV/c

From an exposure of the 30-inch deuterium filled bubble chamber to a 100 GeV/c antiproton-enriched beam at FNAL, we have calculated the topological cross sections for antiproton-neutron interactions with three or more charged particles in the final state. A comparison of our data with pn data at the same momentum allows us to estimate the annihilation contribution to each multiplicity, and hence calculate the average value, and width, of the annihilation multiplicity distribution. Our results are consistent with those from $\text{p}\text{p}$ data at 100 GeV/c, but depart from the trend of lower-energy $\text{p}\text{p}$ data.     

Water-soluble microgels made by radical polymerization in solution

The goal of this study was to prepare and characterize water-soluble, high-molecular-weight microgels. N-Vinylpyrrolidone (NVP) and vinylacetate (VAc) in combination with cross-linkers diethylene glycol dimethacrylate (DEGDMA) or diethylene glycol divinyl ether (DEGDVE) were copolymerized in solution to high conversion. Polymerization was performed in different solvents or solvent mixtures, with solubility parameters ranging from 16.0 to 47.6 J^0.5cm^−1.5, and at different initial monomer concentrations. In solvent mixtures with solubility parameters of 20–40 J^0.5cm^−1.5, macrogelation did not occur below a critical gelation concentration and microgels were formed. For NVP/VAc/DEGDMA (85.0/10.0/5.0 wt%, 84.8/12.9/2.4 mol%) and NVP/VAc/DEGDVE (85.0/10.0/5.0 wt%, 84.8/12.9/3.5 mol%) the critical gelation curves were determined. The molecular weights of the microgels depend on the solvent systems and the initial monomer concentration. Microgels of high molecular weight but low cross-linking density gave aqueous solutions with high viscosities. Increasing the amount of cross-linker to 20 wt% gave high-molecular-weight microgels with lower solution viscosity. Microgels with a monomer composition NVP/VAc/DEGDVE (65.0/15.0/20.0 wt%, 66.2/19.5/14.3 mol%) were prepared in ethanol at different monomer concentrations (3–20 wt%). The molecular weights were determined by a combination of field-flow fractionation and light scattering. By increasing the initial monomer concentration, the molecular weight and the molecular-weight distribution as well as the intrinsic viscosity increased. The exponent of the Mark–Houwink equation was 0.26. Received: 19 March 2001 Accepted: 20 July 2001     

Microstructure and dynamics in lyotropic liquid crystals. Principles and applications of nuclear spin relaxation

Abstract

Nuclear spin relaxation of quadrupolar nuclei provides access to a wide range of properties of lyotropic liquid crystals, ranging from the molecular ordering and dynamics at the interface to the macroscopic viscoelastic behaviour. We emphasize here the unique capability of the spin relaxation method to provide detailed geometric and dynamic information relating to the microstructure of lyotropic liquid crystals, i.e. the metric, curvature, and fluctuations of the dividing interface that separates polar and non-polar regions. This information is conveyed to the spin system via the translational diffusion of surfactants or counterions over the interface. The general principles of the spin relaxation method, as applied to lyotropic liquid crystals, are described, with emphasis on the model-independent information content of the relaxation observables and on the relation to microstructure. Specific results for lamellar, hexagonal, cubic, and nematic phases are also described.     

Combined experimental and theoretical study of the mechanism and enantioselectivity of palladium- catalyzed intermolecular Heck coupling

The asymmetric Heck reaction using P,N-ligands has been studied by a combination of theoretical and experimental methods. The reaction follows Halpern-style selectivity; that is, the major isomer is produced from the least favored form of the pre-insertion intermediate. The initially formed Ph-Pd(P,N) species prefers a geometry with the phenyl trans to N. However, the alternative form, with Ph trans to P, is much less stable but much more reactive. In the preferred transition state, the phenyl moiety is trans to P, but significant electron density has been transferred to the alkene carbon trans to N. The steric interactions in this transition state fully account for the enantioselectivity observed with the ligands studied. The calculations also predict relative reactivity and nonlinear mixing effects for the investigated ligands; these predictions are fully validated by experimental testing. Finally, the low conversion observed with some catalysts was found to be caused by inactivation due to weak binding of the ligand to Pd(0). Adding monodentate PPh3 alleviated the precipitation problem without deteriorating the enantioselectivity and led to one of the most effective catalytic systems to date.     

The mechanism for the rhodium-catalyzed decarbonylation of aldehydes: a combined experimental and theoretical study

The mechanism for the rhodium-catalyzed decarbonylation of aldehydes was investigated by experimental techniques (Hammett studies and kinetic isotope effects) and extended by a computational study (DFT calculations). For both benzaldehyde and phenyl acetaldehyde derivatives, linear Hammett plots were obtained with positive slopes of +0.79 and +0.43, respectively, which indicate a buildup of negative charge in the selectivity-determining step. The kinetic isotope effects were similar for these substrates (1.73 and 1.77 for benzaldehyde and phenyl acetaldehyde, respectively), indicating that similar mechanisms are operating. A DFT (B3LYP) study of the catalytic cycle indicated a rapid oxidative addition into the C(O)-H bond followed by a rate-limiting extrusion of CO and reductive elimination. The theoretical kinetic isotope effects based on this mechanism were in excellent agreement with the experimental values for both substrates, but only when migratory extrusion of CO was selected as the rate-determining step.