Water distributions in polystyrene-block-poly[styrene-g-poly(ethylene oxide)] block grafted copolymer system in aqueous solutions revealed by contrast variation small angle neutron scattering study

We develop an experimental approach to analyze the water distribution around a core-shell micelle formed by polystyrene-block-poly[styrene-g-poly(ethylene oxide (PEO)] block copolymers in aqueous media at a fixed polymeric concentration of 10 mg/ml through contrast variation small angle neutron scattering (SANS) study. Through varying the D(2)O/H(2)O ratio, the scattering contributions from the water molecules and the micellar constituent components can be determined. Based on the commonly used core-shell model, a theoretical coherent scattering cross section incorporating the effect of water penetration is developed and used to analyze the SANS I(Q). We have successfully quantified the intramicellar water distribution and found that the overall micellar hydration level increases with the increase in the molecular weight of hydrophilic PEO side chains. Our work presents a practical experimental means for evaluating the intramacromolecular solvent distributions of general soft matter systems.     

Probing the kinetic performance limits for ion chromatography. II. Gradient conditions for small ions

A gradient kinetic plot method is used for theoretical characterisation of the performance of polymeric particulate anion exchange columns for gradient separations of small inorganic anions. The method employed requires only information obtained from a series of isocratic column performance measurements and in silico predictions of retention time and peak width under gradient conditions. Results obtained under practically constrained conditions provide parameters for the generation of high peak capacities and rapid peak production for fast analysis to be determined. Using this prediction method, a maximum theoretical peak capacity of 84 could be used to achieve separation of 26 components using a 120 min gradient (R_s > 1). This approach provides a highly convenient tool for development of both mono- and multidimensional ion chromatography (IC) methodologies as it yields comprehensive understanding of the influence of gradient slope, analysis time, column length and temperature upon kinetically optimised gradient performance.     

Excited state dynamics and activation parameters of remarkably slow photoinduced CO loss from (η⁶-benzene)Cr(CO)₃ in n-heptane solution: a DFT and picosecond-time-resolved infrared study

The electronic structure of (η?-benzene)Cr(CO)? has been calculated using density functional theory and a molecular orbital interaction diagram constructed based on the Cr(CO)? and benzene fragments. The highest occupied molecular orbitals are mainly metal based. The nature of the lowest energy excited states were determined by time-dependent density functional theory, and the lowest energy excited state was found to have significant metal to carbonyl charge transfer character. The photochemistry of (η?-benzene)Cr(CO)? was investigated by time-resolved infrared spectroscopy with picosecond time resolution. The low energy excited state was detected following irradiation at 400 nm, and this exhibited ν(CO) bands at lower energy than the equivalent ν(CO) bands of (η?-benzene)Cr(CO)?, consistent with metal to carbonyl charge transfer character, and is formed with excess vibrational energy, relaxing to the v = 0 vibrational state within 3 ps. The resulting "cold" excited state decays to form the CO-loss species (η?-benzene)Cr(CO)? in approximately 70% yield and to reform (η?-benzene)Cr(CO)? within 150 ps. The rates of relaxation from the vibrationally hot state to the cold excited state and its subsequent reaction to yield (η?-benzene)Cr(CO)? were measured over a range of temperatures from 274 to 320 K, and the activation parameters for both processes were obtained from Eyring plots. The vibrational relaxation exhibits a negative activation enthalpy ΔH(?) (-10 (±4) kJ mol?1) and a negative activation entropy ΔS(?) (-50 (±16) J mol?1 K?1). A significant barrier (ΔH(?) = +12 (±4) kJ mol?1) was obtained for the formation of (η?-benzene)Cr(CO)? with a ΔS(?) value close to zero. These data are used to propose a model for the CO-loss process to yield (η?-benzene)Cr(CO)? and to explain why low temperature irradiation of (η?-benzene)Cr(CO)? with light of wavelengths greater than 400 nm produced relatively minor amounts of (η?-benzene)Cr(CO)?.     

Accurate modelling of Pd(0) + PhX oxidative addition kinetics

We have used dispersion-corrected DFT (DFT-D) together with solvation to examine possible mechanisms for reaction of PhX (X = Cl, Br, I) with Pd(P(t)Bu(3))(2) and compare our results to recently published kinetic data (F. Barrios-Landeros, B. P. Carrow and J. F. Hartwig, J. Am. Chem. Soc., 2009, 131, 8141-8154). The calculated activation free energies agree near-quantitatively with experimentally observed rate constants.     

New classes of carborane-appended 5-thio-D-glucopyranose derivatives

A series of carborane-appended 5-thio-D-glucopyranose (5-TDGP) derivatives containing one to two 5-TDGP moieties were synthesized via click cycloaddition reaction as well as following the traditional methods. Among the carboranyl-5-TDGP derivatives, the decapitated nido-carboranyl derivative 18 was found to be highly water-soluble and therefore its preliminary biodistribution study was conducted. A comparative biological evaluation of 18 versus its carboranyl-D-glucopyranose analog 19 with human hepatocellular carcinoma cells (SK-Hep1) indicated 5-TDGP to be a better boron carrier than normal D-glucopyranose. The carboranyl-5-TDGP 18 showed a nearly two fold increase in cellular boron accumulation than carboranyl-D-glucopyranose analog 19 over a period of 2 h. The accumulation of both 18 and 19 was found to occur in a temperature dependent manner. The higher accumulation of 18 suggested excellent promise for it to be a candidate for further evaluation as a future BNCT agent.     

Theoretical and experimental investigation of the energetics of cis-trans proline isomerization in peptide models

The energetics of cis-trans proline isomerization in small peptide models have been investigated using the hybrid density functional theory method B3LYP with a 6-31+G* basis set. The molecules studied are models for the phospho-Ser/Thr-Pro substrate for Pin-1, a peptidyl-prolyl isomerase (PPIase) involved in cell division. Pin-1 requires phosphorylation of a Ser or Thr residue adjacent to a Pro residue in the substrate and catalyzes cis-trans isomerization about the proline amide bond. The dihedral angle that would correspond to the reaction coordinate for isomerization of the omega peptide bond was investigated for several small models. Relaxed potential energy scans for this dihedral angle in N-methylacetamide, 1, N,N-dimethylacetamide, 2, acetylpyrrolidine, 3 and acetylproline, 4, were carried out in 20 degrees steps using the B3LYP/6-31+G* level of theory. In addition, similar scans were carried out for 1-4 protonated on the acetylamide carbonyl oxygen. Optimized structures for 1-4 protonated on the amide nitrogen were also obtained at B3LYP/6-31+G*. Relative proton affinities were determined for each site at various angles along the reaction coordinate for isomerization. The relative proton affinities were anchored to experimental gas phase proton affinities, which were taken from the literature for 1 and 2, or determined in an electrospray ionization-quadrupole ion trap instrument using the extended kinetic method for 3 and 4. Proton affinities of 925 +/- 10 and 911 +/- 12 kJ/mol were determined for 3 and 4, respectively. These studies suggest that the nitrogen atom in these amides becomes the most basic site in the molecule at a dihedral angle of ca. 130 degrees . In addition, the nitrogen atoms in 2-4 are predicted to attain basicities in the range 920-950 kJ/mol, making them basic enough to be the preferred site for hydrogen bonding in the Pin-1 active site, in support of the proposed mechanism for PPIases.