Reactions of metal carbonyl derivatives : XIII. Reaction of bis[μ-(tert-butylsulphido)tricarbonyl-iron] with tertiary and ditertiary phosphines and phosphites

The ligands L  P(C₂H₅)₃, P(C₆H₅)₃, P(OCH₃)₃ and P(OC₆H₅)₃ react with [Fe(CO)₃(S-t-C₄H₉)]₂ to give mono-substituted Fe₂(CO)₅L(S-t-C₄H₉)₂ or bis-substituted [Fe(CO)₂L(S-t-C₄H₉)]₂ depending on the reaction conditions. With the exception of [Fe(CO)₂P(C₂H₅)₃(S-t-C₄H₉)]₂, the latter derivatives occur both in solution and in the solid state as a single isomer in which the ligands L are bonded trans to the metal-metal bond. Whereas an asymmetrically bis-substituted product, Fe(CO)₃(S-t-C₄H₉)₂Fe(CO)L' is formed in the reaction of [Fe(CO)₃(S-t-C₄H₉)]₂ with L' &2.dbnd; cis-(C₆H₅)₂PC₂H₂P(C₆H₅)₂, symmetrically bis-substituted derivatives [Fe(CO)₂(S-t-C₄H₉)]₂L', in which the ligand bridges the two iron atoms are produced in the corresponding reactions involving L'  (C₆H₅)₂P(CH_2nP(C₆H₅)₂ (n  1 and 2). The NMR spectrum of [Fe(CO)₂P(OCH₃)₃(S-t-C₄H₉)]₂, as well as those of the complexes [Fe(CO)₂P(OCH₃)₃SR]₂ (R  CH₃ and i-C₃H₇) which have also been synthesised in this study, is interpreted in terms of a virtual coupling effect.     

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.     

Steady-State and Time-Resolved Fluorescence Polarization Behavior of Acenaphthene

Steady-state and time-resolved fluorescence polarization studies have been carried out on acenaphthene (ACE) in low-temperature glass solutions and at room temperature. In the low-temperature glass the fluorescence polarization values vary considerably with both emission and excitation wavelength. There is a time dependence (on the nanosecond time scale) of the fluorescence anisotropy, r(t), at 77 K, which has a strong dependence upon the excitation and emission wavelengths. Under these conditions, the time-dependent decay of the anisotropy is not attributable to chromophoric motion. The observations are consistent with emission from two closely lying and interconverting excited states. Rate constants for the photophysical processes involved have been determined by fitting the data using a model proposed by Fleming et. al. The results are discussed with particular reference to the care required in using dynamic fluorescence polarization measurements to determine energy transfer rates in systems containing this chromophore.     

Simultaneous DSC-reflected Light Intensity of Polymer Systems. Part II

It has been shown that simultaneous measurements of reflected light intensity (RLI) with DSC can give additional and valuable information for the thermal analysis of materials including pure organic and inorganic chemicals, natural and synthetic polymers and foods. This paper extends the range of polymers studied and examines the possibility of improving the reproducibility and quantitative nature of RLI measurements. Polymer crystallisation, the curing of epoxy resins, the action of fire retardant additives and the degradation of polymers and proteins are among the systems studied. This revised version was published online in July 2006 with corrections to the Cover Date.     

The bone volume effect on the dosimetry of plutonium-239 and americium-241 in the skeleton of man and baboon

Studies were undertaken using bone removed from young adult baboons, which had been contaminated with plutonium-239 at various times prior to sacrifice, and human bone from an adult male (USTR Case 246), who had received an internal deposition of americium-241 as a result of a glove-box explosion 11 years prior to his death. The baboon bone was supplied by the CEA, France, and the human bone by the United States Transuranium Registry. The bone samples, examined by qualitative and quantitative autoradiography with CR 39 detectors, demonstrated the rapid redistribution of bone surface-seeking radionuclides in younger primates due to growth and the slower, bone turnover driven redistribution in the adult human bone. In both species, primary and secondary surface deposits of radionuclide remained conspicuous despite bone activity; true volumization of radionuclide was seldom seen. The dosimetric implications of these findings are discussed.     

X-point collapse and saturation in the nonlinear tearing mode reconnection

We study the nonlinear evolution of the resistive tearing mode in slab geometry in two dimensions. We show that, in the strongly driven regime (large delta'), a collapse of the X point occurs once the island width exceeds a certain critical value approximately 1/delta'. A current sheet is formed and the reconnection is exponential in time with a growth rate proportional eta(1/2), where eta is the resistivity. If the aspect ratio of the current sheet is sufficiently large, the sheet can itself become tearing-mode unstable, giving rise to secondary islands, which then coalesce with the original island. The saturated state depends on the value of delta'. For small delta', the saturation amplitude is proportional delta' and quantitatively agrees with the theoretical prediction. If delta' is large enough for the X-point collapse to have occurred, the saturation amplitude increases noticeably and becomes independent of delta'.     

Reflectance anisotropy spectra of the diamond (100)-(2x1) surface: evidence of strongly bound surface state excitons

We compare the results of ab initio calculations with measured reflection anisotropy spectra and show that strongly bound surface-state excitons occur on the clean diamond (100) surface. These excitons are found to have a binding energy close to 1 eV, the strongest ever observed at a semiconductor surface. Important electron-hole interaction effects on the line shape of the optical transitions above the surface-state gap are also found.     

Effect of radial-electric-field polarity on wire-array Z-pinch dynamics

The formation of plasma in wire-array Z-pinch experiments was found to depend upon the polarity of the radial-electric field near the wires. Reversing the radial-electric field midway along the length of an array resulted in the ablation rate of one-half of the array being reduced by 50%, significantly delaying the start of its implosion and altering its acceleration towards the axis. The observed phenomena cannot be explained by the standard magnetohydrodynamic models of array behavior, suggesting that effects such as electron emission may be important, especially during wire initiation.