argE-encoded N-acetyl-L-ornithine deacetylase from Escherichia coli contains a dinuclear metalloactive site

The catalytic and structural properties of the argE-encoded N-acetyl-L-ornithine deacetylase (ArgE) from Escherichia coli were investigated. On the basis of kinetic and ITC (isothermal titration calorimetry) data, Zn(II) binds to ArgE with Kd values that differ by approximately 20 times. Moreover, ArgE exhibits approximately 90% of its full catalytic activity upon addition of one metal ion. Therefore, ArgE behaves similarly to the aminopeptidase from Aeromonas proteolytica (AAP) in that one metal ion is the catalytic metal ion while the second likely plays a structural role. The N-acetyl-L-ornithine (NAO) deacetylase activity of ArgE showed a linear temperature dependence from 20 to 45 degrees C, indicating that the rate-limiting step does not change over this temperature range. The activation energy for NAO hydrolysis by ArgE was 25.6 kJ/mol when loaded with Zn(II) and 34.3 kJ/mol when loaded with Co(II). Electronic absorption and EPR (electron paramagnetic resonance) spectra of [Co x (ArgE)] and [CoCo(ArgE)] indicate that both divalent metal binding sites are five coordinate. In addition, EPR data show clear evidence of spin-spin coupling between the Co(II) ions in the active site but only after addition of a second equivalent of Co(II). Combination of these data provides the first physical evidence that the ArgE from E. coli contains a dinuclear Zn(II) active site, similar to AAP and the carboxypeptidase G2 from Pseudomonas sp. strain RS-16 (CPG2).     

Assessing the risk of vCJD transmission via surgery: models for uncertainty and complexity

Variant Creutzfeldt–Jakob disease (vCJD) is a fatal degenerative brain disease, which probably passed into humans from cattle infected with bovine spongiform encephalopathy (BSE) or ‘mad cow disease’. Whatever its origins, the question arises of whether vCJD could be spread from person to person. Any such risk is very difficult to quantify, not least because the number of people already infected is unknown. This paper reflects on a study assessing the risk of transmission via instruments used in hospital surgery. The study underpinned a number of policy initiatives, including a £200?m programme to improve instrument decontamination. The methodology had to meet two major challenges. That of dealing with very large and multiple uncertainties was initially addressed using simple spreadsheet-based models to generate alternative scenarios. The complexity in the possible dynamics of vCJD within the population was then tackled. This paper outlines the models built and their use in eliciting expert judgement and informing risk management policy.     

α-Substituted diisopropylbenzene hydroperoxides for low-temperature SBR polymerization

Four pure hydroperoxides were evaluated as initiators for the polymerization of styrene–butadiene rubber (SBR) at 5°C. These materials were the meta and para isomers of mono- and dihydroperoxy-diisopropylbenzene. Results showed that the monohydroperoxides produced somewhat faster reactions than a control hydroperoxide, p-menthane hydroperoxide. Response to mercaptan level adjustments was good, providing polymer with a Mooney viscosity in the 50 to 60 range. The reaction rate with the dihydroperoxides was slower, but again provided polymer with a satisfactory viscosity. Basic physical property measurements on polymers prepared with the pure hydroperoxides or control hydroperoxides showed only minor differences. It is anticipated that these differences can be eliminated by small adjustments in the compound recipe.     

Stochastic control of SDEs associated with Lévy generators and application to financial optimization

This paper is concerned with the optimal control of jump type stochastic differential equations associated with (general) Lévy generators. The maximum principle is formulated for the solutions of the equations, which is inspired by N. C. Framstad, B. Øksendal and A. Sulem [J. Optim. Theory Appl., 2004, 121: 77–98] (and a continuation, J. Bennett and J. -L. Wu [Front. Math. China, 2007, 2(4): 539–558]). The result is then applied to optimization problems in financial models driven by Lévy-type processes.     

A note on some piecewise-linear difference equations with Mersenne-type periodic solutions

This paper studies solutions of some piecewise-linear difference equations. In two particular cases, a descent argument is used to show that all solutions are periodic with either prime period 3(2 ^k  ? 1) or 6(2 ^k  ? 1) for some k ≥ 1. The existence of solutions with such periods is also considered.     

Exponentiation of infinite dimensional Z-graded lie algebras

In this article we give a new technique for exponentiating infinite dimensional graded representations of graded Lie algebras that allows for the exponentiation of some non-locally nilpotent elements. Our technique is to naturally extend the representation of the Lie algebra g on the space V naturally to a representation on a subspace £ of the dual space V ^*. After introducing the technique, we prove that it enables the exponentiation of all elements of free Lie Algebras and afhne Kac-Moody Lie algebras.     

Postselective two-photon interference from a continuous nonclassical stream of photons emitted by a quantum dot

We report an electrically driven semiconductor single-photon source capable of emitting photons with a coherence time of up to 400 ps under fixed bias. It is shown that increasing the injection current causes the coherence time to reduce, and this effect is well explained by the fast modulation of a fluctuating environment. Hong-Ou-Mandel-type two-photon interference using a Mach-Zehnder interferometer is demonstrated using this source to test the indistinguishability of individual photons by postselecting events where two photons collide at a beam splitter. Finally, we consider how improvements in our detection system can be used to achieve a higher interference visibility.