On the validity of Fourier's law in systems with spatially varying strain rates

Non-equilibrium molecular dynamics (NEMD) simulations are used to study the generation of heat fluxes in systems with spatially varying shear rates. We show that the kinetic temperature, when used in Fourier's law of heat conduction, does not correctly account for the heat flux, and that the normal temperature as described by Rugh (1997, Phys. Rev. Lett., 78, 772), should be used. Only in the absence of normal temperature gradients can heat fluxes due to strain rate coupling be correctly calculated.     

A STEREOCHEMICAL MODEL FOR THE ACTIVE SITE OF PHOTOSYSTEM II HERBICIDES*

Abstract Photosystem II herbicides act by blocking electron transport at the secondary electron acceptor ‘Q_B’, thought to be a non-covalently bound plastoquinone. Recent evidence suggests that these compounds work by displacing the plastoquinone from its site in the thylakoid. Since the herbicides cannot act as electron carriers, electron transport is then blocked. In this report a model is presented for the site of action of Photosystem II herbicides that encompasses biochemical, biophysical, and structure-activity considerations. The essence of the model is that Photosystem II herbicides are non-reducible analogues of plastoquinone or its semiquinone anion. As examples of the ways in which known herbicidal classes fit the model, the possible interactions of diuron, atrazine, the putative urea-triazine hybrid MBAT (the a-methylbenzyl analogue of atrazine), and dinoseb with the active site are discussed. This model provides a stereochemical basis for herbicidal activity and offers a qualitative approach for the design of novel Photosystem II herbicides.     

Extreme tunnelling in methylamine dehydrogenase revealed by hybrid QM/MM calculations: potential energy surface profile for methylamine and ethanolamine substrates and kinetic isotope effect values

The rate-determining proton transfer step in the amine reduction reaction catalysed by the enzyme methylamine dehydrogenase has been studied using a hybrid quantum mechanical/molecular mechanical (QM/MM) model. Variational transition state theory, combined with multidimensional tunnelling corrections, has been employed to calculate reaction rate constants, and hence deuterium kinetic isotope effects (KIE). To render these calculations computationally feasible, the electronic structure was described using a PM3 method with specific reaction parameters obtained by a fit to energetics obtained at a high level for a small model system. Compared to the use of standard parameters, these revised parameters result in a considerable improvement in the predicted KIE values and activation energy. For both methylamine and ethanolamine substrates, through-barrier, rather than over-barrier, motion is found to dominate with KIE values that are large and close to the experimental values. A major difference between the two substrates is that, for ethanolamine, different hydrogen bonding structures involving the substrate hydroxyl are possible, leading to very different potential energy surfaces with KIE values covering a considerable range. We speculate that this is the origin of the differing temperature behaviour observed for the KIEs of the two substrates.     

PHOTOCHEMICAL TRANSFORMATIONS IN SALICYLANILIDE PHOTOALLERGY

Abstract— The photochemical transformations of 3,3',4',5-tetrachlorosalicylanilide, have been characterized by isolation and spectroscopic determination of the primary and secondary photoproducts. The photoreactivity cleanly involves photochemical dechlorination; three chlorines are ultimately lost in an efficient stepwise fashion. A radical mechanism was implicated by formation of a dimer and hydrogen atom abstraction from cysteine when present during photolysis. In the presence of HSA or BSA, the initial dehalogenation leads to a photoadduct with the protein, but dehalogenation of the primary photoproduct, a trichloro compound, does not lead to an efficient photobinding to protein.     

The benefit to consumers from generic scoring models based on credit reports

Received on 1 July 1991. The benefit to consumers from the use of informative creditreports is demonstrated by showing the improvement in creditdecisions when generic scoring models based on credit reportsare implemented. If these models are highly predictive, thenthe truncation of credit reports will reduce the predictivepower of bureau-based generic scoring systems. As a result,more good credit risks will be denied credit, and more poorcredit risks will be granted credit. It is shown that, evenwhen applied to credit applications that had already been screenedand approved, the use of generic scoring models significantlyimproves credit grantors' ability to predict and eliminate bankruptcies,charge-offs, and delinquencies. As applied to existing accounts,bureau-based generic scores are shown to have predictive valuefor at least 3 months, while scores 12 months old may not bevery powerful. Even though bureau-based scores shift towardsthe high-risk end of the distribution during a recession, theycontinue to rank risk very well. When coupled with application-basedcredit-scoring models, scores based on credit-bureau data furtherimprove the predictive power of the model-the improvements beinggreater with more complete bureau information. We conclude thatgovernment-imposed limits on credit information are anti-consumerby fostering more errors in credit decisions.     

EFFECTS OF INDIVIDUAL HETEROGENEITY IN ESTIMATING THE PERSISTENCE OF SMALL POPULATIONS

ABSTRACT. Population viability models are commonly used to estimate the probability of persistence of small, threatened, or endangered populations. Demographic, temporal, spatial, and individual heterogeneity are important factors affecting the probability of persistence of small populations. Because stochastic process are intractable analytically (Lud-wig [1996]), computer simulation models are often used for estimating population viability via numerical techniques. Although demographic, spatial, and temporal stochasticity have been incorporated into some population viability models, individual heterogeneity has not been included. In this paper we include individual heterogeneity in a simulation model and examine probabilities of population persistence at different levels of heterogeneity and population size. Individual heterogeneity may increase the probability of persistence of small populations. The mechanism for the extension in persistence may be explained by natural selection. Genotypes persisting through a decline may be those that survive better under the conditions causing the decline. These individuals that survive and reproduce in the face of adverse conditions may extend the probability that a small population persists.     

The breakdown of Navier—Stokes hydrodynamics in small periodic systems

The results of direct particle simulations of 2-dimensional, convective flow are compared with truncated solutions of the Navier—Stokes equation, and show that the Navier—Stokes predictions are quite accurate throughout the transient development of the flow, if the correct value of the viscosity is used. The viscosity determines the steady values of the flow velocities, and the Navier—Stokes predictions are accurate for systems with multiple k vector forcing. At higher values of the fields, there is a limiting value of the forcing for which the character of the response matches the character of the forcing. Beyond this point there is a breakdown in hydrodynamic behaviour where the particle nature of the fluid dominates. Both multiphase solid and fluid regions are observed, plus cavitation, depending upon the forcing and the system geometry.     

PORPHYRIN-SENSITIZED PHOTOREACTIONS IN THE PRESENCE OF ASCORBATE: OXIDATION OF CELL MEMBRANE LIPIDS AND HYDROXYL RADICAL TRAPS

Abstract— Photooxidation reactions in ascorbate (AH)-containing erythrocyte membrane suspensions have been studied in broad perspective by simultaneously monitoring lipid peroxidation in the membrane compartment and formation of hydrogen peroxide (H₂O₂) and hydroxyl radical (OH) in the aqueous compartment. Non-bound uroporphyrin (UP) and membrane-bound protoporphyrin (PP) were used as sensitizers. Photoreduction of UP to the radical anion (UP^-) was detected by electron spin resonance when UP/AH/membrane mixtures were irradiated anaerobically. Aerobic irradiation resulted in a strong AH^--stimulation of lipid peroxidation, H₂O₂ formation, and OH^- generation (detected with 2-deoxyribose (DOR) and the spin trap 5,5-dimethyl-l-pyrroline-N-oxide). Use of diagnostic agents (e.g. catalase, desferrioxamine, mannitol) revealed that OH^- is involved in light-stimulated DOR oxidation, but not in lipid peroxidation. Similar irradiation in the presence of PP resulted in far greater lipid peroxidation than observed with UP, but less DOR oxidation, and insignificant accumulation of H₂O₂. This suggests that photoreduction of membrane-bound PP is less efficient, possibly due to hindered access of AH^-.