Exploring the hydroxylation-dehydrogenation connection: novel catalytic activity of castor stearoyl-ACP Delta(9) desaturase

The novel product profile obtained by incubating chiral fluorinated substrate analogues with castor stearoyl-ACP Delta(9) desaturase has been rationalized through a series of labeling studies. It was found that the introduction of the Z-double bond between C-9 and C-10 of the parent substrate occurs with pro-R enantioselectivity--a result that accounts for the observed stereochemistry of oxidation products derived from (9R)- and (9S)-9-fluorostearoyl-ACP. Oxidation of (9R)-9-fluorostearoyl-ACP occurs via at least two rapidly interchanging substrate conformations in the active site as detected by reaction pathway branching induced by deuteration at C-10 and C-11. Hydroxylation and desaturation of this substrate share the same site of initial oxidative attack.     

Universality in lattice models of dynamic arrest: introduction of an order parameter

We introduce an order parameter for dynamical arrest. Dynamically available volume (unoccupied space that is available to the motion of particles) is expressed as holes for the simple lattice models we study. Near the arrest transition the system is dilute in holes, so we expand dynamical quantities in a series of hole density. Unlike the situation when presented in particle density, all cases of simple models that we examine have a quadratic dependence of the diffusion constant on hole density. This observation implies that in certain regimes ideal dynamical arrest transitions may possess a hitherto unnoticed degree of universality.     

The interaction of oxygen with polycrystalldve niobium studied using aes and low-energy sims

The interaction of oxygen with polycrystalline niobium has been studied using both Auger electron spectroscopy and low-energy secondary ion mass spectrometry in the temperature range from 300–1250 K. At higher temperatures there is oxygen dissolution into the bulk but a preferential surface segregation on recooling. Between 300 and 1250 K, there is a rapid initial adsorption into a very stable state which is associated with increases in the Nb⁺ and NbO⁺ yields that are linear with coverage. At 1250 K, further changes are very slow. At 900 K, the initial stage is followed by the adsorption with a lower sticking coefficient (<0.1) as coverage increases from θ = 0.5 to 0.7. This produces an additional larger increase in the yield of Nb⁺ but a much smaller change in NbO⁺. At 300 K, the sticking probability falls more slowly with coverage above θ = 0.5 and the amount of oxygen continues to increase slowly with exposure. The SIMS spectrum shows dramatic increases in Nb⁺, NbO⁺ and NbO⁺₂ yields and the successive appearance of small yields of ions such as Nb₂O⁺₂ and Nb₂O⁺₃ as oxide formation begins. The Nb⁺ yield slowly decreases as further oxidation occurs. Each stage of oxidation has a characteristic secondary ion mass spectrum.     

Mapping properties of matrix summability methods

Summary Conditions on a complex matrix A are given which are necessary and sufficient for A to have the property that if x belongs to the complex linear space generated by the set of all convergent real sequences having nonnegative kth differences, then Ax belongs to the complex linear space generated by the set of all convergent real sequences having nonnegative jth differences. Entrata in Redazione il 18 giugno 1970.     

Qualitative behavior of geostochastic systems

Stochastic partial differential equations with polynomial coefficients have many applications in the study of spatially distributed populations in genetics, epidemiology, ecology, and chemical kinetics. The purpose of this paper is to describe some methods for investigating the qualitative behavior of spatially homogenous solutions of nonlinear stochastic partial differential equations. The stability and bifurcation of solutions, as well as the behavior near critical points are discussed. The methods employed are the nonlinear Markov approximation, moment density inequalities for identifying invariant sets of behaviors and rescaling and quasi-linearization around critical points.     

A model for streamer propagation

A model is presented to describe the propagation of positive corona streamers in the low field region of a non-uniform field gap in atmospheric air. It has been assumed that the growth is a property solely of the streamer tip, uninfluenced by the channel conductivity. Calculations from the model indicate that the criterion for propagation of a streamer in zero external field is that the number of ions in the tip be 10⁸ and the radius about 3×10^?3 cm. It is proposed that the streamer ceases to propagate as a result of the loss of energy of the tip due to the formation of ion pairs in the channel. The results of previous experimental observations of streamers are compared with calculations derived from the model, and a prediction from the model of the lifetime of streamers after voltage removal is discussed.     

Do light colored scalars exist?

The general question of the properties of light, neutral colored spin-zero particles in QCD is examined. Models with spontaneous breaking of QCD at very large distances, such as that of DeRújula, Giles, and Jaffe and the SO(3) scheme of Slansky, Goldman, and Shaw, require such light colored Higgs scalars. These scalars will form color-singlet hadronic bound states at short distances and estimates are given of bound state masses, decay widths, and production rates in processes such as ψ → γ + X within the MIT bag model. The resulting states are expected in the mass neighborhood ～1.5 GeV and should resemble glueballs.     

Mechanisms of radiation-induced surface composition modification in VCr alloys at high and low temperature

The effect of 0.5 to 3 keV argon ion sputtering on the surface composition of several alloys of vanadium and chromium has been investigated at temperatures in the range 300 to 950 K. At room temperature, sputtering depletes the alloys in chromium and the relative V/Cr sputter rate constant ratio is found to be 1.20, consistent with the ratio of the pure element sputter yields. Annealing clean alloys at temperatures in the range 550 to 670°C further enriches the surface in vanadium and also oxygen segregates to the surface. This behaviour is attributed to co-segregation of oxygen and vanadium, an interpretation supported by the observation of vanadium chemical shifts analogous to those observed in vanadium oxides, the depth profiles of the segregated surfaces and the bond energies in the VCrO system. Sputtering the alloys at temperatures in the range 410 to 620°C enriches the surface in chromium to a depth which increases with sputter annealing time. Similar behaviour has been reported on sputtering a V/Cr alloy at these temperatures but with much higher energy V⁺ ions, and has been attributed to radiation-induced segregation of chromium. The results presented in this paper offer a different interpretation. Annealing enriches the surface in vanadium which co-segregates with oxygen. Thus on sputter- annealing the surface will become depleted in vanadium at a rate which is probably enhanced by an increased sputter yield of VO over either V or Cr. This interpretation is further supported by the observation that the alloy is still depleted in vanadium when the sputtering occurs at room temperature when performed in a cycle with intervening annealing intervals.