Vanadium (V) is reduced by the 'as isolated' nitrogenase Fe-protein at neutral pH

Orthovanadate has been investigated in the presence of the nitrogenase Fe-protein. Electron paramagnetic resonance (EPR) spectra demonstrate that vanadium (V) is reduced by the reduced Fe-protein to vanadium (IV) which then probably binds to the nucleotide binding site in place of the Mg2+ which is normally present. In contrast, the oxidized Fe-protein is unable to reduce vanadate. In this case vanadate has potential for use as a phosphate analogue where it acts as transition state mimic for hydrolysis.     

Mechanism of reaction of hydrogen peroxide with horseradish peroxidase: identification of intermediates in the catalytic cycle.

The mechanism of the reaction of horseradish peroxidase isoenzyme C (HRPC) with hydrogen peroxide to form the reactive enzyme intermediate compound I has been studied using electronic absorbance, rapid-scan stopped-flow, and electron paramagnetic resonance (EPR) spectroscopies at both acid and basic pH. The roles of the active site residues His42 and Arg38 in controlling heterolytic cleavage of the H(2)O(2) oxygen-oxygen bond have been probed with site-directed mutant enzymes His42 --> Leu (H42L), Arg38 --> Leu (R38L), and Arg38 --> Gly (R38G). The biphasic reaction kinetics of H42L with H(2)O(2) suggested the presence of an intermediate species and, at acid pH, a reversible second step, probably due to a neutral enzyme-H(2)O(2) complex and the ferric-peroxoanion-containing compound 0. EPR also indicated the formation of a protein radical situated more than approximately 10 A from the heme iron. The stoichiometry of the reaction of the H42L/H(2)O(2) reaction product and 2,2'-azinobis(3-ethylbenzothiazolinesulfonic acid) (ABTS) was concentration dependent and fell from a value of 2 to 1 above 0.7 mM ABTS. These data can be explained if H(2)O(2) undergoes homolytic cleavage in H42L. The apparent rate of compound I formation by H42L, while low, was pH independent in contrast to wild-type HRPC where the rate falls at acid pH, indicating the involvement of an ionizable group with pK(a) approximately 4. In R38L and R38G, the apparent pK(a) was shifted to approximately 8 but there is no evidence that homolytic cleavage of H(2)O(2) occurs. These data suggest that His42 acts initially as a proton acceptor (base catalyst) and then as a donor (acid catalyst) at neutral pH and predict the observed slower rate and lower efficiency of heterolytic cleavage observed at acid pH. Arg38 is influential in lowering the pK(a) of His42 and additionally in aligning H(2)O(2) in the active site, but it does not play a direct role in proton transfer.     

Seeing luminescence appear as crystals crumble. Isolation and subsequent self-association of individual [(C6H11NC)2Au]+ ions in crystals

Non-luminescent, isostructural crystals of [(C₆H₁₁NC)₂Au](EF₆)·C₆H₆ (E = As, Sb) lose benzene upon standing in air to produce green luminescent (E = As) or blue luminescent (E = Sb) powders. Previous studies have shown that the two-coordinate cation, [(C₆H₁₁NC)₂Au]⁺, self-associates to form luminescent crystals that contain linear or nearly linear chains of cations and display unusual polymorphic, vapochromic, and/or thermochromic properties. Here, we report the formation of non-luminescent crystalline salts in which individual [(C₆H₁₁NC)₂Au]⁺ ions are isolated from one another. In [(C₆H₁₁NC)₂Au](BArF₂₄) ((BArF₂₄)⁻ is tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) each cation is surrounded by two anions that prohibit any close approach of the gold ions. Crystallization of [(C₆H₁₁NC)₂Au](EF₆) (E = As or Sb, but not P) from benzene solution produces colorless, non-emissive crystals of the solvates [(C₆H₁₁NC)₂Au](EF₆)·C₆H₆. These two solvates are isostructural and contain columns in which cations and benzene molecules alternate. With the benzene molecules separating the cations, the shortest distances between gold ions are 6.936(2) Å for E = As and 6.9717(19) Å for E = Sb. Upon removal from the mother liquor, these crystals crack due to the loss of benzene from the crystal and form luminescent powders. Crystals of [(C₆H₁₁NC)₂Au](SbF₆)·C₆H₆ that powder out form a pale yellow powder with a blue luminescence with emission spectra and powder X-ray diffraction data that show that the previously characterized [(C₆H₁₁NC)₂Au](SbF₆) is formed. In the process, the distances between the gold(i) ions decrease to ∼3 Å and half of the cyclohexyl groups move from an axial orientation to an equatorial one. Remarkably, when crystals of [(C₆H₁₁NC)₂Au](AsF₆)·C₆H₆ stand in air, they lose benzene and are converted into the yellow, green-luminescent polymorph of [(C₆H₁₁NC)₂Au](AsF₆) rather than the colorless, blue-luminescent polymorph. Paradoxically, the yellow, green-luminescent powder that forms as well as authentic crystals of the yellow, green-luminescent polymorph of [(C₆H₁₁NC)₂Au](AsF₆) are sensitive to benzene vapor and are converted by exposure to benzene vapor into the colorless, blue-luminescent polymorph.

Non-luminescent crystals of [(C₆H₁₁NC)₂Au](AsF₆)·C₆H₆ readily lose benzene to form the green-luminescent polymorph of [(C₆H₁₁NC)₂Au](AsF₆) rather than the blue-luminescent polymorph.     

Polymerizations Mediated by Well‐Defined Rhodium Complexes

This Minireview details the current state‐of‐the‐art relating to (co)polymerizations mediated by well‐defined Rh^I‐ethynyl, vinyl, and aryl complexes. In particular, we focus on Rh^I species suitable for the (co)polymerization of phenylacetylenes, arylisocyanides, as well as propargyl esters and amides.     

Curing theory of A_f-A_g type free radical polymerization (I)──Distribution function and its invariant property

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Experimental Results on the New Techniques for Integer Programming Formulations

We present experimental results on some new formulations of problems as mixed integer programmes.On a family of small problems involving ‘either/or’ constraints, the newer formulations performed distinctly better than does a ‘textbook formulation’, with an advantage that increases with problem size. On a second set of small problems involving separable programmes with fixed charges imposed on a piecewise-linear functions, the advantage of the newer formulations was again evident.     

Aqueous solution properties of pH‐responsive AB diblock acrylamido–styrenic copolymers synthesized via aqueous reversible addition–fragmentation chain transfer

Here we report the synthesis and solution characterization of a novel series of AB diblock copolymers with neutral, water‐soluble A blocks consisting of N,N‐dimethylacrylamide and pH‐responsive B blocks of N,N‐dimethylvinylbenzylamine. To our knowledge, this represents the first example of an acrylamido–styrenic block copolymer prepared directly in a homogeneous aqueous solution. The best blocking order [with poly(N,N‐dimethylacrylamide) as a macro‐chain‐transfer agent] yielded well‐defined block copolymers with minimal homopolymer impurities. The reversible aggregation of these block copolymers in aqueous media was studied with ¹H NMR spectroscopy and dynamic light scattering. Finally, an example of core‐crosslinked micelles was demonstrated by the addition of a difunctional crosslinking agent to a micellar solution of the parent block copolymer. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1724–1734, 2004     

Nonparametric bandit methods

Bandits are a finite collection of random variables. Bandit problems are Markov decision problems in which, at each decision time, the decision maker selects a random variable (referred to as a bandit arm) and observes an outcome. The selection is based on the observation history. The objective is to sequentially choose arms so as to minimize growth (with decision time) rate of the number of suboptimal selections.The appellation bandit refers to mechanical gambling machines, and the tradition stems from the question of allocating competing treatments to a sequence of patients having the same disease. Our motivation is machine learning in which a game-playing or assembly-line adjusting computer is faced with a sequence of statistically-similar decision problems and, as resource, has access to an expanding data base relevant to these problems.The setting for the present study is nonparametric and infinite horizon. The central aim is to relate a methodology which postulates finite moments or, alternatively, bounded bandit arms. Under these circumstances, strategies proposed are shown to be asymptotically optimal and converge at guaranteed rates. In the bounded-arm case, the rate is optimal.We extend the theory to the case in which the bandit population is infinite, and share some computational experience.