Formation and structure of 1:1 complexes between aryl hydroxamic acids and vanadate at neutral pH

Although aryl hydroxamic acids are well-known to form coordination complexes with vanadate (V(V)), the nature of these complexes at neutral pH and submillimolar concentrations, the conditions under which such complexes inhibit various serine amidohydrolases, is not well established. A series of qualitative and quantitative experiments, involving UV/vis, (1)H NMR, and (51)V NMR spectroscopies, established that both 1:1 and 1:2 vanadate/hydroxamate complexes form at pH 7.5, with the former dominating at submillimolar concentrations. Formation constants for the complexes of several aryl and alkyl hydroxamic acids were determined; for example, for benzohydroxamic acid, the stepwise formation constants of the 1:1 and 1:2 complexes were 3000 and 400 M(-1), respectively. The (51)V chemical shift of the 1:1 4-nitrobenzohydroxamic acid complex was -497 ppm, and that of its unsubstituted analogue was -498 ppm. A (1)H-(15)N HSQC spectrum of the 4-nitrobenzo-(15)N-hydroxamic acid/vanadate complex indicated the presence of an N-H group with (15)N and (1)H chemical shifts of 115 and 5.83 ppm, respectively. A (13)C NMR spectrum of the complex of 4-nitrobenzo-(13)C-hydroxamic acid with vanadate displayed a resonance at 170.1 ppm and thus a coordination-induced shift (CIS) of +3.8 ppm. In contrast, the CIS value of an established 1:2 complex, thought to contain chelated hydroxamic acid ligands, was +11.9 ppm. These spectral data led to the following structural picture of 1:1 complexes of vanadate and aryl hydroxamic acids. They contain penta- or hexa-coordinated vanadium. The ligand is in the hydroxamate rather than hydroximate form. The ligand is presumably bound to vanadium through the hydroxamic hydroxyl oxygen, but the hydroxamic acid carbonyl oxygen interacts weakly with vanadium. These species are the most likely candidates for the inhibitors of serine amidohydrolases found in vanadate/hydroxamic acid mixtures.     

Solid-State 13C-NMR spectral evidence for charge transfer complex formation in aromatic diimides and dianhydrides

Solution and solid-state proton decoupled ¹³C-NMR spectra were determined on two diimides derived from 4, 4′-oxydiphthalic anhydride. Comparison of the individual diimide spectra to that of a mixture of the two diimides indicates that ordering of these materials occurs in the solid state via charge transfer complex formation. A similar study was conducted using two isomeric dianhydrides, 4, 4′-isophthaloyldiphthalic anhydride (IDPA) and 4, 4′-terephthaloyldiphthalic anhydride (TDPA). The solution spectra of these compounds are similar and are those which would be expected for these compounds. However, their solid state spectra differ from each other. The solid-state spectrum of TDPA resembles its solution spectrum, whereas, that of IDPA differs greatly from its solution spectrum and indicates charge transfer complex formation occurs with this molecule. This difference is explained in terms of the stereochemistry of the two isomeric dianhydrides.     

Activity of Ni?Mo hydrodesulfurization catalysts supported on mixed alumina-zirconia

The hydrodesulfurization (HDS) of thiophene by nickel-molybdenum catalysts supported on alumina-zirconia has been investigated. The effect of varying the ratios of oxides in the support is to produce a minimum in the activity for HDS, and the coimpregnated catalysts are shown to be more active than the analogous co-precipitated catalysts.

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Mechanistic insights from reactions between copper(II)-phenoxyl complexes and substrates with activated C-H bonds

The reactivities of two copper(II)-phenoxyl analogues of the oxidized, active form of the metalloenzyme galactose oxidase, [1tBu2]+ and [2tBu2]+, have been studied using the substrates benzyl alcohol and 9,10-dihydroanthracene, for a total of four reactions. The reaction stoichiometries in all cases show a 2:1 ratio of oxidant to benzaldehyde or anthracene product, indicating that [1tBu2]+ and [2tBu2]+ behave ultimately as only one-electron oxidants, but the reaction kinetics each indicate that only a single copper(II)-phenoxyl complex is involved in the rate-determining step. For each substrate, rate laws indicate that [1tBu2]+ and [2tBu2]+ react by different mechanisms: one proceeds by a simple bimolecular reaction, while the other first enters into a substrate-binding equilibrium before subsequently reacting by an intramolecular reaction. The reactions proceeding by the latter mechanism have faster overall rates, which correlates to a lower entropic barrier for the substrate-binding mechanism. Correlation of the reaction rates with the C-H bond dissociation energies of substrates as well as significant deuterium kinetic isotope effects indicates that the rate-determining steps involve hydrogen atom abstraction from the activated C-H bonds. A variable-temperature study (268-308 K) of the nonclassical KIE of the [1tBu2]+/benzyl alcohol reaction (kH/kD = 15 at 298 K) failed to show evidence for quantum tunneling. The rapid sequence by which a second 1 equiv of copper(II)-phenoxyl oxidant completes the reaction after the rate- and product-determining hydrogen atom abstraction step cannot be probed kinetically. Comparisons are made to the reactivities of other copper(II)-phenoxyl complexes reported in the literature and to galactose oxidase itself.     

Existence of near-zero minima in elastic photon–atom scattering

We consider near-zero minima in the photon–atom elastic scattering cross section, possible signatures of the presence of a particular element in a target, using the S-matrix approach. However, the usual formulation of this approach performs an averaging over magnetic substates at the level of the scattering amplitude that is approximate (except for the case of fully filled subshells). We consider whether the predicted near-zero minima persist in a more proper treatment. We find that near-zero minima seen below thresholds in scattering from ground-state atoms generally persist, but near-zero minima seen above thresholds in scattering from excited states may be spurious.     

High resolution electronic spectroscopy of p-vinylphenol in the gas phase

A recent controversy regarding the proper assignment of two closely spaced bands in the S₁ ← S₀ electronic transition of trans-p-coumaric acid (pCA) has been addressed by recording their spectra at full rotational resolution. The results show unambiguously that the carrier of these two bands is p-vinylphenol (pVP), a thermal decomposition product of pCA. The two bands belong to two conformers of pVP; trans-pVP at 33,207.3 cm⁻¹ and cis-pVP at 33,211.8 cm⁻¹.     

Site-Specific Differences in Proteasome-Dependent Degradation of Monoubiquitinated α-Synuclein

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Magnetism in geometrically frustrated YMnO3 under hydrostatic pressure studied with muon spin relaxation

The ferroelectromagnet YMnO3 consists of weakly coupled triangular layers of S=2 spins. Below T(N) approximately equal to 70 K muon-spin relaxation data show two oscillatory relaxing signals due to magnetic order, with no purely relaxing signals resolvable (which would require different coexisting spin distributions). The transition temperature T(N) increases with applied hydrostatic pressure, even though the ordered moment decreases. These results suggest that pressure increases both the exchange coupling between the layers and the frustration within the layers.     

Bacterial Spores Survive Electrospray Charging and Desolvation

The survivability of Bacillus subtilis spores and vegetative Escherichia coli cells after electrospray from aqueous suspension was tested using mobility experiments at atmospheric pressure. E. coli did not survive electrospray charging and desolvation, but B. subtilis did. Experimental conditions ensured that any surviving bacteria were de-agglomerated, desolvated, and electrically charged. Based on mobility measurements, B. subtilis spores survived even with 2,000–20,000 positive charges. B. subtilis was also found to survive introduction into vacuum after either positive or negative electrospray. Attempts to measure the charge distribution of viable B. subtilis spores using electrostatic deflection in vacuum were inconclusive; however, viable spores with low charge states (less than 42 positive or less than 26 negative charges) were observed.