Characterization of phosphate conversion layers by combined Auger electron spectroscopy and X-ray microanalysis

Combined high-resolution AES and EDX studies have been performed of a number of industrial phosphate conversion layers on steel in order to determine the importance of the surface composition for the corrosion protection of the phosphated steel.

The well-known Cr(III) rinse of zinc-iron phosphate which improves its long-term paint adhesion performance was found to result in an exchange of Zn²⁺ with Cr³⁺ ions in the surface layers of the crystals. Corrosion improvement is caused by a slower rate of attack of the Cr-containing crystals due to the low solubility of CrP0₄ in aqueous media.

A commercial calcium-zinc-iron phosphate system was studied for corrosion resistance in relation to its conditions of formation. Phosphate layers with optimum corrosion resistance were found to have a strongly enriched iron content in the surface layers.     

Temperature and magnetic field dependent correlations in the singlet ground state system CsFeBr3

The magnetic excitations in CsFeBr₃ have been measured with inelastic scattering of cold neutrons to high precision at 80 mK. The fact that the lowest frequency mode softens with decreasing temperature but stabilizes at 0.11 THz below 2.5 K is the indication that CsFeBr₃ remains a SGS system forT0.From dispersion curves measured earlier in a magnetic field along the chain axis experimental intensities were derived and in turn correlation lengths. Correlation lengths were also calculated using the new parameters for exchange and anisotropy. The experimental results and the calculations both show that the correlation lengths increase for increasing magnetic field, flatten off around the phase transformation at 4.1 Tesla with a maximum of about 35 Å and then decrease again for higher fields. No critical phenomena connected with this phase transformation could be detected.The Mirror Mode published earlier was found to be of spurious origin due to double scattering. It does not exist.     

How does product isotope effect prove the operation of a two-state "rebound" mechanism in C-H hydroxylation by cytochrome P450?

C-H hydroxylation is a fundamental process. In Nature it is catalyzed by the enzyme cytochrome P450, in a still-debated mechanism that poses a major intellectual challenge for both experiment and theory; currently, the opinions keep swaying between the original single-state rebound mechanism, a two-oxidant mechanism (where ferric peroxide participates as a second oxidant, in addition to the primary active species, the high-valent iron-oxo species), and two-state reactivity (TSR) mechanism (where two spin states are involved). Recent product isotope effect (PIE) measurements for the trans-2-phenyl-methyl cyclopropane probe (1), led Newcomb and co-workers (Newcomb, M.; Aebisher, D.; Shen, R.; Esala, R.; Chandrasena, P.; Hollenberg, P.; Coon, M. J. J. Am. Chem. Soc. 2003, 125, 6064-6065) to rule out TSR in favor of the two-oxidant scenario, since the direction of the PIE was at odds with the one predicted from calculations on methane hydroxylation. The present report describes a density functional theoretical study of C-H hydroxylation of the Newcomb probe, 1, leading to rearranged (3) and unrearranged (2) products. Our study shows that the reaction occurs via TSR in which the high-spin pathway gives dominant rearranged products, whereas the low-spin pathway favors unrearranged products. The calculated PIE(2/3) values based on TSR are found to be in excellent agreement with the experimental data of Newcomb and co-workers. This match between experiment and theory makes a strong case that the reaction occurs via TSR mechanism.     

Uranyl coordination environment in hydrophobic ionic liquids: an in situ investigation

Different inner-sphere coordination environments are observed for the uranyl nitrate complexes formed with octyl-phenyl-N,N-diisobutylcarbamoylmethylphosphine oxide and tributyl phosphate in dodecane and in the hydrophobic ionic liquids (ILs) [C(4)mim][PF(6)] and [C(8)mim][N(SO(2)CF(3))(2)]. Qualitative differences in the coordination environment of the extracted uranyl species are implied by changes in peak intensity patterns and locations for uranyl UV-visible spectral bands when the solvent is changed. EXAFS data for uranyl complexes in dodecane solutions is consistent with hexagonal bipyramidal coordination and the existence of UO(2)(NO(3))(2)(CMPO)(2). In contrast, the complexes formed when uranyl is transferred from aqueous nitric acid solutions into the ILs exhibit an average equatorial coordination number of approximately 4.5. Liquid/liquid extraction results for uranyl in both ILs indicate a net stoichiometry of UO(2)(NO(3))(CMPO)(+). The concentration of the IL cation in the aqueous phase increases in proportion to the amount of UO(2)(NO(3))(CMPO)(+) in the IL phase, supporting a predominantly cation exchange mechanism for partitioning in the IL systems.     

Time-resolved fluorescence analysis of the mobile flavin cofactor in p -hydroxybenzoate hydroxylase

Conformational heterogeneity of the FAD cofactor in p-hydroxybenzoate hydroxylase (PHBH) was investigated with time-resolved polarized flavin fluorescence. For binary enzyme/substrate (analogue) complexes of wild-type PHBH and Tyr222 mutants, crystallographic studies have revealed two distinct flavin conformations; the ‘in’ conformation with the isoalloxazine ring located in the active site, and the ‘out’ conformation with the isoalloxazine ring disposed towards the protein surface. Fluorescence-lifetime analysis of these complexes revealed similar lifetime distributions for the ‘in’ and ‘out’ conformations. The reason for this is twofold. First, the active site of PHBH contains various potential fluorescence-quenching sites close to the flavin. Fluorescence analysis of uncomplexed PHBH Y222V and Y222A showed that Tyr222 is responsible for picosecond fluorescence quenching free enzyme. In addition, other potential quenching sites, including a tryptophan and two tyrosines involved in substrate binding, are located nearby. Since the shortest distance between these quenching sites and the isoalloxazine ring differs only little on average, these aromatic residues are likely to contribute to fluorescence quenching. Second, the effect of flavin conformation on the fluorescence lifetime distribution is blurred by binding of the aromatic substrates: saturation with aromatic substrates induces highly efficient fluorescence quenching. The flavin conformation is therefore only reflected in the small relative contributions of the longer lifetimes.     

Water Relaxation Measurements on Semiquinones of Various Flavoproteins

The water relaxation rates of several flavoproteins in the semiquinone state have been investigated by the spin echo technique. The results indicate a rather unspecific interaction between water and the protein-bound flavosemiquinones. An average interaction distance of 0.3-0.5 nm has been estimated. From the temperature dependence of the rate constants the free energy of activation for proton exchange is calculated to be about 17 kJ/mol. The rate of proton exchange is around 10¹¹ s?¹ for the flavosemiquinones investigated are accessible to water regardless of their ionic state. The large difference in relaxation rates of water protons between D - and L - amino-acid oxidases is noticeable. Oxynitrilase exhibits the highest whereas Azotobacter vinelandii flavodoxin shows the lowest water relaxation rate of the flavoproteins studied. The results are discussed in relation to the visible-light absorption properties of the flavoproteins.