Cooperative energy transfer

The energy transfer in a system of two-level molecules is analyzed. It is shown that this system leads to supertransfer of energy in analogy with Dicke theory of superradiance.     

Formation of n-1 substituted alloxazine derivative from 7,8-dimethyl-10-formylmethylisoalloxazine in cetic acid.

The new alloxazine derivative formed from 7,8-dimethyl-10-formylmethyl-isoalloxazine in acetic acid has been isolated; structure and mechanism of formation have been proposed.     

Investigations of Sm6(Mn1-xFex)23 system

The new Sm₆(Mn_1-xFe_x)₂₃(0?x?1.0) system hasbeen synthesized and investigated in a wide temperature range by the X-ray, magnetometric and Mössbauer effect methods. The X-ray studies show that the system forms solid solutions which are isostructural with the Th₆Mn₂₃ type crystal structure throughout the entire compositional range. Both Fe-rich and Mn-rich regions of the system are magnetically ordered and are separated from each other by the non-magnetically ordered 0.22?x?0.33 region. The substitution of Fe atoms for Mn atoms in the Mn-rivh region and similarly of Mn atoms for Fe atoms in the Fe-rich region decreases both the Curie temperature and the value of the magnetic moment per molecule. The temperature dependence of the reciprocal susceptibility obeys the Néel law. The Mössbauer absorption spectra reflect wide distributions of the ⁵⁷Fe hyperfine interaction parameters, and disappearance of long range magnetic coupling of Fe atoms in the magnetically ordered x=0 to 0.22 composition range.     

Magnetic phase diagram of MnAs: Effect of magnetic field on structural and magnetic transitions

The boundary, in the T?H₀ plane, separating the MnP-type and NiAs-type crystallographic phases of MnAs was determined from magnetization and volume measurements. Transitions on one part of the boundary are of first order, but are of second order on the remaining part. The two parts are separated by a tricitical point.     

Ordering of nonnegative definite matrices with application to comparison of linear models

For any real nonnegative definite matrices M₁ and M₂ several necessary and sufficient conditions for M₁ ? M₂ are presented. The conditions are used for the comparison of linear models.     

Dependence of work function on coverage for aluminum/tungsten in the field electron microscope

Aluminum adsorption on tungsten has been investigated by field electron microscope techniques. Changes in average work function and work functions of the (001) plane and (111) region were determined as a function of aluminum deposition. The average work function exhibits a minimum of about (4.11 × 0.04)eV (relative to an assumed value of 4.50 eV for the tungsten substrate) and reaches a value of (4.21 ± 0.04) eV for thick deposits. The (001) plane and (111) region exhibit complicated work function dependences on aluminum coverage. Some qualitative observations of aluminum on tungsten surface diffusion are also reported.     

Rayleigh light scattering in molecular crystals of succinonitrile and trimethylacetonitrile

Two representatives of molecular crystals exhibiting plastic phase : succinonitrile NCCH₂CH₂CN and trimethylacetonitrile |CH₃|₃CCN were studied. For these crystals the horizontally and vertically polarized components of the scattered radiation were measured in the temperature range from the plastic phase to liquid, and the depolarization degrees were determined. The isotropic component V_V was also measured as a function of temperature changing from liquid to plastic phase. In both phases the squares of effective anisotropy of optical polarizability were found from the measurements of the anisotropic component H_V.     

Electron transfer and catalytic control by the iron-sulfur clusters in a respiratory enzyme, E. coli fumarate reductase

Factors governing the efficacy of long-range electron relays in enzymes have been examined using protein film voltammetry in conjunction with site-directed mutagenesis. Investigations of the fumarate reductase from Escherichia coli, in which three Fe-S clusters relay electrons over more than 30 A, lead to the conclusion that varying the medial [4Fe-4S] cluster potential over a 100 mV range does not have a significant effect on the inherent kinetics of electron transfer to and from the active-site flavin. The results support a proposal that the reduction potential of an individual electron relay site in a multicentered enzyme is not a strong determinant of activity; instead, as deduced from the potential dependence of catalytic electron transfer, electron flow through the intramolecular relay is rapid and reversible, and even uphill steps do not limit the catalytic rate.     

Enzymatic removal of carboxyl protecting groups. 1. Cleavage of the tert-butyl moiety

[reaction: see text] A recent discovery that a certain amino acid motif (GGG(A)X-motif) in lipases and esterases determines their activity toward tertiary alcohols prompted us to investigate the use of these biocatalysts in the mild and selective removal of tert-butyl protecting groups in amino acid derivatives and related compounds. An esterase from Bacillus subtilis (BsubpNBE) and lipase A from Candida antarctica (CAL-A) were identified as the most active enzymes, which hydrolyzed a range of tert-butyl esters of protected amino acids (e.g., Boc-Tyr-O(t)Bu, Z-GABA-O(t)Bu, Fmoc-GABA-O(t)Bu) in good to high yields and left Boc, Z, and Fmoc-protecting groups intact.     

Investigations of vanadium oxide bronzes θ-(Fe1−yAly)xV2O5

The vanadium oxide bronzes θ-(Fe_1?yAl_y)_xV₂O₅ are Curie-Weiss paramagnets and hopping semiconductors. The samples studied were synthesized by direct solid-state reaction and investigated by the X-ray diffraction, differential thermal analysis, electrical resistivity, magnetic susceptibility, and Mössbauer techniques. The crystal lattice parameters, effective magnetic moments of Fe³⁺ cations, Curie-Weiss temperatures, and the values of ⁵⁷Fe hyperfine interaction parameters were determined. Endothermic effects were observed for some of the samples.