Resistivity of positive-J spin glasses: Study ofLaGd under hydrostatic pressure

Results of a new calculation of the resistivity in the noise model of spin glasses, based on the –J S interaction forpositive J, are compared with measurements of the resistivity in an fcc La-8at% Gd alloy under hydrostatic pressures up to 12 kbar, the first positive-J resistivity investigations under pressure. A positive value ofJ that decreases with increasing pressure, in accord with other experiments, leads to quantitative agreement between theory and experiment if one uses a large electronic density of states at the Fermi energy, in accord with specific heat measurements and electronic band structure calculations in fcc La. Impurity potential scattering is found to contribute significantly to the resistivity. Its size and pressure variation is determined.This work is supported in part by the Deutsche Forschungs-gemeinschaft.     

On non-analytic thermodynamic behaviour of the classical linear chain

The free energy ? per particle and the two-particle distribution function g have been expressed as functionals of the nearest-neighbour interaction Φ(x) for a variety of boundary conditions. For some boundary conditions and potentials Φ(x), ? and g behave singularly at a critical density

$\text{ρ}{}_{\mathrm{<mtext>c</mtext>}}\text{=}\text{lim}\text{s→0}{}^{\mathrm{+}}\text{∫}\text{0}\text{∞}\text{e}{}^{\mathrm{?sx?\beta \Phi (x)}}\text{d}\text{x?}\text{∫}\text{0}\text{∞}\text{x}\text{e}{}^{\mathrm{?sx?\beta \Phi (x)}}\text{d}\text{x.}$

     

9-Anthraldehyde acetals as protecting groups

Anthraldehyde acetals can be introduced regioselectively to carbohydrates in high yields. Advantages over conventional acetal protecting groups are increased crystallinity and strong absorbance and fluorescence which facilitate purification and reaction monitoring. The anthraldehyde acetals can be deprotected selectively in the presence of benzylidene acetals and can be cleaved regioselectively to yield 6-O-(9-anthracenyl)methyl ethers.     

SEDIMENTATION ANALYSIS OF DNA PHOTOOXIDIZED IN THE PRESENCE OF THIOPYRONINE

Abstract. Illumination of single-stranded φ×174 phage DNA with visible lightλ > 500 nm) in the presence of the sensitizer thiopyronine results in both chain scissions detectable by velocity sedimentation in neutral medium and in alkali-labile bonds which yield secondary strand breaks after alkaline treatment. Compared with the generation of primary strand breaks, the formation of alkali-labile sites seems to be the predominant reaction.
Photodynamic treatment of homogeneous double-stranded Ta, phage DNA leads to changes in the overall conformation of DNA as revealed by an initial increase of the sedimentation coefficient. The simultaneous occurrence of different effects (decrease of molecular weight, increase of effective DNA flexibility) is particularly evident from changes in the sedimentation coefficient distribution. The fact that both processes influence the sedimentation behaviour questions the common procedure of calculating double-strand break numbers from sedimentation coefficient distributions on the basis of s#-M relations which are valid for intact DNA only.
Photooxidized double-stranded DNA exhibits an increased sensitivity against shear forces.     

Low-temperature vibrational spectra of cis-dichloro-(meso-2,3-diaminobutane)palladium(II) and platinum(II)

IR and Raman spectra of MCl₂(meso-2,3-diaminobutane), (M = Pd, Pt), have been recorded down to liquid nitrogen temperature or lower. It is shown that correlation coupling occurs between closely spaced hydrogen-bonded pairs of molecules, which form a “super molecule,” but not between all eight molecules in the unit cell. The lattice mode region is also understood in outline on the basis of motions of the “super molecules” Methyl torsional modes appear to be near 140 cm^?1.     

Locoselective [4 + 2] Cycloadditions of Vinylindoles with Inverse Electron Demand: A new access of indolyl-substituted and annellated pyridazines

2-Vinylindole ( 1a ) and its donor- and acceptor-substituted (E)-derivatives 1b – e react highly locoselectively with dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate ( 3 ) to form the novel (indol-2-yl)-1,4-dihydropyridazines 4a and 7 as well as the heterocyclic annellated pyridazines 4b , 5 , and 6 . The reactions of the structurally related 3-vinylindoles 2a – e with 3 also gave rise to new indol-3-ylpyridazines 8 , 9 , and 10 . The locoselectivities of these Diels-Alder reactions were controlled mainly by steric effects.     

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.     

Plutonium(IV) mononitrate and dinitrate complex formation in acid solutions as a function of ionic strength

Titrations of Pu(IV) with HNO₃ in a series of aqueous HClO₄ solutions ranging in ionic strength from 2 to 19 molal were followed using visible and near-infrared absorption spectrophotometry. The Pu 5f-5f spectra in the visible and near IR range change with complex formation. At each ionic strength, a series of spectra were obtained by varying nitrate concentration. Each series was deconvoluted into spectra of Pu⁴⁺ (aq), Pu(NO₃)³⁻ and Pu(NO₃)₂ ²⁺ complexes, and simultaneously their formation constants were determined. When corrected for the incomplete dissociation of nitric acid, the ionic strength dependence of each formation constant can be described by two parameters, β⁰ and Δε using the formulae of specific ion interaction theory.     

The association energy of aggregates of divalent impurities and vacancies in NaCl-type crystals (point charge approximation)

The formation and association energies for the different aggregates of divalent metal impurities and cation vacancies are derived by considerations of the sums over all lattice vectors. This gives a simple formula in the point charge approximation if an infinite lattice is assumed. Because of the neglection of the polarization of the ions this method yields a higher value for these energies, as a comparison with the literature for the case of dipoles shows. But nevertheless it is a first possibility for a correct estimation of these energies also for higher aggregates.