774 research outputs found
Application of Chiral Lanthanide-induced Shift Reagents to Optically Active Cations: the Use of Tris[3-(trifluoromethylhydroxymethylene)-( + )-camphorato]europium(III) to Determine the Enantiomeric Purity of Tris(phenanthroline)ruthenium(II) Dichloride
In non-polar solvents, chiral europium complexes provide attractive n. m. r. shift reagents to resolve spectra of optically active cations, and, in particular, for tris(phenanthroline)ruthenium dichloride,^1H n. m. r. shift differences of up to 0.7 p.p.m. between isomers easily permit the determination of absolute enantiomeric purity
Hopping and clustering of oxygen vacancies in SrTiO3 by anelastic relaxation
The complex elastic compliance s11(w,T) of SrTiO3-d has been measured as a
function of the O deficiency d < 0.01. The two main relaxation peaks in the
absorption are identified with hopping of isolated O vacancies over a barrier
of 0.60 eV and reorientation of pairs of vacancies involving a barrier of 1 eV.
The pair binding energy is ~0.2 eV and indications for additional clustering,
possibly into chains, is found already at d ~0.004. The anistropic component of
the elastic dipole of an O vacancy is Deltalambda = 0.026.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Non-linear Elastic Response in Solid Helium: critical velocity or strain
Torsional oscillator experiments show evidence of mass decoupling in solid
4He. This decoupling is amplitude dependent, suggesting a critical velocity for
supersolidity. We observe similar behavior in the elastic shear modulus. By
measuring the shear modulus over a wide frequency range, we can distinguish
between an amplitude dependence which depends on velocity and one which depends
on some other parameter like displacement. In contrast to the torsional
oscillator behavior, the modulus depends on the magnitude of stress, not
velocity. We interpret our results in terms of the motion of dislocations which
are weakly pinned by 3He impurities but which break away when large stresses
are applied
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Amyloid β-sheet mimics that antagonize protein aggregation and reduce amyloid toxicity.
The amyloid protein aggregation associated with diseases such as Alzheimer's, Parkinson's and type II diabetes (among many others) features a bewildering variety of β-sheet-rich structures in transition from native proteins to ordered oligomers and fibres. The variation in the amino-acid sequences of the β-structures presents a challenge to developing a model system of β-sheets for the study of various amyloid aggregates. Here, we introduce a family of robust β-sheet macrocycles that can serve as a platform to display a variety of heptapeptide sequences from different amyloid proteins. We have tailored these amyloid β-sheet mimics (ABSMs) to antagonize the aggregation of various amyloid proteins, thereby reducing the toxicity of amyloid aggregates. We describe the structures and inhibitory properties of ABSMs containing amyloidogenic peptides from the amyloid-β peptide associated with Alzheimer's disease, β(2)-microglobulin associated with dialysis-related amyloidosis, α-synuclein associated with Parkinson's disease, islet amyloid polypeptide associated with type II diabetes, human and yeast prion proteins, and Tau, which forms neurofibrillary tangles
Quantitative treatment of the creep of metals by dislocation and rate-process theories
An equation for the steady-state rate of creep has been derived by applying the theory of dislocations to the creep of pure metals. The form of this equation is in agreement with empirical equations describing creep rates. The theory was also used to predict the dependence of steady-state rate of creep on physical constants of the material and good agreement was obtained with data in the literature for pure annealed metals. The rate of creep was found to decrease with increasing modulus of rigidity. This relation suggest that one of the requirements for a heat-resisting alloy is that its matrix be a metal that has a high modulus of rigidity and therefore a high modulus of elasticity
Correlation between stick-slip frictional sliding and charge transfer
A decade ago, Budakian and Putterman (Phys. Rev. Lett., {\bf 85}, 1000
(2000)) ascribed friction to the formation of bonds arising from contact
charging when a gold tip of a surface force apparatus was dragged on
polymethylmethacrylate surface. We propose a stick-slip model that captures the
observed correlation between stick-slip events and charge transfer, and the
lack of dependence of the scale factor connecting the force jumps and charge
transfer on normal load. Here, stick-slip dynamics arises as a competition
between the visco-elastic and plastic deformation time scales and that due to
the pull speed with contact charging playing a minor role. Our model provides
an alternate basis for explaining most experimental results without ascribing
friction to contact charging.Comment: 8 pages, 4 figures, To be appeared in Physical Review
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