Force-induced unfolding of a homopolymer on fractal lattice: exact results vs. mean field predictions

We study the force-induced unfolding of a homopolymer on the three dimensional Sierpinski gasket. The polymer is subject to a contact energy between nearest neighbour sites not consecutive along the chain and to a stretching force. The hierarchical nature of the lattice we consider allows for an exact treatment which yields the phase diagram and the critical behaviour. We show that for this model mean field predictions are not correct, in particular in the exact phase diagram there is {\em not} a low temperature reentrance and we find that the force induced unfolding transition below the theta temperature is second order.Comment: 15 pages, 5 eps figure

Transferable Pair Potentials for CdS and ZnS Crystals

A set of interatomic pair potentials is developed for CdS and ZnS crystals. We show that a simple energy function, which has been used to describe the properties of CdSe [J. Chem. Phys. 116, 258 (2002)], can be parametrized to accurately describe the lattice and elastic constants, and phonon dispersion relations of bulk CdS and ZnS in the wurtzite and rocksalt crystal structures. The predicted coexistence pressure of the wurtzite and rocksalt structures, as well as the equation of state are in good agreement with experimental observations. These new pair potentials enable the study of a wide range of processes in bulk and nanocrystalline II-VI semiconductor materials

Two types of bone necrosis in the middle triassic pistosaurus longaevus bones: The results of integrated studies

Avascular necrosis, diagnosed on the basis of either a specific pathological modification of the articular surfaces of bone or its radiologic appearance in vertebral centra, has been recognized in many Mesozoic marine reptiles as well as in present-day marine mammals. Its presence in the zoological and paleontologic record is usually associated with decompression syndrome, a disease that affects secondarily aquatic vertebrates that could dive. Bone necrosis can also be caused by infectious processes, but it differs in appearance from decompression syndrome-associated aseptic necrosis. Herein, we report evidence of septic necrosis in the proximal articular surface of the femur of a marine reptile, Pistosaurus longaevus, from the Middle Triassic of Poland and Germany. This is the oldest recognition of septic necrosis associated with septic arthritis in the fossil record so far, and the mineralogical composition of pathologically altered bone is described herein in detail. The occurrence of septic necrosis is contrasted with decompression syndrome-associated avascular necrosis, also described in Pistosaurus longaevus bone from Middle Triassic of Germany

Galileo dust data from the jovian system: 2000 to 2003

The Galileo spacecraft was orbiting Jupiter between Dec 1995 and Sep 2003. The Galileo dust detector monitored the jovian dust environment between about 2 and 370 R_J (jovian radius R_J = 71492 km). We present data from the Galileo dust instrument for the period January 2000 to September 2003. We report on the data of 5389 particles measured between 2000 and the end of the mission in 2003. The majority of the 21250 particles for which the full set of measured impact parameters (impact time, impact direction, charge rise times, charge amplitudes, etc.) was transmitted to Earth were tiny grains (about 10 nm in radius), most of them originating from Jupiter's innermost Galilean moon Io. Their impact rates frequently exceeded 10 min^-1. Surprisingly large impact rates up to 100 min^-1 occurred in Aug/Sep 2000 when Galileo was at about 280 R_J from Jupiter. This peak in dust emission appears to coincide with strong changes in the release of neutral gas from the Io torus. Strong variability in the Io dust flux was measured on timescales of days to weeks, indicating large variations in the dust release from Io or the Io torus or both on such short timescales. Galileo has detected a large number of bigger micron-sized particles mostly in the region between the Galilean moons. A surprisingly large number of such bigger grains was measured in March 2003 within a 4-day interval when Galileo was outside Jupiter's magnetosphere at approximately 350 R_J jovicentric distance. Two passages of Jupiter's gossamer rings in 2002 and 2003 provided the first actual comparison of in-situ dust data from a planetary ring with the results inferred from inverting optical images.Comment: 59 pages, 13 figures, 6 tables, submitted to Planetary and Space Scienc

Reversible stretching of homopolymers and random heteropolymers

We have analyzed the equilibrium response of chain molecules to stretching. For a homogeneous sequence of monomers, the induced transition from compact globule to extended coil below the $\theta$-temperature is predicted to be sharp. For random sequences, however, the transition may be smoothed by a prevalence of necklace-like structures, in which globular regions and coil regions coexist in a single chain. As we show in the context of a random copolymer, preferential solvation of one monomer type lends stability to such structures. The range of stretching forces over which necklaces are stable is sensitive to chain length as well as sequence statistics.Comment: 14 pages, 4 figure

Action-derived molecular dynamics in the study of rare events

We present a practical method to generate classical trajectories with fixed initial and final boundary conditions. Our method is based on the minimization of a suitably defined discretized action. The method finds its most natural application in the study of rare events. Its capabilities are illustrated by non-trivial examples. The algorithm lends itself to straightforward parallelization, and when combined with molecular dynamics (MD) it promises to offer a powerful tool for the study of chemical reactions.Comment: 7 Pages, 4 Figures (3 in color), submitted to Phys. Rev. Let

Mechanical response of random heteropolymers

We present an analytical theory for heteropolymer deformation, as exemplified experimentally by stretching of single protein molecules. Using a mean-field replica theory, we determine phase diagrams for stress-induced unfolding of typical random sequences. This transition is sharp in the limit of infinitely long chain molecules. But for chain lengths relevant to biological macromolecules, partially unfolded conformations prevail over an intermediate range of stress. These necklace-like structures, comprised of alternating compact and extended subunits, are stabilized by quenched variations in the composition of finite chain segments. The most stable arrangements of these subunits are largely determined by preferential extension of segments rich in solvophilic monomers. This predicted significance of necklace structures explains recent observations in protein stretching experiments. We examine the statistical features of select sequences that give rise to mechanical strength and may thus have guided the evolution of proteins that carry out mechanical functions in living cells.Comment: 10 pages, 6 figure

Looking into the future, what do we see?

This series of short communications introduces and celebrates the occasion of World Nutrition Rio2012, and looks ahead. There will be another series published next month in our May issue, which will be on-line early, at 0001 GMT on Thursday 26 April, the day before Rio2012 begins. The questions and answers: Our contributors, this month and next month, have been asked to write within a standard framework, based on their knowledge and experience, in a style comparable with those of the news and comment sections of journals concerned with public health. Their contributions are judgements, as any policy statements are and must be. The first question is about Rio2012 itself. The second question is addressed to young colleagues. All the questions are designed to look forward and to advise, warn and encourage. What do I hope Rio2012 will achieve? My advice to a young public health nutritionist When I am optimistic, what is my vision? When I am pessimistic, what do I foresee? My most highly recommended writer One choice of my own publication

Prediction of huge X-ray Faraday rotation at the Gd N_4,5 threshold

X-ray absorption spectra in a wide energy range around the 4d-4f excitation threshold of Gd were recorded by total electron yield from in-plane magnetized Gd metal films. Matching the experimental spectra to tabulated absorption data reveals unprecedented short light absorption lengths down to 3 nm. The associated real parts of the refractive index for circularly polarized light propagating parallel or antiparallel to the Gd magnetization, determined through the Kramers-Kronig transformation, correspond to a magneto-optical Faraday rotation of 0.7 degrees per atomic layer. This finding shall allow the study of magnetic structure and magnetization dynamics of lanthanide elements in nanosize systems and dilute alloys.Comment: 4 pages, 2 figures, final version resubmitted to Phys. Rev. B, Brief Reports. Minor change