The dynamical balance, transport and circulation of the Antarctic Circumpolar Current

The physical ingredients of the ACC circulation are reviewed. A picture of thecirculation is sketched by means of recent observations of the WOCE decade. Wepresent and discuss the role of forcing functions (wind stress, surfacebuoyancy flux) in the balance of the (quasi)-zonal flow, the meridionalcirculation and their relation to the ACC transport. Emphasis will be on theinterrelation of the zonal momentum balance and the meridional circulation, theimportance of diapycnal mixing and eddy processes. Finally, new model conceptsare described: a model of the ACC transport dependence on wind stress andbuoyancy flux, based on linear wave theory; and a model of the meridionaloverturning of the Southern Ocean, based on zonally averaged dynamics with eddyparameterization

Computer simulation of protein systems

Ligand binding to dihydrofolate reductase (DHFR) is discussed. This is an extremely important enzyme, as it is the target of several drugs (inhibitors) which are used clinically as antibacterials, antiprotozoals and in cancer chemotherapy. DHFR catalyzes the NADPH (reduced nicotinamide adenine dinucleotide phosphate) dependent reduction of dihydrofolate to tetrahydrofolate, which is used in several pathways of purine and pyrimidine iosynthesis, including that of thymidylate. Since DNA synthesis is dependent on a continuing supply of thymidylate, a blockade of DHFR resulting in a depletion of thymidylate can lead to the cessation of growth of a rapidly proliferating cell line. DHFR exhibits a significant species to species variability in its sensitivity to various inhibitors. For example, trimethoprim, an inhibitor of DHFR, binds to bacterial DHFR's 5 orders of magnitude greater than to vertebrate DHFR's. The structural mechanics, dynamics and energetics of a family of dihydrofolate reductases are studied to rationalize the basis for the inhibitor of these enyzmes and to understand the molecular basis of the difference in the binding constants between the species. This involves investigating the conformational changes induced in the protein on binding the ligand, the internal strain imposed by the enzyme on the ligand, the restriction of fluctuations in atom positions due to binding and the consequent change in entropy

Energy Distribution in disordered elastic Networks

Disordered networks are found in many natural and artificial materials, from gels or cytoskeletal structures to metallic foams or bones. Here, the energy distribution in this type of networks is modeled, taking into account the orientation of the struts. A correlation between the orientation and the energy per unit volume is found and described as a function of the connectivity in the network and the relative bending stiffness of the struts. If one or both parameters have relatively large values, the struts aligned in the loading direction present the highest values of energy. On the contrary, if these have relatively small values, the highest values of energy can be reached in the struts oriented transversally. This result allows explaining in a simple way remodeling processes in biological materials, for example, the remodeling of trabecular bone and the reorganization in the cytoskeleton. Additionally, the correlation between the orientation, the affinity, and the bending-stretching ratio in the network is discussed

On the Four-Dimensional Diluted Ising Model

In this letter we show strong numerical evidence that the four dimensional Diluted Ising Model for a large dilution is not described by the Mean Field exponents. These results suggest the existence of a new fixed point with non-gaussian exponents.Comment: 9 pages. compressed ps-file (uufiles

Tubulin subunit carboxyl termini determine polymerization efficiency

Cleavage of tubulin by subtilisin removes a small (Mr < 2000) fragment from the C-terminal end of both α and β subunits. The resulting protein is much reduced in negative charge. The cleaved, less acidic protein retains its competence to polymerize in a GTP-dependent and cold-, GDP-, and podophyllotoxin-sensitive manner and assembles into sheets or bundles of twisted filaments. The critical concentration for polymerization of the cleaved protein is about 50-fold lower than that for intact tubulin. It is proposed that the C termini of the subunits normally impede polymerization

The adsorption and desorption of ethanol ices from a model grain surface

Reflection absorption infrared spectroscopy (RAIRS) and temperature programed desorption (TPD) have been used to probe the adsorption and desorption of ethanol on highly ordered pyrolytic graphite (HOPG) at 98 K. RAIR spectra for ethanol show that it forms physisorbed multilayers on the surface at 98 K. Annealing multilayer ethanol ices (exposures > 50 L) beyond 120 K gives rise to a change in morphology before crystallization within the ice occurs. TPD shows that ethanol adsorbs and desorbs molecularly on the HOPG surface and shows four different species in desorption. At low coverage, desorption of monolayer ethanol is observed and is described by first-order kinetics. With increasing coverage, a second TPD peak is observed at a lower temperature, which is assigned to an ethanol bilayer. When the coverage is further increased, a second multilayer, less strongly bound to the underlying ethanol ice film, is observed. This peak dominates the TPD spectra with increasing coverage and is characterized by fractional-order kinetics and a desorption energy of 56.3 +/- 1.7 kJ mol(-1). At exposures exceeding 50 L, formation of crystalline ethanol is also observed as a high temperature shoulder on the TPD spectrum at 160 K. (c) 2008 American Institute of Physics

Three-dimensional Ising model in the fixed-magnetization ensemble: a Monte Carlo study

We study the three-dimensional Ising model at the critical point in the fixed-magnetization ensemble, by means of the recently developed geometric cluster Monte Carlo algorithm. We define a magnetic-field-like quantity in terms of microscopic spin-up and spin-down probabilities in a given configuration of neighbors. In the thermodynamic limit, the relation between this field and the magnetization reduces to the canonical relation M(h). However, for finite systems, the relation is different. We establish a close connection between this relation and the probability distribution of the magnetization of a finite-size system in the canonical ensemble.Comment: 8 pages, 2 Postscript figures, uses RevTe