468 research outputs found
Continuous RSB mean-field solution of the Potts glass
We investigate the p-state mean-field model of the
Potts glass () below the continuous phase transition to a
glassy phase. We find that apart from a solution with a first hierarchical
level of replica-symmetry breaking (1RSB), locally stable close to the
transition point, there is a continuous full replica-symmetry breaking (FRSB)
solution. The latter is marginally stable and has a higher free energy than the
former. We argue that the true equilibrium is reached only by FRSB, being
globally thermodynamically homogeneous, whereas 1RSB is only locally
homogeneous.Comment: REVTeX4.1, 4 pages, 1 figur
Evidence against a glass transition in the 10-state short range Potts glass
We present the results of Monte Carlo simulations of two different 10-state
Potts glasses with random nearest neighbor interactions on a simple cubic
lattice. In the first model the interactions come from a \pm J distribution and
in the second model from a Gaussian one, and in both cases the first two
moments of the distribution are chosen to be equal to J_0=-1 and Delta J=1. At
low temperatures the spin autocorrelation function for the \pm J model relaxes
in several steps whereas the one for the Gaussian model shows only one. In both
systems the relaxation time increases like an Arrhenius law. Unlike the
infinite range model, there are only very weak finite size effects and there is
no evidence that a dynamical or a static transition exists at a finite
temperature.Comment: 9 pages of Latex, 4 figure
Finite-size scaling at the dynamical transition of the mean-field 10-state Potts glass
We use Monte Carlo simulations to study the static and dynamical properties
of a Potts glass with infinite range Gaussian distributed exchange interactions
for a broad range of temperature and system size up to N=2560 spins. The
results are compatible with a critical divergence of the relaxation time tau at
the theoretically predicted dynamical transition temperature T_D, tau \propto
(T-T_D)^{-\Delta} with Delta \approx 2. For finite N a further power law at
T=T_D is found, tau(T=T_D) \propto N^{z^\star} with z^\star \approx 1.5 and for
T>T_D dynamical finite-size scaling seems to hold. The order parameter
distribution P(q) is qualitatively compatible with the scenario of a first
order glass transition as predicted from one-step replica symmetry breaking
schemes.Comment: 8 pages of Latex, 4 figure
Synthesis of Potential Anti-Cancer Agents. XVII. Urea Nitrogen Mustards
The synthesis of nitrogen mustard ur eas from various isocyamates
and the free base of NN-bis-(2-chloroe thyla mine) is
described. Prelimina ry pharmacological tests show that so m e of
these compounds have a cytostatic activity
The Glassy Potts Model
We introduce a Potts model with quenched, frustrated disorder, that enjoys of
a gauge symmetry that forbids spontaneous magnetization, and allows the glassy
phase to extend from down to T=0. We study numerical the 4 dimensional
model with states. We show the existence of a glassy phase, and we
characterize it by studying the probability distributions of an order
parameter, the binder cumulant and the divergence of the overlap
susceptibility. We show that the dynamical behavior of the system is
characterized by aging.Comment: 4 pages including 4 (color) ps figures (all on page 4
Synthesis of Potential Anti-Cancer Agents. XVI. Nitrogen Mustards from 1-Aminophenazine and 8-Aminoquinoline
The synthesis of nitrogen mustard amides from 1-aminophenazine
and 8-aminoquinoline is described
Some non perturbative calculations on spin glasses
Models of spin glasses are studied with a phase transition discontinuous in
the Parisi order parameter. It is assumed that the leading order corrections to
the thermodynamic limit of the high temperature free energy are due to the
existence of a metastable saddle point in the replica formalism. An ansatz is
made on the form of the metastable point and its contribution to the free
energy is calculated. The Random Energy Model is considered along with the
p-spin and the p-state Potts Models in their p < infinity expansion.Comment: 12 pages, LaTe
Proteolytic processing of imported chloroplast proteins
Three proteins located in the thylakoid lumen, plastocyanin and the 23KDa and 33KDa oxygen evolving polypeptides of photosystem II, are synthesised in the cytoplasm as higher molecular weight precursors, with N-terminal transit peptides. Import of these proteins involves removal of the first part of the transit peptide by a stromal processing peptidase to yield an intermediate. Maturation, by removal of the remaining transit peptide is performed by a thylakoidal processing peptidase (TPP). TPP has been partially purified and characterised from pea thylakoids and found to be an integral thylakoid membrane protein with the active site on the lumenal (trans) side of the membrane. TPP has a molecular weight of less than 250 000 and is not associated with any supra-molecular complex. Partial purification has yielded ten bands on a Coomassie stained SDS-PAGE gel; however TPP has not been attributed to any of these bands.
TPP displays specificity for chloroplast protein precursors with transit peptides containing a thylakoid transfer domain; however, no species specificity is displayed. TPP exhibits similarities in reaction specificity to Escherichia coli leader peptidase (LEP) in that both peptidases cleave the same eukaryotic and bacterial precursor as well as cleaving higher plant lumenal precursors at the predicted cleavage site.
No standard protease inhibitor has been found to abolish TPP activity; however, a synthetic signal sequence polypeptide will inhibit TPP and LEP.
A thylakoidal endopeptidase (EPS) has been discovered which cleaves lumenal precursors to a size slightly larger than the mature size. EP5 displays different inhibitor sensitivities to TPP and is either a high molecular weight protein or associated with a supramolecular complex. EPS is assumed to be involved in the turnover of thylakoid proteins
Critical Behavior of Three-Dimensional Disordered Potts Models with Many States
We study the 3D Disordered Potts Model with p=5 and p=6. Our numerical
simulations (that severely slow down for increasing p) detect a very clear spin
glass phase transition. We evaluate the critical exponents and the critical
value of the temperature, and we use known results at lower values to
discuss how they evolve for increasing p. We do not find any sign of the
presence of a transition to a ferromagnetic regime.Comment: 9 pages and 9 Postscript figures. Final version published in J. Stat.
Mec
Evidence for elevated alkalinity in the glacial Southern Ocean
An increase in whole ocean alkalinity during glacial periods could account, in part, for the drawdown of atmospheric CO2 into the ocean. Such an increase was inevitable due to the near elimination of shelf area for the burial of coral reef alkalinity. We present evidence, based on downcore measurements of benthic foraminiferal B/Ca and Mg/Ca from a core in the Weddell Sea, that the deep ocean carbonate ion concentration, [CO32-], was elevated by similar to 25 mu mol/kg during each glacial period of the last 800 kyr. The heterogeneity of the preservation histories in the different ocean basins reflects control of the carbonate chemistry of the deep glacial ocean in the Atlantic and Pacific by the changing ventilation and chemistry of Weddell Sea waters. These waters are more corrosive than interglacial northern sourced waters but not as undersaturated as interglacial southern sourced waters. Our inferred increase in whole ocean alkalinity can be reconciled with reconstructions of glacial saturation horizon depth and the carbonate budget if carbonate burial rates also increased above the saturation horizon as a result of enhanced pelagic calcification. The Weddell records display low [CO32-] during deglaciations and peak interglacial warmth, coincident with maxima in percent CaCO3 in the Atlantic and Pacific oceans. Should the burial rate of alkalinity in the more alkaline glacial deep waters outstrip the rate of alkalinity supply, then pelagic carbonate production by the coccolithophores at the end of the glacial maximum could drive a decrease in ocean [CO32-] and act to trigger the deglacial rise in pCO(2)
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