Hidden symmetries in the asymmetric exclusion process

We present a spectral study of the evolution matrix of the totally asymmetric exclusion process on a ring at half filling. The natural symmetries (translation, charge conjugation combined with reflection) predict only two fold degeneracies. However, we have found that degeneracies of higher order also exist and, as the system size increases, higher and higher orders appear. These degeneracies become generic in the limit of very large systems. This behaviour can be explained by the Bethe Ansatz and suggests the presence of hidden symmetries in the model. Keywords: ASEP, Markov matrix, symmetries, spectral degeneracies, Bethe Ansatz.Comment: 16 page

A computer-assisted motivational social network intervention to reduce alcohol, drug and HIV risk behaviors among Housing First residents.

BackgroundIndividuals transitioning from homelessness to housing face challenges to reducing alcohol, drug and HIV risk behaviors. To aid in this transition, this study developed and will test a computer-assisted intervention that delivers personalized social network feedback by an intervention facilitator trained in motivational interviewing (MI). The intervention goal is to enhance motivation to reduce high risk alcohol and other drug (AOD) use and reduce HIV risk behaviors.Methods/designIn this Stage 1b pilot trial, 60 individuals that are transitioning from homelessness to housing will be randomly assigned to the intervention or control condition. The intervention condition consists of four biweekly social network sessions conducted using MI. AOD use and HIV risk behaviors will be monitored prior to and immediately following the intervention and compared to control participants' behaviors to explore whether the intervention was associated with any systematic changes in AOD use or HIV risk behaviors.DiscussionSocial network health interventions are an innovative approach for reducing future AOD use and HIV risk problems, but little is known about their feasibility, acceptability, and efficacy. The current study develops and pilot-tests a computer-assisted intervention that incorporates social network visualizations and MI techniques to reduce high risk AOD use and HIV behaviors among the formerly homeless. CLINICALTRIALS.Gov identifierNCT02140359

Exactly solvable model with two conductor-insulator transitions driven by impurities

We present an exact analysis of two conductor-insulator transitions in the random graph model. The average connectivity is related to the concentration of impurities. The adjacency matrix of a large random graph is used as a hopping Hamiltonian. Its spectrum has a delta peak at zero energy. Our analysis is based on an explicit expression for the height of this peak, and a detailed description of the localized eigenvectors and of their contribution to the peak. Starting from the low connectivity (high impurity density) regime, one encounters an insulator-conductor transition for average connectivity 1.421529... and a conductor-insulator transition for average connectivity 3.154985.... We explain the spectral singularity at average connectivity e=2.718281... and relate it to another enumerative problem in random graph theory, the minimal vertex cover problem.Comment: 4 pages revtex, 2 fig.eps [v2: new title, changed intro, reorganized text

Distribution of exchange energy in a bond-alternating S=1 quantum spin chain

The quasi-one-dimensional bond-alternating S=1 quantum antiferromagnet NTENP is studied by single crystal inelastic neutron scattering. Parameters of the measured dispersion relation for magnetic excitations are compared to existing numerical results and used to determine the magnitude of bond-strength alternation. The measured neutron scattering intensities are also analyzed using the 1st-moment sum rules for the magnetic dynamic structure factor, to directly determine the modulation of ground state exchange energies. These independently determined modulation parameters characterize the level of spin dimerization in NTENP. First-principle DMRG calculations are used to study the relation between these two quantities.Comment: 10 pages, 10 figure

Site-Directed Mutagenesis of the Cysteine Ligands to the [4Fe−4S] Cluster of Escherichia coli MutY †

International audienceThe Escherichia coli DNA repair enzyme MutY plays an important role in the recognition and repair of 7,8-dihydro-8-oxo-2′-deoxyguanosine:2′-deoxyadenosine (OG:A) mismatches in DNA [Michaels et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 7022-7025]. MutY prevents DNA mutations resulting from the misincorporation of A opposite OG by using N-glycosylase activity to remove the adenine base. An interesting feature of MutY is that it contains a [4Fe-4S] 2+ cluster that has been shown to play an important role in substrate recognition [Porello, S. L., Cannon, M. J., David, S. S. (1998) Biochemistry 37, 6465-6475]. Herein, we have used site-directed mutagenesis to individually replace the cysteine ligands to the [4Fe-4S] 2+ cluster of E. coli MutY with serine, histidine, and alanine. The extent to which the various mutations reduce the levels of protein overexpression suggests that coordination of the [4Fe-4S] 2+ cluster provides stability to MutY in vivo. The ability of the mutated enzymes to bind to a substrate analogue DNA duplex and their in vivo activity were evaluated. Remarkably, the effects are both substitution and position dependent. For example, replacement of cysteine 199 with histidine provides a mutated enzyme that is expressed at high levels and exhibits DNA binding and in vivo activity similar to the WT enzyme. These results suggest that histidine coordination to the iron-sulfur cluster may be accommodated at this position in MutY. In contrast, replacement of cysteine 192 with histidine results in less efficient DNA binding and in vivo activity compared to the WT enzyme without affecting levels of overexpression. The results from the site-directed mutagenesis suggest that the structural properties of the iron-sulfur cluster coordination domain are important for both substrate DNA recognition and the in vivo activity of MutY

Power Spectra of a Constrained Totally Asymmetric Simple Exclusion Process

To synthesize proteins in a cell, an mRNA has to work with a finite pool of ribosomes. When this constraint is included in the modeling by a totally asymmetric simple exclusion process (TASEP), non-trivial consequences emerge. Here, we consider its effects on the power spectrum of the total occupancy, through Monte Carlo simulations and analytical methods. New features, such as dramatic suppressions at low frequencies, are discovered. We formulate a theory based on a linearized Langevin equation with discrete space and time. The good agreement between its predictions and simulation results provides some insight into the effects of finite resoures on a TASEP.Comment: 4 pages, 2 figures v2: formatting change

Meanders and the Temperley-Lieb algebra

The statistics of meanders is studied in connection with the Temperley-Lieb algebra. Each (multi-component) meander corresponds to a pair of reduced elements of the algebra. The assignment of a weight $q$ per connected component of meander translates into a bilinear form on the algebra, with a Gram matrix encoding the fine structure of meander numbers. Here, we calculate the associated Gram determinant as a function of $q$, and make use of the orthogonalization process to derive alternative expressions for meander numbers as sums over correlated random walks.Comment: 85p, uuencoded, uses harvmac (l mode) and epsf, 88 figure

Impurity Energy Level Within The Haldane Gap

An impurity bond $J{'}$ in a periodic 1D antiferromagnetic, spin 1 chain with exchange $J$ is considered. Using the numerical density matrix renormalization group method, we find an impurity energy level in the Haldane gap, corresponding to a bound state near the impurity bond. When $J{'}<J$ the level changes gradually from the edge of the Haldane gap to the ground state energy as the deviation $dev=(J-J{'})/J$ changes from 0 to 1. It seems that there is no threshold. Yet, there is a threshold when $J{'}>J$. The impurity level appears only when the deviation $dev=(J{'}-J)/J{'}$ is greater than $B_{c}$, which is near 0.3 in our calculation.Comment: Latex file,9 pages uuencoded compressed postscript including 4 figure

Calcium and phosphate homeostasis in dogs with newly diagnosed naturally occurring hypercortisolism

Background: Hypercortisolism affects calcium and phosphate metabolism in dogs; however, the exact mechanisms are not completely understood. Objectives: To evaluate circulating concentrations of whole parathormone (wPTH), 25-hydroxyvitamin D (25-(OH)D), calcitriol, and fibroblast growth factor-23 (FGF-23) in dogs with naturally occurring hypercortisolism (NOHC) and healthy dogs, and their association with calcium and phosphate homeostasis. Animals: Twenty-three client-owned dogs with NOHC, and 12 client or staff-owned healthy dogs. Methods: Prospective cross-sectional study. The circulating concentrations of total calcium, ionized calcium (iCa), phosphate, wPTH, 25-(OH)D, calcitriol and FGF-23, and the urinary fractional excretion of phosphate (FEP) and calcium (FECa) were compared between dogs with NOHC before treatment and healthy dogs. Results: Dogs with NOHC had higher mean serum phosphate concentrations (4.81 mg/dL, SD ± 0.71 vs 3.86 mg/dL, SD ± 0.60; P &lt;.001), median FECa (0.43%, range, 0.03-2.44 vs 0.15%, range, 0.06-0.35; P =.005), and median serum wPTH concentrations (54.6 pg/mL, range, 23.7-490 vs 24.6 pg/mL, range, 5.5-56.4; P =.003) as compared to the controls. Circulating concentrations of total calcium, iCa, and calcitriol and the FEP did not differ between groups, whereas the serum 25-(OH)D concentrations were lower in dogs with NOHC as compared to the controls (70.2 pg/mL, SD ± 42.3 vs 106.3 pg/mL, SD ± 35.3; P =.02). The dogs with NOHC had lower plasma FGF-23 concentrations than controls (316.6 pg/mL, range, 120.8-575.6 vs 448.7 pg/mL, range, 244.8-753; P =.03). Conclusions and Clinical Importance: Urine loss of calcium and hyperphosphatemia could contribute to the adrenal secondary hyperparathyroidism