Biomedical subjects

Y Kafri

Publications and source records attributed to Y Kafri.

10 recordsLinked to original sources

PubMed2003

Griffiths singularities in unbinding of strongly disordered polymers.

Griffiths singularities occurring in the unbinding of strongly disordered heteropolymers are studied. A model with two randomly distributed binding energies, -1 and -v, is introduced and studied analytically by analyzing the Lee-Yang zeros of the partition sum. It is demonstrated that in the limit v--> infinity the model exhibits a Griffiths type singularity at a temperature T(G)=O(1) corresponding to melting of long homogeneous domains of the low binding energy. For finite v>>1 the model is expected to exhibit an additional, unbinding, transition at a high temperature T(M)=O(v).

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PubMed2003

Interstrand distance distribution of DNA near melting.

The distance distribution between complementary base pairs of the two strands of a DNA molecule is studied near the melting transition. Scaling arguments are presented for a generalized Poland-Scheraga-type model that includes self-avoiding interactions. At the transition temperature and for a large distance r, the distribution decays as 1/r(kappa) with kappa=1+(c-2)/nu. Here nu is the self-avoiding walk correlation length exponent and c is the exponent associated with the entropy of an open loop in the chain. Results for the distribution function just below the melting point are also presented. Numerical simulations that fully take into account the self-avoiding interactions are in good agreement with the scaling approach.

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PubMed2002

Criterion for phase separation in one-dimensional driven systems.

A general criterion for the existence of phase separation in driven density-conserving one-dimensional systems is proposed. It is suggested that phase separation is related to the size dependence of the steady-state currents of domains in the system. A quantitative criterion for the existence of phase separation is conjectured using a correspondence made between driven diffusive models and zero-range processes. The criterion is verified in all cases where analytical results are available, and predictions for other models are provided.

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PubMed2001

Ordering dynamics of the driven lattice-gas model.

The evolution of a two-dimensional driven lattice-gas model is studied on an LxxL(y) lattice. Scaling arguments and extensive numerical simulations are used to show that starting from random initial configuration the model evolves via two stages: (a) an early stage in which alternating stripes of particles and vacancies are formed along the direction y of the driving field, and (b) a stripe coarsening stage, in which the number of stripes is reduced and their average width increases. The number of stripes formed at the end of the first stage is shown to be a function of L(x)/L(straight phi)(y), with straight phi approximately 0.2. Thus, depending on this parameter, the resulting state could be either single or multistriped. In the second, stripe coarsening stage, the coarsening time is found to be proportional to L(y), becoming infinitely long in the thermodynamic limit. This implies that the multistriped state is thermodynamically stable. The results put previous studies of the model in a more general framework.

Journal Article↗

PubMed2001

Search for correlates of protective immunity conferred by anthrax vaccine.

Vaccination by anthrax protective antigen (PA)-based vaccines requires multiple immunization, underlying the need to develop more efficacious vaccines or alternative vaccination regimens. In spite of the vast use of PA-based vaccines, the definition of a marker for protective immunity is still lacking. Here we describe studies designed to help define such markers. To this end we have immunized guinea pigs by different methods and monitored the immune response and the corresponding extent of protection against a lethal challenge with anthrax spores. Active immunization was performed by a single injection using one of two methods: (i) vaccination with decreasing amounts of PA and (ii) vaccination with constant amounts of PA that had been thermally inactivated for increasing periods. In both studies a direct correlation between survival and neutralizing-antibody titer was found (r(2) = 0.92 and 0.95, respectively). Most significantly, in the two protocols a similar neutralizing-antibody titer range provided 50% protection. Furthermore, in a complementary study involving passive transfer of PA hyperimmune sera to naive animals, a similar correlation between neutralizing-antibody titers and protection was found. In all three immunization studies, neutralization titers of at least 300 were sufficient to confer protection against a dose of 40 50% lethal doses (LD(50)) of virulent anthrax spores of the Vollum strain. Such consistency in the correlation of protective immunity with anti-PA antibody titers was not observed for antibody titers determined by an enzyme-linked immunosorbent assay. Taken together, these results clearly demonstrate that neutralizing antibodies to PA constitute a major component of the protective immunity against anthrax and suggest that this parameter could be used as a surrogate marker for protection.

Animals↗

PubMed2000

Why is the DNA denaturation transition first order?

We study a model for the denaturation transition of DNA in which the molecules are considered as being composed of a sequence of alternating bound segments and denaturated loops. We take into account the excluded-volume interactions between denaturated loops and the rest of the chain by exploiting recent results on scaling properties of polymer networks of arbitrary topology. The phase transition is found to be first order in d = 2 dimensions and above, in agreement with experiments and at variance with previous theoretical results, in which only excluded-volume interactions within denaturated loops were taken into account. Our results agree with recent numerical simulations.

Animals↗

PubMed2000

Stochastic ballistic annihilation and coalescence

We study a class of stochastic ballistic annihilation and coalescence models with a binary velocity distribution in one dimension. We obtain an exact solution for the density which reveals a universal phase diagram for the asymptotic density decay. By universal we mean that all models in the class are described by a single phase diagram spanned by two reduced parameters. The phase diagram reveals four regimes, two of which contain the previously studied cases of ballistic annihilation. The two new phases are a direct consequence of the stochasticity. The solution is obtained through a matrix product approach and builds on properties of a q-deformed harmonic oscillator algebra.

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PubMed2000

Coarsening of a class of driven striped structures

The coarsening process in a class of driven systems exhibiting striped structures is studied. The dynamics is governed by the motion of the driven interfaces between the stripes. When two interfaces meet they coalesce thus giving rise to a coarsening process in which l(t), the average width of a stripe, grows with time. This is a generalization of the reaction-diffusion process A+A-->A to the case of extended coalescing objects, namely, the interfaces. Scaling arguments which relate the coarsening process to the evolution of a single driven interface are given, yielding growth laws for l(t), for both short and long times. We introduce a simple microscopic model for this process. Numerical simulations of the model confirm the scaling picture and growth laws. The results are compared to the case where the stripes are not driven and different growth laws arise.

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PubMed1999

Boundary effects in reaction-diffusion processes.

The effects of a boundary on reaction systems are examined in the framework of the general single-species reaction/coalescence process. The boundary naturally represents the reactants' container, but is applicable to exciton dynamics in a doped TMMC crystal. We show that a density excess, which extends into the system diffusively from the boundary, is formed in two dimensions and below. This implies a surprising result for the magnetization near a fixed spin in the coarsening of the one-dimensional critical Ising model. The universal, dimensionally dependent functional forms of this density excess are given by an exact solution and the field-theoretic renormalization group.

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PubMed1992

Viral neuroinvasion and encephalitis induced by lipopolysaccharide and its mediators.

The present study was designed to test the effect of bacterial endotoxin on penetration of viruses into the central nervous system (CNS). As a model we used two neurovirulent viruses that lack neuroinvasive capacity: West Nile virus-25 (WN-25) and neuroadapted Sindbis virus (SVN). Administration of lipopolysaccharide (LPS, 100 micrograms/mouse) to CD-1 mice, followed by WN-25 inoculation resulted in 83% encephalitis and death, compared with less than 5% in controls. The results in SVN-inoculated CD-1 mice were quite similar. LPS-treated mice suffered 62% mortality compared with 6% in the nontreated group. No changes in viral neuroinvasiveness were demonstrated in viruses isolated from brains of encephalitic mice, suggesting that neuroinvasion is not due to a selection process for an invasive variant, but to direct penetration of the viruses through the blood-brain barrier (BBB). LPS did not induce WN-25 encephalitis in LPS-insensitive C3H/HeJ mice, compared with 100% neuroinvasion in C3H/HeB mice. Induction of neuroinvasion could be transferred to C3H/HeJ mice by transfusion with serum obtained from LPS-treated, LPS-responsive mice. Passive immunization of CD-1 mice with anti-mTNF antibodies before LPS administration did not prevent LPS-induced WN-25 encephalitis. Furthermore, neutralization of tumor necrosis factor activity in the serum of LPS-treated mice did not abolish its activity, and transfusion-associated encephalitis was observed after the administration of the neutralized serum with WN-25. We suggest that LPS can contribute to virus penetration from the blood into the CNS, a process which turns a mild viral infection into a severe lethal encephalitis. This effect is mediated by soluble factors, and is probably achieved by injury to cerebral microvascular endothelium and modulation of BBB permeability.

Animals↗