Collapse of Randomly Self-Interacting Polymers

We use complete enumeration and Monte Carlo techniques to study self--avoiding walks with random nearest--neighbor interactions described by $v_0q_iq_j$, where $q_i=\pm1$ is a quenched sequence of ``charges'' on the chain. For equal numbers of positive and negative charges ($N_+=N_-$), the polymer with $v_0>0$ undergoes a transition from self--avoiding behavior to a compact state at a temperature $\theta\approx1.2v_0$. The collapse temperature $\theta(x)$ decreases with the asymmetry $x=|N_+-N_-|/(N_++N_-)$Comment: 8 pages, TeX, 4 uuencoded postscript figures, MIT-CMT-

Larga batalla de un paciente con sida y tuberculosis recurrente: ¿reinfección o reactivación?

The advent of Mycobacterium tuberculosis strain genotyping has allowed differentiation between disease relapse and exogenous re-infection. We report here a remarkable case of multiply recurrent tuberculosis in a patient living with HIV. Between 1995 and 2009, a young HIV-infected intravenous drug user, who was reluctant to comply with anti-retroviral treatment, underwent at least five tuberculosis episodes caused by three distinct M. tuberculosis strains sharply differentiated by drug susceptibility profile, genotype and infectious source. Eventually, the patient died during a relapse of tuberculosis due to a notorious multidrug-resistant outbreak-strain, which infected him during a prolonged hospitalization in the epicentre of such outbreak. Whether recurrent tuberculosis is due to a new infection or to reactivation of a previous one is a century-long controversial question. In our patient, both conditions alternated throughout his 15 years of living with HIV. Cases such as this might not be exceptional in certain underprivileged suburban areas of Argentina and should raise concern over three pending issues in tuberculosis control policies, namely secondary preventive therapy, institutional infection control and patient follow-up throughout the health network system.La genotipificación de aislamientos clínicos de Mycobacterium tuberculosis permite diferenciar entre recaída y reinfección exógena. Presentamos un notable caso de tuberculosis recurrente asociado a sida. Entre 1995 y 2009, un usuario de drogas ilegales por vía endovenosa, con infección por el VIH pero reacio al tratamiento antirretroviral, sufrió cinco episodios de tuberculosis causados por tres cepas de M. tuberculosis. Las cepas se diferenciaron por los genotipos, los perfiles de sensibilidad a antibióticos y las fuentes de infección. El paciente murió de tuberculosis por una cepa multirresistente, responsable de un extenso brote en Argentina, que lo infectó durante una prolongada internación en el hospital que fue el epicentro de dicho brote. La controversia sobre el origen de la tuberculosis recurrente –reinfección o reactivación– lleva ya un siglo y continúa vigente. En el caso que aquí se describe, las dos situaciones se alternaron a lo largo de los 15 años de lucha contra el sida que atravesó este paciente. Esta situación puede no ser excepcional en áreas suburbanas desfavorecidas de Argentina, y pone en evidencia tres temas insuficientemente atendidos en nuestro medio: el tratamiento preventivo de la tuberculosis secundaria, el control de su transmisión en hospitales y cárceles, y el seguimiento de los pacientes a través del sistema de salud.Fil: Reniero, A.. Hospital Municipal de San Isidro; ArgentinaFil: Beltrán, M.. Hospital Municipal de San Isidro; ArgentinaFil: de Kantor, I. N.. PAHO/WHO consultant; ArgentinaFil: Ritacco, Gloria Viviana. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Dirección Nacional de Institutos de Investigación. Administración Nacional de Laboratorios e Institutos de Salud. Instituto Nacional de Enfermedades Infecciosas; Argentin

Theta-point universality of polyampholytes with screened interactions

By an efficient algorithm we evaluate exactly the disorder-averaged statistics of globally neutral self-avoiding chains with quenched random charge $q_i=\pm 1$ in monomer i and nearest neighbor interactions $\propto q_i q_j$ on square (22 monomers) and cubic (16 monomers) lattices. At the theta transition in 2D, radius of gyration, entropic and crossover exponents are well compatible with the universality class of the corresponding transition of homopolymers. Further strong indication of such class comes from direct comparison with the corresponding annealed problem. In 3D classical exponents are recovered. The percentage of charge sequences leading to folding in a unique ground state approaches zero exponentially with the chain length.Comment: 15 REVTEX pages. 4 eps-figures . 1 tabl

Larga batalla de un paciente con sida y tuberculosis recurrente: ¿reinfección o reactivación?

The advent of Mycobacterium tuberculosis strain genotyping has allowed differentiation between disease relapse and exogenous re-infection. We report here a remarkable case of multiply recurrent tuberculosis in a patient living with HIV. Between 1995 and 2009, a young HIV-infected intravenous drug user, who was reluctant to comply with anti-retroviral treatment, underwent at least five tuberculosis episodes caused by three distinct M. tuberculosis strains sharply differentiated by drug susceptibility profile, genotype and infectious source. Eventually, the patient died during a relapse of tuberculosis due to a notorious multidrug-resistant outbreak-strain, which infected him during a prolonged hospitalization in the epicentre of such outbreak. Whether recurrent tuberculosis is due to a new infection or to reactivation of a previous one is a century-long controversial question. In our patient, both conditions alternated throughout his 15 years of living with HIV. Cases such as this might not be exceptional in certain underprivileged suburban areas of Argentina and should raise concern over three pending issues in tuberculosis control policies, namely secondary preventive therapy, institutional infection control and patient follow-up throughout the health network system.La genotipificación de aislamientos clínicos de Mycobacterium tuberculosis permite diferenciar entre recaída y reinfección exógena. Presentamos un notable caso de tuberculosis recurrente asociado a sida. Entre 1995 y 2009, un usuario de drogas ilegales por vía endovenosa, con infección por el VIH pero reacio al tratamiento antirretroviral, sufrió cinco episodios de tuberculosis causados por tres cepas de M. tuberculosis. Las cepas se diferenciaron por los genotipos, los perfiles de sensibilidad a antibióticos y las fuentes de infección. El paciente murió de tuberculosis por una cepa multirresistente, responsable de un extenso brote en Argentina, que lo infectó durante una prolongada internación en el hospital que fue el epicentro de dicho brote. La controversia sobre el origen de la tuberculosis recurrente –reinfección o reactivación– lleva ya un siglo y continúa vigente. En el caso que aquí se describe, las dos situaciones se alternaron a lo largo de los 15 años de lucha contra el sida que atravesó este paciente. Esta situación puede no ser excepcional en áreas suburbanas desfavorecidas de Argentina, y pone en evidencia tres temas insuficientemente atendidos en nuestro medio: el tratamiento preventivo de la tuberculosis secundaria, el control de su transmisión en hospitales y cárceles, y el seguimiento de los pacientes a través del sistema de salud.Fil: Reniero, A.. Hospital Municipal de San Isidro; ArgentinaFil: Beltrán, M.. Hospital Municipal de San Isidro; ArgentinaFil: de Kantor, I. N.. PAHO/WHO consultant; ArgentinaFil: Ritacco, Gloria Viviana. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Dirección Nacional de Institutos de Investigación. Administración Nacional de Laboratorios e Institutos de Salud. Instituto Nacional de Enfermedades Infecciosas; Argentin

Two-Dimensional Polymers with Random Short-Range Interactions

We use complete enumeration and Monte Carlo techniques to study two-dimensional self-avoiding polymer chains with quenched ``charges'' $\pm 1$. The interaction of charges at neighboring lattice sites is described by $q_i q_j$. We find that a polymer undergoes a collapse transition at a temperature $T_{\theta}$, which decreases with increasing imbalance between charges. At the transition point, the dependence of the radius of gyration of the polymer on the number of monomers is characterized by an exponent $\nu_{\theta} = 0.60 \pm 0.02$, which is slightly larger than the similar exponent for homopolymers. We find no evidence of freezing at low temperatures.Comment: 4 two-column pages, 6 eps figures, RevTex, Submitted to Phys. Rev.

Harmonic Superspace, Minimal Unitary Representations and Quasiconformal Groups

We show that there is a remarkable connection between the harmonic superspace (HSS) formulation of N=2, d=4 supersymmetric quaternionic Kaehler sigma models that couple to N=2 supergravity and the minimal unitary representations of their isometry groups. In particular, for N=2 sigma models with quaternionic symmetric target spaces of the form G/HXSU(2) we establish a one-to-one mapping between the Killing potentials that generate the isometry group G under Poisson brackets in the HSS formulation and the generators of the minimal unitary representation of G obtained by quantization of its geometric realization as a quasiconformal group. Quasiconformal extensions of U-duality groups of four dimensional N=2, d=4 Maxwell-Einstein supergravity theories (MESGT) had been proposed as spectrum generating symmetry groups earlier. We discuss some of the implications of our results, in particular, for the BPS black hole spectra of 4d, N=2 MESGTs.Comment: 20 pages; Latex file: references added; minor cosmetic change

Collapse of Stiff Polyelectrolytes due to Counterion Fluctuations

The effective elasticity of highly charged stiff polyelectrolytes is studied in the presence of counterions, with and without added salt. The rigid polymer conformations may become unstable due to an effective attraction induced by counterion density fluctuations. Instabilities at the longest, or intermediate length scales may signal collapse to globule, or necklace states, respectively. In the presence of added-salt, a generalized electrostatic persistence length is obtained, which has a nontrivial dependence on the Debye screening length.Comment: 4 pages RevTex, 3 ps figures included using epsf, final version as appeared in PR

Anomalous Dynamics of Translocation

We study the dynamics of the passage of a polymer through a membrane pore (translocation), focusing on the scaling properties with the number of monomers $N$. The natural coordinate for translocation is the number of monomers on one side of the hole at a given time. Commonly used models which assume Brownian dynamics for this variable predict a mean (unforced) passage time $\tau$ that scales as $N^2$, even in the presence of an entropic barrier. However, the time it takes for a free polymer to diffuse a distance of the order of its radius by Rouse dynamics scales with an exponent larger than 2, and this should provide a lower bound to the translocation time. To resolve this discrepancy, we perform numerical simulations with Rouse dynamics for both phantom (in space dimensions $d=1$ and 2), and self-avoiding (in $d=2$) chains. The results indicate that for large $N$, translocation times scale in the same manner as diffusion times, but with a larger prefactor that depends on the size of the hole. Such scaling implies anomalous dynamics for the translocation process. In particular, the fluctuations in the monomer number at the hole are predicted to be non-diffusive at short times, while the average pulling velocity of the polymer in the presence of a chemical potential difference is predicted to depend on $N$.Comment: 9 pages, 9 figures. Submitted to Physical Review

Sequence randomness and polymer collapse transitions

Contrary to expectations based on Harris' criterion, chain disorder with frustration can modify the universality class of scaling at the theta transition of heteropolymers. This is shown for a model with random two-body potentials in 2D on the basis of exact enumeration and accurate Monte Carlo results. When frustration grows beyond a certain finite threshold, the temperature below which disorder becomes relevant coincides with the theta one and scaling exponents definitely start deviating from those valid for homopolymers.Comment: 4 pages, 4 eps figure