A review of Monte Carlo simulations of polymers with PERM

In this review, we describe applications of the pruned-enriched Rosenbluth method (PERM), a sequential Monte Carlo algorithm with resampling, to various problems in polymer physics. PERM produces samples according to any given prescribed weight distribution, by growing configurations step by step with controlled bias, and correcting "bad" configurations by "population control". The latter is implemented, in contrast to other population based algorithms like e.g. genetic algorithms, by depth-first recursion which avoids storing all members of the population at the same time in computer memory. The problems we discuss all concern single polymers (with one exception), but under various conditions: Homopolymers in good solvents and at the $\Theta$ point, semi-stiff polymers, polymers in confining geometries, stretched polymers undergoing a forced globule-linear transition, star polymers, bottle brushes, lattice animals as a model for randomly branched polymers, DNA melting, and finally -- as the only system at low temperatures, lattice heteropolymers as simple models for protein folding. PERM is for some of these problems the method of choice, but it can also fail. We discuss how to recognize when a result is reliable, and we discuss also some types of bias that can be crucial in guiding the growth into the right directions.Comment: 29 pages, 26 figures, to be published in J. Stat. Phys. (2011

Interaction of Mesoporous Silica Nanoparticles with Human Red Blood Cell Membranes: Size and Surface Effects

The interactions of mesoporous silica nanoparticles (MSNs) of different particle sizes and surface properties with human red blood cell (RBC) membranes were investigated by membrane filtration, flow cytometry, and various microscopic techniques. Small MCM-41-type MSNs (∼100 nm) were found to adsorb to the surface of RBCs without disturbing the membrane or morphology. In contrast, adsorption of large SBA-15-type MSNs (∼600 nm) to RBCs induced a strong local membrane deformation leading to spiculation of RBCs, internalization of the particles, and eventual hemolysis. In addition, the relationship between the degree of MSN surface functionalization and the degree of its interaction with RBC, as well as the effect of RBC−MSN interaction on cellular deformability, were investigated. The results presented here provide a better understanding of the mechanisms of RBC−MSN interaction and the hemolytic activity of MSNs and will assist in the rational design of hemocompatible MSNs for intravenous drug delivery and in vivo imaging

Electroconvulsive Therapy in a Patient with Chronic Catatonia: Clinical Outcomes and Cerebral 18[F]Fludeoxyglucose Positron Emission Tomography Findings

Catatonia is a psychomotor syndrome that can be associated with both psychiatric diseases (mainly mood disorders, but also psychotic disorders) and medical conditions. Lorazepam (6-21 mg/day, occasionally up to 30md/day) is the first choice treatment and electroconvulsive therapy (ECT) is the second line, regardless of the underlying clinical condition. There are some evidences also for effectiveness of other medications. Patients treated acutely usually show rapid and full therapeutic response but even longstanding catatonia can improve. However, some authors suggested that chronic catatonia in the context of schizophrenia is phenomenologically different and less responsive to lorazepam and ECT, especially if associated with echophenomena. We present here the case of a patient with longstanding catatonic schizophrenia treated with antipsychotics who significantly improved after ECT. Improvement regarded mainly catatonia, but also negative symptoms, cognition and psychosocial functioning. A slight amelioration in prefrontal metabolism (Brain(18)[F]FDG PET) one month following the ECT course was also noted