Effective medium approach for stiff polymer networks with flexible cross-links

Recent experiments have demonstrated that the nonlinear elasticity of in vitro networks of the biopolymer actin is dramatically altered in the presence of a flexible cross-linker such as the abundant cytoskeletal protein filamin. The basic principles of such networks remain poorly understood. Here we describe an effective medium theory of flexibly cross-linked stiff polymer networks. We argue that the response of the cross-links can be fully attributed to entropic stiffening, while softening due to domain unfolding can be ignored. The network is modeled as a collection of randomly oriented rods connected by flexible cross-links to an elastic continuum. This effective medium is treated in a linear elastic limit as well as in a more general framework, in which the medium self-consistently represents the nonlinear network behavior. This model predicts that the nonlinear elastic response sets in at strains proportional to cross-linker length and inversely proportional to filament length. Furthermore, we find that the differential modulus scales linearly with the stress in the stiffening regime. These results are in excellent agreement with bulk rheology data.Comment: 12 pages, 8 figure

The bend stiffness of S-DNA

We formulate and solve a two-state model for the elasticity of nicked, double-stranded DNA that borrows features from both the Worm Like Chain and the Bragg--Zimm model. Our model is computationally simple, and gives an excellent fit to recent experimental data through the entire overstretching transition. The fit gives the first value for the bending stiffness of the overstretched state as about 10 nm*kbt, a value quite different from either B-form or single-stranded DNA.Comment: 7 pages, 1 figur

Nonlinear elasticity of composite networks of stiff biopolymers with flexible linkers

Motivated by recent experiments showing nonlinear elasticity of in vitro networks of the biopolymer actin cross-linked with filamin, we present an effective medium theory of flexibly cross-linked stiff polymer networks. We model such networks by randomly oriented elastic rods connected by flexible connectors to a surrounding elastic continuum, which self-consistently represents the behavior of the rest of the network. This model yields a crossover from a linear elastic regime to a highly nonlinear elastic regime that stiffens in a way quantitatively consistent with experiment.Comment: 4 pages, 3 figure

Off-lattice Monte Carlo Simulation of Supramolecular Polymer Architectures

We introduce an efficient, scalable Monte Carlo algorithm to simulate cross-linked architectures of freely-jointed and discrete worm-like chains. Bond movement is based on the discrete tractrix construction, which effects conformational changes that exactly preserve fixed-length constraints of all bonds. The algorithm reproduces known end-to-end distance distributions for simple, analytically tractable systems of cross-linked stiff and freely jointed polymers flawlessly, and is used to determine the effective persistence length of short bundles of semi-flexible worm-like chains, cross-linked to each other. It reveals a possible regulatory mechanism in bundled networks: the effective persistence of bundles is controlled by the linker density.Comment: 4 pages, 4 figure

Reply to "Comment on 'Theory of high-force DNA stretching and overstretching'"

In his Comment to an earlier paper [Phys. Rev. E 67, 051906 (2003)] Lam points out an error in Eq. (20) of the original paper. Here we show that use of the corrected expression produces results very similar to those presented in our original paper, so our qualitative conclusions are unchanged

Theory of High-Force DNA Stretching and Overstretching

Single molecule experiments on single- and double stranded DNA have sparked a renewed interest in the force-extension of polymers. The extensible Freely Jointed Chain (FJC) model is frequently invoked to explain the observed behavior of singlestranded DNA. We demonstrate that this model does not satisfactorily describe recent high-force stretching data. We instead propose a model (the Discrete Persistent Chain, or “DPC”) that borrows features from both the FJC and the Wormlike Chain, and show that it resembles the data more closely. We find that most of the high-force behavior previously attributed to stretch elasticity is really a feature of the corrected entropic elasticity; the true stretch compliance of single-stranded DNA is several times smaller than that found by previous authors. Next we elaborate our model to allow coexistence of two conformational states of DNA, each with its own stretch and bend elastic constants. Our model is computationally simple, and gives an excellent fit through the entire overstretching transition of nicked, double-stranded DNA. The fit gives the first values for the elastic constants of the stretched state. In particular we find the effective bend stiffness for DNA in this state to be about 10 nm · kBT, a value quite different from either B-form or single-stranded DNA

Efficacy of pheromone-based control system, Exosex™ SPTab, against moth pests in European food processing facilities

The Exosex™ SPTab auto-confusion system is a novel pheromone-based method for control of stored product moth pests in both food and tobacco processing and storage facilities. The method uses a femaleproduced sex pheromone, (Z,E)-9,12-tetradecadienyl acetate, combined with a patented electrostatic powder delivery system known as Entostat™ to disrupt mating and interrupt the lifecycle of several important moth pests: Plodia interpunctella, Ephestia kuehniella, Ephestia cautella and Ephestia elutella. Male moths are attracted to a compressed tablet of the powder, which contains the sex pheromone. The powder releases pheromone at a slow enough rate to attract males to make contact with the tablet. The powder adheres to the moth cuticle via electrostatic attraction and the moth leaves the tablet coated in female sex pheromone. Flight tunnel studies have shown that this disrupts their ability to locate female moths and they become attractive sources for other males. Here we will present findings from full scale, long term trials that were conducted under real conditions at commercial food processing facilities across Europe. Populations of target moth species were monitored alongside deployment of the SPTab system and compared with untreated control areas and historical data from the test areas in the years prior to deployment. In all cases populations were reduced compared to the same area in the previous year and compared to untreated control areas under local pest control practices. The SPTab auto-confusion system could offer the opportunity to actively reduce the use of pesticides and its use as an integrated pest management tool within the food and tobacco processing and storage industry is discussed. Keywords: Mating disruption, Plodia interpunctella, Ephestia kuehniella, SPTab, Sex pheromone