Interplay between writhe and knotting for swollen and compact polymers

The role of the topology and its relation with the geometry of biopolymers under different physical conditions is a nontrivial and interesting problem. Aiming at understanding this issue for a related simpler system, we use Monte Carlo methods to investigate the interplay between writhe and knotting of ring polymers in good and poor solvents. The model that we consider is interacting self-avoiding polygons on the simple cubic lattice. For polygons with fixed knot type we find a writhe distribution whose average depends on the knot type but is insensitive to the length $N$ of the polygon and to solvent conditions. This "topological contribution" to the writhe distribution has a value that is consistent with that of ideal knots. The standard deviation of the writhe increases approximately as $\sqrt{N}$ in both regimes and this constitutes a geometrical contribution to the writhe. If the sum over all knot types is considered, the scaling of the standard deviation changes, for compact polygons, to $\sim N^{0.6}$. We argue that this difference between the two regimes can be ascribed to the topological contribution to the writhe that, for compact chains, overwhelms the geometrical one thanks to the presence of a large population of complex knots at relatively small values of $N$. For polygons with fixed writhe we find that the knot distribution depends on the chosen writhe, with the occurrence of achiral knots being considerably suppressed for large writhe. In general, the occurrence of a given knot thus depends on a nontrivial interplay between writhe, chain length, and solvent conditions.Comment: 10 pages, accepted in J.Chem.Phy

Adsorbed self-avoiding walks subject to a force

We consider a self-avoiding walk model of polymer adsorption where the adsorbed polymer can be desorbed by the application of a force. In this paper the force is applied normal to the surface at the last vertex of the walk. We prove that the appropriate limiting free energy exists where there is an applied force and a surface potential term, and prove that this free energy is convex in appropriate variables. We then derive an expression for the limiting free energy in terms of the free energy without a force and the free energy with no surface interaction. Finally we show that there is a phase boundary between the adsorbed phase and the desorbed phase in the presence of a force, prove some qualitative properties of this boundary and derive bounds on the location of the boundary

Hatching Strategies in Monogenean (Platyhelminth) Parasites that Facilitate Host Infection

In parasites, environmental cues may influence hatching of eggs and enhance the success of infections. The two major endoparasitic groups of parasitic platyhelminths, cestodes (tapeworms) and digeneans (flukes), typically have high fecundity, infect more than one host species, and transmit trophically. Monogeneans are parasitic flatworms that are among the most host specific of all parasites. Most are ectoparasites with relatively low fecundity and direct life cycles tied to water. They infect a single host species, usually a fish, although some are endoparasites of amphibians and aquatic chelonian reptiles. Monogenean eggs have strong shells and mostly release ciliated larvae, which, against all odds, must find, identify, and infect a suitable specific host. Some monogeneans increase their chances of finding a host by greatly extending the hatching period (possible bet-hedging). Others respond to cues for hatching such as shadows, chemicals, mechanical disturbance, and osmotic changes, most of which may be generated by the host. Hatching may be rhythmical, larvae emerging at times when the host is more vulnerable to invasion, and this may be combined with responses to other environmental cues. Different monogenean species that infect the same host species may adopt different strategies of hatching, indicating that tactics may be more complex than first thought. Control of egg assembly and egg-laying, possibly by host hormones, has permitted colonization of frogs and toads by polystomatid monogeneans. Some monogeneans further improve the chances of infection by attaching eggs to the host or by retaining eggs on, or in, the body of the parasite. The latter adaptation has led ultimately to viviparity in gyrodactylid monogeneans

Self-avoiding walks and polygons confined to a square

We prove several rigorous results about the asymptotic behaviour of the numbers of polygons and self-avoiding walks confined to a square on the square lattice. Specifically we prove that the dominant asymptotic behaviour of polygons confined to an LxL square is identical to that of self-avoiding walks that cross an LxL square from one corner vertex to the opposite corner vertex. We also prove a result about the subdominant asymptotic behaviour of self-avoiding walks crossing a square and extend this result to polygons confined to a square. In addition, we investigate the problems of self-avoiding walks and polygons in a hypercube in the d-dimensional hypercubic lattice.Comment: Minor errors corrected and some additional results adde

Translocation of links through a pore: effects of link complexity and size

We have used Langevin dynamics to simulate the forced translocation of linked polymer rings through a narrow pore. For fixed size (i.e. fixed number of monomers) the translocation time depends on the link type and on whether the rings are knotted or unknotted. For links with two unknotted rings, the crossings between the rings can slow down the translocation and are responsible for a delay as the crossings pass through the pore. The results fall on a set of relatively smooth curves for different link families with the translocation time not always increasing with crossings number within the same family. When one ring is knotted the results depend on whether the link is prime or composite and, for the composite case, they depend on whether the knotted or unknotted ring enters the pore first. We find a similar situation for 3-component links where the results depend on whether the link is prime or composite. These results contribute to our understanding of how the entanglement complexity between filaments impacts their translocation dynamics and should be useful for extending nanopore-sensing techniques to probe the topological properties of these systems.Comment: 15 pages, 7 figure

Preweaning and Postweaning Performance of Crossbred Calves--0, 1 or 2 Ralgro Implants

The preweaning performance of 221 steer and heifer calves given differing numbers of Ralgro implants was evaluated. The steer and heifer calves given one implant had an additional 23 and 33 lb weaning weight compared to those receiving no implant. No additional response was shown by steer calves receiving a second implant 100 days after the first was given. Average daily gains during the 105-day growing period were not significantly faster than those receiving two implants during the 103-day finishing phase. This work supports other studies indicating that implanting calves at weaning time is an economical management practice. The work further indicates to feeders that calfhood implants have little or no effect on subsequent performance in the feedlot

Synthesis and characterization of amorphous astrominerals

Abstract only availableAsymptotic Giant Branch (AGB) stars are the main contributors of solid material (dust) to the interstellar medium. The dust forms as gas escaping from the star cools and condenses to form a circumstellar dust shell. Around oxygen-rich stars, most of the dust particles are silicates, but important questions remain about which minerals are formed and whether the grains are crystalline or amorphous (glassy). Dust grains that form below the glass transition temperature (Tg) of a particular mineral should be amorphous, while those that form much above Tg should be crystalline. The most refractory silicates predicted to form are members of the Melilite group of minerals, whose end-members are Gehlenite (Ca2Al2SiO7), Sodium Melilite (CaNaAlSi2O7) and Åkermanite (Ca2MgSiO7). Synthetic glasses corresponding to these end-member compositions were synthesized from oxide and carbonate powders by melting in Pt crucibles in a muffle furnace at temperatures up to 1650˚C. The viscosity of the melts was measured by the parallel-plate technique in the temperature range 655 to 900˚C, over the viscosity range 2x107 to 91012 Pa s. The viscosity-temperature data were interpolated to determine Tg for each glass, taken to be the temperature at which the viscosity is 1012 Pa s. Two naturally occurring mineral samples, of åkermanite and gehlenite, were also melted. At higher temperatures and lower viscosities than about 108 Pa s, crystallization is expected to be rapid relative to the timescale of cooling in circumstellar dust shells. Our experimentally determined glass transition temperatures therefore provide constraints on the nature (crystalline vs amorphous) of several refractory astrominerals predicted to be found in these shells. Future studies of these glasses will include infrared spectroscopy, in order to better interpret astronomical spectra and test for the presence of amorphous dust with these compositions. The results will be used to test competing models for circumstellar dust formation.Missouri Academy at Northwest Missouri State Universit