Theory of Crosslinked Bundles of Helical Filaments: Intrinsic Torques in Self-Limiting Biopolymer Assemblies

Inspired by the complex influence of the globular crosslinking proteins on the formation of biofilament bundles in living organisms, we study and analyze a theoretical model for the structure and thermodynamics of bundles of helical filaments assembled in the presence of crosslinking molecules. The helical structure of filaments, a universal feature of biopolymers such as filamentous actin, is shown to generically frustrate the geometry of crosslinking between the "grooves" of two neighboring filaments. We develop a coarse-grained model to investigate the interplay between the geometry of binding and mechanics of both linker and filament distortion, and we show that crosslinking in parallel bundles of helical filaments generates {\it intrinsic torques}, of the type that tend to wind bundle superhelically about its central axis. Crosslinking mediates a non-linear competition between the preference for bundle twist and the size-dependent mechanical cost of filament bending, which in turn gives rise to feedback between the global twist of self-assembled bundles and their lateral size. Finally, we demonstrate that above a critical density of bound crosslinkers, twisted bundles form with a thermodynamically preferred radius that, in turn, increases with a further increase in crosslinking bonds. We identify the {\it stiffness} of crosslinking bonds as a key parameter governing the sensitivity of bundle structure and assembly to the availability and affinity of crosslinkers.Comment: 15 pages, 9 figures, Appendi

Targeted protein delivery: carbodiimide crosslinking influences protein release from microparticles incorporated within collagen scaffolds

open access articleTissue engineering response may be tailored via controlled, sustained release of active agents from protein-loaded degradable microparticles incorporated directly within three-dimensional (3D) ice-templated collagen scaffolds. However, the effects of covalent crosslinking during scaffold preparation on the availability and release of protein from the incorporated microparticles have not been explored. Here, we load 3D ice-templated collagen scaffolds with controlled additions of poly-(DL-lactide-co-glycolide) microparticles. We probe the effects of subsequent N-(3-dimethylaminopropyl)- N0-ethylcarbodiimide hydrochloride crosslinking on protein release, using microparticles with different internal protein distributions. Fluorescein isothiocyanate labelled bovine serum albumin is used as a model protein drug. The scaffolds display a homogeneous microparticle distribution, and a reduction in pore size and percolation diameter with increased microparticle addition, although these values did not fall below those reported as necessary for cell invasion. The protein distribution within the microparticles, near the surface or more deeply located within the microparticles, was important in determining the release profile and effect of crosslinking, as the surface was affected by the carbodiimide crosslinking reaction applied to the scaffold. Crosslinking of microparticles with a high proportion of protein at the surface caused both a reduction and delay in protein release. Protein located within the bulk of the microparticles, was protected from the crosslinking reaction and no delay in the overall release profile was seen

Crosslinking in parallel

A crosslink is a double link established between the two entries of an edge in an adjacency list representation of a graph. Crosslinks play important roles in several parallel algorithms as they provide constant time access between the two entries of an edge; the existence of crosslinks is usually assumed. We consider the problem of establishing crosslinks in a crosslink-less adjacency list for graphs that belong to a class of graphs called the linearly contractible graphs, and show that cross-links can be established optimally in O(log n log*n) time using a CREW PRAM and optimally in O(log n) time using a CRCW PRAM for such graphs

Studying synthesis confinement effects on the internal structure of nanogels in computer simulations

We study the effects of droplet finite size on the structure of nanogel particles synthesized by random crosslinking of molecular polymers diluted in nanoemulsions. For this, we use a bead-spring computer model of polymer-like structures that mimics the confined random crosslinking process corresponding to irradiation- or electrochemically-induced crosslinking methods. Our results indicate that random crosslinking under strong confinement can lead to unusual nanogel internal structures, with a central region less dense than the external one, whereas under moderate confinement the resulting structure has a denser central region. We analyze the topology of the polymer networks forming nanogel particles with both types of architectures, their overall structural parameters, their response to the quality of the solvent and compare the cases of non-ionic and ionic systems

Spontaneous thermal expansion of nematic elastomers

We study the monodomain (single-crystal) nematic elastomer materials, all side-chain siloxane polymers with the same mesogenic groups and crosslinking density, but differing in the type of crosslinking. Increasing the proportion of long di-functional segments of main-chain nematic polymer, acting as network crosslinking, results in dramatic changes in the uniaxial equilibrium thermal expansion on cooling from isotropic phase. At higher concentration of main chains their behaviour dominates the elastomer properties. At low concentration of main-chain material, we detect two distinct transitions at different temperatures, one attributed to the main-chain, the other to the side-chain component. The effective uniaxial anisotropy of nematic rubber, r(T) proportional to the effective nematic order parameter Q(T), is given by the average of the two components and thus reflects the two-transition nature of thermal expansion. The experimental data is compared with the theoretical model of ideal nematic elastomers; applications in high-amplitude thermal actuators are discussed in the end

Heparin insolubilized with crosslinking agent

New plastic compositions, involving the systhesis of a polymeric system containing heparin insolubilized with crosslinking agents, show appreciable promise in human body implant technology

Crosslinking of dermal sheep collagen using hexamethylene diisocyanate

The use of hexamethylene diisocyanate (HMDIC) as a crosslinking agent for dermal sheep collagen (DSC) was studied. Because HMDIC is only slightly water soluble, a surfactant was used to obtain a clear and micellar crosslinking solution and to promote the penetration of HMDIC in the DSC matrix. Using optimized conditions treatment of non-crosslinked DSC (N-DSC) with HMDIC (H-DSC) increased the shrinkage temperature (Ts) of N-DSC from 56°C to 74°C for H-DSC. A linear relation between the decrease in free amine group content and the increase in Ts was observed. Crosslinking with HMDIC did not influence the tensile strength of the N-DSC samples but increased the elongation at break from 141% to 163% and decreased the high-strain modulus from 26 MPa to 16 MPa for the H-DSC samples, respectively

The influence of tetraethoxysilane sol preparation on the electrospinning of silica nanofibers

The critical parameters determining the electrospinning of silica nanofibers starting from tetraethoxysilane sols are reported. By controlling the reaction conditions, the rheological properties of the sol allowed for electrospinning without needing the addition of an organic polymer. This allows the polymer removal step, which is deleterious to the fibers and an economic and ecological inconvenience, to be skipped. The effects on the electrospinning process of the viscosity of the sol, the concentration of ethanol, the degree of crosslinking and the size of the colloidal species were studied in depth with ATR-FTIR, Si-29 NMR, H-1 NMR and DLS. Moreover, to separate the contributions of the different parameters three different set-ups for sol preparation were used. An optimum amount of 9 mol L-1 ethanol for electrospinning was determined. In addition, the optimum degree of crosslinking and size of colloidal particles, approximately 3.5-7 nm, were obtained for stable electrospinning and for producing uniform, beadless nanofibers that were stable in time. The optimum viscosity range is in between 100 and 200 mPa s, which is in line with previous work. Using these optimum conditions, continuous electrospinning was carried out for 3 h, resulting in large flexible silica nanofibrous membranes