Session S24: Focus Session: Interaction of Polymers with Biological Structures

Theoretical and Numerical Modeling of faceted Ionic crystalline vesicles

Icosahedral shape is found in several natural structures including large viruses, large fullerenes and cationic-anionic vesicles. Faceting into icosahedral shape can occur in large crystalline membranes via elasticity theory. Icosahedral symmetry is found in small systems of particles with short-range interactions on a sphere. Dr G. Vernizzi and I show a novel electrostatic-driven mechanism of ionic crystalline shells faceting into icosahedral shapes even for systems with a small number of particles. Icosahedral shape is possible in cationic and anionic molecules adsorbed onto spherical interfaces, such as emulsions or other immiscible liquid droplets because the large concentration of charges at the interface can lead to ionic crystals on the curved interface. Such self-organized ionic structures favors the formation of flat surfaces. We find that these ionic crystalline shells can have lower energy when faceted into icosahedra along particular directions. Indeed, the ``ionic'' buckling is driven by preferred bending directions of the planar ionic structure, along which is more likely for the icosahedral shape to develop an edge. Since only certain orientations are allowed, rotational symmetry is broken. One can hope to exploit this mechanism to generate functional materials where, for instance, proteins with specific charge groups can orient at specific directions along an icosahedral cationic-anionic vesicle.   

Microchannels with adhesive posts trap cells with specific mechanical properties

In order to perform various biological assays and tissue engineering studies, there is a critical need for microfluidic devices that can be used to trap cells with specific mechanical properties. Here, we model cells as fluid filled elastic shells, which also represent polymeric microcapsules. Using a combined approach based on lattice Boltzmann and lattice spring models, we study the motion of cells within a channel with two adhesive posts on the opposite walls. The distance between the posts is comparable to the diameter of the cell. The cells are driven to move through the channel by an imposed pressure gradient. We probe the effect of post compliance and the adhesion strength on the dynamics of the cells. We isolate the conditions at which all cells with shell stiffness lying within a specified range can be trapped in between the posts. Thus, our study can facilitate the design of simple and robust devices for analyzing mechanical properties of biological cells and synthetic microcapsules.   

Biomimetic Micellar Networks

The self-assembly of amphiphilic block copolymers in dilute aqueous solution has been used to prepare structural analogues of fibrous materials common in physiology. The dependence of aggregate structure on amphiphile composition has been documented for a number of polymeric systems and by controlling the relative extent of hydrophilicity to hydrophobicity, block copolymers can be designed to target specific morphologies. Cell interactions with self-assembled structures can be promoted through conjugation of peptides or other targeting moieties to the constituent amphiphiles. The covalent attachment of RDG-containing peptides to the hydrophilic terminus of poly(ethylene oxide)-b-polybutadiene and the dilute solution behavior of these modified polymeric amphiphiles has been studied. An overall amphiphile composition conducive to worm-like micelle formation was targeted, and cross-linking of the hydrophobic core of these aggregate structures resulted in solution properties akin to fibrillar collagen gels.   

Post-Functionalized Polymer Brushes for Bio-Separation: Tuning GFP Adsorption via Functional Group Display

An inexpensive and robust biosensor platform that can be tuned to separate and/or detect complex mixtures of biomolecules while minimizing reagents would be of great use for military, homeland security, and medical diagnostic applications. Gradient surfaces of poly(2-hydroxyethyl methacrylate) (PHEMA) brushes have been previously shown to spatially localize biomolecule binding, while minimizing non-specific adsorption of the same biomolecule on other regions of the gradient specimen. In order to further improve the specificity and to provide latent functionality for detection of the binding events, post-polymerization modification of PHEMA with various functional groups has been investigated. Using standard succinimide-based coupling, hydroxyl pendants of PHEMA brushes were conjugated to oligo-peptides, alkanes and oligo(ethylene glycol) (OEG) through an alpha-terminus primary amine. Ellipsometry, contact angle, XPS and ER-FTIR spectroscopy indicated that coupling occurred with efficiencies ranging from 10-40{\%}. Post-functionalization of PHEMA with OEG and hexadecane allows manipulation of the hydrophilicity of the surface and thus tuning of Green Fluorescent Protein (GFP) binding.   

Structure and dynamics of water near the interface with oligo(ethylene oxide) self-assembled monolayers

Oligo(ethylene oxide) self-assembled monolayers (OEO SAM's) deposited on Au are the prototypical materials used to study protein resistance. Recently, protein resistance has been shown to vary as a function of surface coverage and to be maximal at about two-thirds coverage, not complete coverage. We use molecular dynamics simulations to study the nature of the interface between water and the OEO SAM for a range of SAM coverages. As SAM coverage decreases, the amount of water within the OEO monolayer increases monotonically; however, the penetration depth of the water shows a maximum near the experimentally-found maximal coverage. As the water content increases, the SAM-water mixture becomes harder to distinguish from bulk water. Since the oxygen atoms of OEO are hydrogen bond acceptors, a hydrogen bond network forms within the SAM-water mixture. The water molecules diffuse freely within the monolayer and exchange with the bulk water. Because the monolayer becomes increasingly like bulk water as the coverage decreases, proteins stay in their bulk soluble conformation and do not adsorb. {\em \small{Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000.}}   

Development of novel antibiofouling materials from natural phenol compounds

Biofilms consist of a gelatinous matrix formed on a solid surface by microbial organisms.Biofilm is caused due to the adhesion of microbes to solid surfaces with production of extracellular polymers and the process of the biofilm formation is reffered to as biofouling.Biofouling causes serious problems in chemical, medical and pharmaceutical industries.Although there have been some antibiofouling materials developed over the years,no plausible results have been found yet.Natural polyphenolic compounds like flavanoids,cathechins have strong antioxidant and antimicrobial properties.Recently,apocynin,a phenol derivative,was polymerized to form oligomers,which can regulate intracellular pathways in cancer cells preventing cell proliferation and migration.These natural phenolic compounds have never been applied to solid surfaces to prevent biofouling.It is thought that probably because of the difficulty to crosslink them to form a stable coating.In this study,some novel polyphenolic compounds synthesized using enzymatic technique from cashew nut shell liquid,a cheap and renewable byproduct of the cashew industry are used as coating materials to prevent biofouling.The interaction of these materials with microbes preventing fouling on surfaces and the chemico-physical properties of the materials causing the antibiofouling effect will be discussed.It is critical to understand the antibiofouling mechanism of these materials for better design and application in various fields.   

Conformation Distributions in Adsorbed Proteins.

While the structural basis of protein function is well understood in the biopharmaceutical and biotechnology industries, few methods for the characterization and comparison of protein conformation distributions are available. New methods capable of measuring the stability of protein conformations and the integrity of protein-protein, protein-ligand and protein-surface interactions both in solution and on surfaces are needed to help the development of protein-based products. We are developing infrared spectroscopy methods for the characterization and comparison of molecular conformation distributions in monolayers and in solutions. We have extracted an order parameter describing the orientational and conformational variations of protein functional groups around the average molecular values from a single polarized spectrum. We will discuss the development of these methods and compare them to amide hydrogen/deuterium exchange methods for albumin in solution and on different polymer surfaces to show that our order parameter is related to protein stability.   

Strain-stiffening response in organogels assembled using steroidal biomolecules

The phenomenon of strain-stiffening or strain-hardening refers to an increase in the elastic modulus (stiffness) of a material with increasing strain amplitude. While this response is exhibited by many biological materials, including gels of biopolymers such as actin, it is rarely seen in other types of soft matter. Here, we report strain-stiffening in a new class of self- assembled organogels being studied in our laboratory. These gels are formed in nonpolar organic liquids by combining a lipid (lecithin) or two-tailed surfactant (AOT) with a type of naturally occurring steroidal amphiphile called a bile salt. Based on rheological and scattering data, we deduce that the gel structure comprises a network of semiflexible filaments. Interestingly, gels induced by small organic molecules other than bile salts do not show strain-stiffening. We suggest that the bile salt molecules confer an intrinsic stiffness to the filaments in the gel, which is important for strain-stiffening.   

Solvent Viscosity at the Protein Surface

Biochemical activity of biological macromolecules depends on solvent's viscosity, $\eta $, at their surface. The latter might differ from the bulk solvent viscosity due to preferential hydration. In order to estimate $\eta $ at the protein surface, we studied dielectric relaxation spectra of lysozyme-water-glycerol mixtures. Additional relaxation process that appears in the presence of proteins has been assigned to their rotation. Employing Debye-Stokes-Einstein relationship [$\tau _{R}$ = (4$\pi $R$_{R}^{3}\eta $/KT)],$^{ }$and assuming that hydrodynamics radius of protein, R$_{R}$, does not change, we estimated $\eta $ at the protein surface. Analysis of the obtained results indeed reveals a significant difference between bulk solvent's viscosity and the viscosity experienced by a protein. The water concentration appears to be significantly enhanced at the protein surface, in agreement with earlier thermodynamics study. Using the viscosity data, we estimate solvent composition at the protein surface.$^{ }$We expect that the developed approach will help to unravel the role of the solvent and its viscosity in dynamics, stability and biochemical activity of proteins.   

Correlation of chitosan's rheological properties to its ability to electrospin

Chitosan, derived from chitin found in the exoskeleton of crustaceans, has been investigated extensively for use in biomedical applications ranging from drug delivery to scaffolds for tissue engineering. Therefore, forming nanofibers of this linear polysaccharide is desirable for use in such applications, because the nanofibers can be tailored to mimic the size and porosity of the extracellular matrix. Electrostatic spinning (electrospinning) is a convenient method to produce nonwoven mats of nanofibers. The ability of the solutions to successfully electospin is closely correlated with the rheological properties of the solutions. Chitosan is challenging to electrospin due to its relatively high viscosity at modest concentrations. Solutions of chitosan blended with poly(ethylene oxide) (PEO) have been electrospun successfully with freshly prepared solutions. If the blended solutions are stored, they do not readily electrospin. Moreover, chitosan/PEO blend solutions show a drastic decrease in zero shear rate viscosity over time, which can be attributed to phase separation. The challenges associated with electrospinning charged biopolymers (chitosan is cationic) will be discussed in terms of their rheological properties. Successes and failures will be highlighted and compared results for readily electrospun neutral polymers.   

Diblock Copolymer as a Surface Delivery Vehicle for DNA Chip Construction

A generic DNA sensor is made of a substrate, a coupling layer built on the substrate and the DNA attached to the coupling layer. Previously a DNA chip was constructed using a small molecule bi-functional linker via 1,3-dipolar zaide-alkyne cycloaddition coupling chemistry. The reaction efficiency of the cycloaddition coupling chemistry is high but there are some disadvantages such as low DNA coverage and low mobility of DNA due to the use of the small molecule linker. In this paper, a newly synthesized asymmetric diblock copolymer poly(methyl methacrylate-b-tert butyl acrylate) [poly(MMA-b-tBA)] with alkyne functional groups at the end of tBA block will be used as the coupling layer for the DNA chip construction. As will be shown in this paper, the attached DNA will have more mobility and higher surface coverage because of the use of the alkyne-end functionalized diblock copolymer as the coupling layer. More importantly, the areal density of the DNA molecules can be tuned by the thickness of the film simply made by the spin-coating method. The copolymer thin film was characterized by angle-dependent X-ray photoelectron spectroscopy, ellipsometry measurement and contact angle measurement. The thickness of tBA block was estimated using the substrate-overlayer model of ADXPS. The dye-labeled DNA chemically bonded to the surface was characterized by fluorescence measurement.   

Effect of copolymer microstructure on single chain collapse

We present dynamic Monte Carlo simulations of the collapse of copolymers containing sticky comonomers, $c$. There is a qualitative difference in the transition depending on $c$ content. For $c$ content $>\sim $ 50{\%}, copolymer collapse is qualitatively similar to that observed for homopolymers, when rescaled to account for comonomer solvophobicity. However, collapse of copolymers with $c \quad < \quad \sim $50{\%} is qualitatively steeper than for homopolymers. We show that the change in the nature of collapse is due to the formation of an intermediate structure after the theta-point. The pathway to collapse is also strongly influenced by the distribution of comonomers along the chain. For uniform copolymer chains (viz. equispaced $c$ units), collapse happens at lower temperatures than random copolymers. Further, uniform copolymers, but not random, appear to collapse cooperatively. Our results have relevance to protein folding where specific amino acid sequences lead to collapse and folding to a unique native structure.