Session Y5: Charged and Ion-Containing Polymers II

Manipulating Assembly, Disassembly and Exchange in Responsive Polyelectrolyte Multilayers

Polyelectrolyte multilayer assembly is based on the alternating adsorption of multilvalent positively and negatively charged species to create ionically crosslinked thin films with nanoscale control of film composition and function. We have utilized this method of assembly to manipulate ion transport, molecular transport, and electrochemical transport in these films, enabling the generation of a range of organic and organic-inorganic devices. Biological materials applications are also derived from such films, enabling their use as drug delivery devices. In each of these applications, it is desired to control interdiffusion and exchange within the multilayer systems to maintain desired function and generate isolated regions of composition and function within the z-direction of the film. Here we address these applications and means of controlling this phenomenon. Furthermore, it is desirable to induce controlled means of disassembly of these multilayer thin films. We will address a number of approaches for achieving this, including hydrolytic degradation, hydrogen bond dissociation, and controlled deconstruction on electrochemical impulse.   

Using Folding Pathways to Predict Protein Structure

Since the demonstration that the amino acid sequence of a protein encodes its structure, the prediction of structure from sequence remains an outstanding problem that impacts numerous scientific disciplines. By iteratively fixing secondary structure assignments of residues during Monte Carlo simulations of folding, a coarse grained model without homology information or explicit side chains outperforms current homology-based secondary structure prediction methods. The computationally rapid algorithm also generates tertiary structures with backbone conformations of comparable accuracy to existing all-atom methods for many small proteins, particularly for low homology sequences. Given appropriate search strategies and scoring functions, reduced representations can accurately predict secondary structure as well as three-dimensional structures, thereby increasing the size of proteins approachable by \textit{ab initio} methods and the accuracy of template-based methods, in particular for sequences with low homology. In addition, we will discuss recent advances in understanding non-linear electrostatic contributions to transfer free energies in continuum electrostatic models.   

Temporal and Spatial Distributions of Water in Ion-Containing Perfluorosulfonic Polymers

The spatial distribution and the molecular dynamics of water in perfluorinated ionomer polymeric membranes (Nafion 11x in the acid form) were quantified at several hydration levels by Atomic Force Microscopy and Dielectric Relaxation Spectroscopy, respectively. A variety of concurrent AFM modes, including interleave and intermittent contact methods, is necessary to map the water-containing domains on the polymer surfaces, whereas at least two different dielectric relaxation setups are needed to record the range of water dynamics that develop in these systems as the hydration level changes. The competition between sulfonic-group/water attraction and water/water hydrogen-bonding, in addition to confinement effects, give rise to at least three ``states'' of water, manifested through distinct dynamical behaviors: The fastest process observed was identified as the cooperative picosecond relaxation of free/isotropic, bulk-like water, whereas the slowest process --with microsecond relaxation times-- corresponds to water molecules strongly bound to the charged sulfonic groups. An intermediate relaxation, in the picosecond range and about three times slower than those of bulk water, is shown to contain substantial dynamical heterogeneities and most probably corresponds to a variety of local environments that are cumulatively defined as ``loosely bound'' water. AFM studies, probing the same surfaces at various hydration levels, provides insights on the location and geometry of the water domains that contribute to the various dynamical ``states''. Both the spatial and temporal distributions of water are sensitive to the sample preparation conditions, especially with respect to the geometry and dynamics of the ``loosely bound'' water domains.   

Microrheological studies of solvent-response dynamics of polyelectrolytes

We have developed a dialysis cell for microrheology, which provides unique capabilities for studying microstructural dynamics of macromolecular solutions due to sudden changes in solvent composition (e.g., ionic strength, pH, solvent/cosolvent ratio). The device was used to determine the response of sodium sulphonated polystyrene (NaPSS) solutions of different molecular weights to changes in counterion concentration. In general, polyelectrolyte chains collapse upon addition of counter-ions, but recent numerical simulations by Hsiao and Luijten (PRL 97, 2006) predicted reexpansion at high concentrations of multivalent counterions. We tested and confirmed these predictions for trivalent chloride salts (Al, In, Tl), although the effect is subtle and strongly varies between the cations investigated. Another study employed the dialysis cell to characterize pH-induced swelling and deswelling of colloidal microgel particles of a poly(N-isopropylacrylamide)-co-(acrylic acid) copolymer. The acid copolymer caases pH-responsiveness, swelling the particles at high pH due to deprotonation. In dilute suspensions of these particles, we studied the swelling response for different AAc fractions as a function of pH. In the concentrated suspensions, pH-induced particle expansion can cause transitions between fluid, glassy and crystalline phases. Data will be presented on the dynamics of the observed phase behavior.   

Ion- and pH-dependent volume transitions in biopolymer gels

Swelling and collapse of polyelectrolyte gels are the result of a balance of different interactions that control the osmotic pressure and network elasticity. In biopolymer systems ions often play a central role in determining the phase behavior. For example, DNA condensation induced by multivalent cations is crucial for its packaging. It is known that biological processes, such as nerve excitation and muscle contraction, are mediated by divalent cations. In general, relatively little is known about the interaction between multivalent ions and charged biopolymers due to the lack of an appropriate theory and the absence of a sufficiently broad base of experimental data. Recent experimental observations made by anomalous small-angle X-ray scattering indicate that the spatial extent of the counterion cloud is significantly reduced in the case of divalent ions relative to the monovalent ions. An understanding of ion induced swelling/collapse transition in polyelectrolyte gels may shed light on the mechanism of important physiological processes. We compare the effects of pH, ionic strength and counterion valence on the structure and osmotic properties of biopolymer gels. Systematic studies made on DNA gels indicate that monovalent salts gradually reduce gel swelling but do not cause discontinuous volume transition. Introducing calcium ions into the gels produces a reversible volume change. Similarly, decreasing the pH in the surrounding environment leads to shrinkage of the swollen networks. Scattering observations reveal that cations mediate the equilibrium properties by modifying the local environment and the organization of the polymer chains. Osmotic pressure measurements detect significant differences between the effects of pH and ion valence.