A molecular theory to study the properties of end tethered polymer layers, in which the polymers have the ability to form hydrogen bonds with water is presented. The approach combines the ideas of the single-chain mean-field theory to treat tethered layers with the approach of Dormidontova (Macromolecules, 2002 35,987) to include hydrogen bonds. The generalization includes the consideration of position dependent polymer-water and water-water hydrogen bonds. The theory is applied to model poly ethylene oxide (PEO) and the predictions are compared with equivalent polymer layers that do not form hydrogen bonds. It is found that increasing the temperature lowers the solubility of the PEO and results in a collapse of the layer at high enough temperatures. The properties of the layer and their temperature dependence are shown to be the result of the coupling between the conformational entropy of the chains, the ability of the polymer to form hydrogen bonds, and the intermolecular interactions. The structural and thermodynamic properties of the PEO layers, such as the lateral pressure-area isotherms and polymer chemical potentials, are studied as a function of temperature and type of tethering surface. The possibility of phase separation of the PEO layer at high enough temperature is predicted, due to the reduced solubility induced by breaking of polymerwater hydrogen bonds. A discussion of the advantages and limitations of the theory, together with how to apply the approach to different hydrogen bonding polymers is presented.

Exploring the mechanism of specific salt effects in electrolyte solutions is an old and attractive subject. It has been gradually realized that the competition at the molecular level plays an important role. Aiming to include molecular details as many as possible, we combine molecular dynamics (MD) simulations with a molecular theory to study specific salt effects on poly(ethylene oxide) (PEO) solutions with the addition of monovalent salt. Radial distribution functions obtained from MD simulations provide microscopic structures of different components as well as interactions between various species. On the basis of these interactions, we construct the molecular theory with four assumptions: (1) an ion along with bound water in the first shell works as a single entity; (2) short-ranged interactions among various species are modeled as hydrogen-bonding interactions; (3) the ability of a hydrated ion to provide donors/acceptors for hydrogen bonding is governed by the charge density; (4) contact ion pairs are included, especially in the cases of small cations. The molecular theory is generalized with the explicit inclusion of ion-PEO, ion-water, ion-ion, water-water, and water-PEO hydrogen bonds. This means the molecular-scale structure and interaction are included within the frame of the theory. Theoretically calculated cloud points verify that the salting-out ability for alkali metal ions follows the series ofwhich is in agreement with the experimental observations. Here, the competition among ion-PEO, ion-water, and water-PEO interactions and the impact of steric repulsions induced by the introduction of ions are two essential factors determining the phase behavior of PEO solutions. The combined methods bridge the microscopic interactions and structures to the macroscopic behavior.

The binding of streptavidin to biotin located at the terminal ends of poly(ethylene oxide) tethered to a planar surface is studied using molecular theory. The theoretical model is applied to mimic experiments (Langmuir 2008(Langmuir , 24, 2472 performed using drop-shape analysis to study receptorligand binding at the oil/water interface. Our theoretical predictions show very good agreements with the experimental results. Furthermore, the theory enables us to study the thermodynamic and structural behavior of the PEO-biotin+streptavidin layer. The interfacial structure, shown by the volume fraction profiles of bound proteins and polymers, indicates that the proteins form a thick layer supported by stretched polymers, where the distribution of bound proteins is greater than the thickness of the height of one layer of proteins. When the polymer spacer is composed of PEO (3000), a thick layer with multi-layers of proteins is formed, supported by the stretched polymer chains. It was found that thick multi-layers of proteins are formed when long spacers are present or at very high protein surface coverages on short spacers. This shows that the flexibility of the polymer spacer plays an important role in determining the structure of the bound proteins due to their ability to accommodate highly distorted conformations to optimize binding and protein interactions. Protein domains are predicted when the amount of bound proteins is small due to the existence of streptavidinstreptavidin attractive interactions. As the number of proteins is increased, the competition between attractive interactions and steric repulsions determines the stability and structure of the bound layer. The theory predicts that the competition between these two forces leads to a phase separation at higher protein concentrations. The point where this transition happens depends on both spacer length and protein surface coverage and is an important consideration for practical applications of these and other similar systems. If the goal is to maximize protein binding, it is favorable to be above the layer transition, as multiple layers can accommodate greater bound protein densities. On the other hand, if the goal is to use these bound proteins as a linker group to build more complex structures, such as when avidin or streptavidin serves as a linker between two biotinylated polymers or proteins, the optimum is to be below the layer transition such that all bound linker proteins are available for further binding. II. INTRODUCTIONLigand-receptor interactions are an important control mechanism in many biological systems. For example, the binding of extracellular matrix proteins to specific receptors on the cell surface triggers signal transduction pathways allowing the cell to sense and react to its environment in Correspondence to: Igal Szleifer. NIH Public Access Author ManuscriptLangmuir. Author manuscript; available in PMC 2010 October 20. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscriptways such as aggregating with ot...