Counterion-Specific Protein Adsorption on Polyelectrolyte Brushes

All-atom molecular dynamics simulations of polymer and polyelectrolyte brushes

Densely grafted polymer and polyelectrolyte (PE) brushes, owing to their significant abilities to functionalize surfaces for a plethora of applications in sensing, diagnostics, current rectification, surface wettability modification, drug delivery, and oil recovery, have attracted significant attention over the past several decades. Unfortunately, most of the attention has primarily focused on understanding the properties of the grafted polymer and the PE chains with little attention devoted to studying the behavior of the brush-supported ions (counterions needed to screen the PE chains) and water molecules. Over the past few years, our group has been at the forefront of addressing this gap: we have employed all-atom molecular dynamics (MD) simulations for studying a wide variety of polymer and PE brush systems with specific attention to unraveling the properties and behavior of the brush-supported water molecules and ions. Our findings have revealed some of the most fascinating properties of such brush-supported ions and water molecules, including the most remarkable control of nanofluidic transport afforded by the specific ion and water responses induced by the PE brushes grafted on the inner walls of the nanochannel. This feature article aims to summarize some of our key contributions associated with such atomistic simulations of polymer and PE brushes and brush-supported water molecules and counterions.

Interactions on Proteins Arising from the Self-Assembly of a Polyelectrolyte Brush

Surfaces grafted with polyelectrolyte chains for excellent performance in protein antifouling are highly desired in many applications, such as biomedical implants and devices. In general, the adsorbing/resisting behaviors of proteins can be mainly attributed to the electrostatic interactions that are associated with the charge properties of proteins and polyelectrolytes. By coarse-grained molecular dynamics simulations, we examined the self-assembled structures of polyanion and polyzwitterion brushes as well as the interactions on negatively and positively charged proteins. We found that in addition to charges, the structural polarization induced by self-assembly with a certain charge distribution shows significant influences on protein behavior. The large-scale dipole-dipole interactions between brushes and proteins can dominate the behavior of proteins on the brushes under certain circumstances. To ensure simulation accuracy, we compared two models and found a polar Martini model that explicitly treats electrostatic interactions as long-ranged ones, giving a more reasonable structural description compared with the normal Martini model that truncates electrostatic interactions.

Tuning the Light Response of Strong Polyelectrolyte Brushes with Counterions

In this work, a negatively charged poly(3-sulfopropyl methacrylate potassium) (PSPMA) brush has been employed as a model system to demonstrate the tuning of the light response of strong polyelectrolyte brushes (SPBs) with counterions. The substitution of K⁺ counterions by azobenzene-containing counterions (Azo-N⁺) renders the PSPMA brush light-responsive in aqueous solutions. Nevertheless, the strength of the light response of the PSPMA brush is weak due to the inefficient disassembly of the micelle-like aggregates in the brush upon irradiation with ultraviolet light. Counterion mixtures of Azo-N⁺ and K⁺ are employed to realize a strong light response of the PSPMA brush by incorporating a reasonable amount of Azo-N⁺ counterions into the brush. The strength of the light response of the PSPMA brush can be tuned by the mole ratio of Azo-N⁺ to K⁺. Furthermore, properties including the hydration and conformation of the PSPMA brush can be reversibly switched via alternating ultraviolet and visible light irradiation. This work opens up the opportunities available for the use of counterions to tune the light response of SPBs.

Ionic effects on synthetic polymers: from solutions to brushes and gels

The ionic effects on synthetic polymers have attracted extensive attention due to the crucial role of ions in the determination of the properties of synthetic polymers. This review places the focus on specific ion effects, multivalent ion effects, and ionic hydrophilicity/hydrophobicity effects in synthetic polymer systems from solutions to brushes and gels. The specific ion effects on neutral polymers are determined by both the direct and indirect specific ion-polymer interactions, whereas the ion specificities of charged polymers are mainly dominated by the specific ion-pairing interactions. The ionic cross-linking effect exerted by the multivalent ions is widely used to tune the properties of polyelectrolytes, while the reentrant behavior of polyelectrolytes in the presence of multivalent ions still remains poorly understood. The ionic hydrophilicity/hydrophobicity effects not only can be applied to make strong polyelectrolytes thermosensitive, but also can be used to prepare polymeric nano-objects and to control the wettability of polyelectrolyte brush-modified surfaces. The not well-studied ionic hydrogen bond effects are also discussed in the last section of this review.

Influence of charge sequence on the adsorption of polyelectrolytes to oppositely-charged polyelectrolyte brushes

When a solution of polyanionic chains is placed in contact with a polycationic brush, the polyanions adsorb into the brush. We investigate the influence of the charge sequences of the free and bound species on the thermodynamics of polyelectrolyte adsorption. As model systems, we consider free and brush polyelectrolytes with either block or alternating charge sequences, and study the adsorption process using coarse-grained Langevin dynamics with implicit solvent, explicit counterions, and excess salt. Free energy, internal energy, and entropy of adsorption are computed using umbrella sampling methods. When the number of polyanions exceed the number of polycations, the brush becomes overcharged. Free chains adsorb most strongly when both free and tethered chains have a block charge sequence, and most weakly when both species have an alternating sequence. Adsorption is stronger when the free polyanion has a block sequence and the tethered polycation is alternating than in the reverse case of an alternating free polymer and a tethered block copolymer. Sequence-dependent effects are shown to be largely energetic, rather than entropic, in origin.

Tuning the Properties of Charged Polymers at the Solid/Liquid Interface with Ions

In conventional theories, where ions are treated as point charges, the properties of charged polymers can be tuned using ions via the ionic strength. However, this article will show that the properties of charged polymers at the solid/liquid interface, including charged polymer brushes and polyelectrolyte multilayers, can be tuned by ions beyond ionic strength effects. Ion specificity, multivalency, ionic hydrogen bonding, and ionic hydrophobicity/hydrophilicity are used to tune a range of properties of charged polymers at the solid/liquid interface such as hydration, conformation, stiffness, surface wettability, lubricity, adhesion, and protein adsorption. The ionic effects demonstrated here greatly broaden our understanding of the use of ions to tune the interfacial properties of charged polymers. It is anticipated that these ionic effects can be further expanded by incorporating new types of important ion-charged polymer interactions and can also be extended to neutral polymer systems.

Anion Specificity in Dimethyl Sulfoxide-Water Mixtures Exemplified by a Thermosensitive Polymer

In the present work, we have investigated the anion-specific upper critical solution temperature (UCST) behavior of polymer-supported borinic acid (PBA) in dimethyl sulfoxide-water (DMSO-H₂O) mixtures. An inverted V-shaped series CH₃COO^- < Cl^- < salt-free > NO₃^- > ClO₄^- > SCN^- is observed in terms of the anion-specific UCST of PBA in the DMSO-H₂O mixtures. Both direct anion-polymer interactions and indirect solvent-mediated anion-polymer interactions are involved in the specific anion effect on the UCST behavior of PBA. The direct binding of anions to the PBA surface generates a salting-in effect on PBA, causing the UCST for the different types of anions to increase from chaotropic to kosmotropic anions due to the stronger binding of the more chaotropic anions. On the other hand, the indirect anionic polarization of hydrogen bonding between PBA and DMSO molecules also produces a salting-in effect on PBA, leading the UCST for the different types of anions to increase from kosmotropic to chaotropic anions because of the stronger capability of the more kosmotropic anions to polarize the hydrogen bonding. Thus, the dominating anion-PBA interactions change from the direct anion binding to the indirect anionic polarization of hydrogen bonding as the anions change from chaotropes to kosmotropes. The observed inverted V-shaped series suggests that the specific anion effect on the UCST behavior of PBA in the DMSO-H₂O mixtures is determined by the combined effects of the binding of anions to the PBA surface and the anionic polarization of hydrogen bonding between PBA and DMSO molecules.

Measuring the Interactions between Protein-Coated Microspheres and Polymer Brushes in Aqueous Solutions

Hydrophilic or zwitterionic polymer-functionalized surfaces have become attractive biomaterials in bioscience and technology due to their excellent protein-resistant ability. Understanding the fundamental interactions between proteins and polymers plays an essential role in the surface design of biomaterials. In this work, we studied the interactions between bovine serum albumin (BSA) and two sorts of polymer brushes including zwitterionic poly(carboxybetaine methacrylate) (PCBMA) and hydrophilic poly[oligo(ethylene glycol) methyl ether methacrylate] (POEGMA) in NaCl aqueous solutions directly with a self-established total internal reflection microscope (TIRM) to provide a better understanding of the underlying nonfouling mechanism of polymers. Our results indicate that both the surface charge and brushes length can affect protein adsorption through electrostatic and steric repulsions, respectively. Both PCBMA- and POEGMA-coated surfaces display negative charge properties due to incomplete coverage and ionic adsorption. As a result, strong electrostatic repulsions between proteins and negatively charged polymer-coated surfaces could contribute to the resistance of protein-coated particles in solutions with low ionic strength (0.1, 0.5, and 1 mM) and disappear in solutions with high ionic strength (10 mM). The measured interaction profiles demonstrate that PCBMA brushes could provide apparent steric forces only at high ionic strength (10 mM), where zwitterionic brushes exhibit a relatively extended conformation with a lack of electrostatic forces between intra- and interpolymers. In contrast, the steric repulsion between proteins and POEGMA brushes appears when particles diffuse at low positions in all salt concentrations (0.1-10 mM) with similar steric decay lengths, which results from the unresponsiveness of POEGMA brushes to the salt stimulus.

Binding and Release between Polymeric Carrier and Protein Drug: pH-Mediated Interplay of Coulomb Forces, Hydrogen Bonding, van der Waals Interactions, and Entropy

The accelerating search for new types of drugs and delivery strategies poses challenge to understanding the mechanism of delivery. To this end, a detailed atomistic picture of binding between the drug and carrier is quintessential. Although many studies focus on the electrostatics of drug-vector interactions, it has also been pointed out that entropic factors relating to water and counterions can play an important role. By carrying out extensive molecular dynamics simulations and subsequently validating with experiments, we shed light herein on the binding in aqueous solution between a protein drug and polymeric carrier. We examined the complexation between the polymer poly(ethylene glycol) methyl ether acrylate-b-poly(carboxyethyl acrylate (PEGMEA-b-PCEA) and the protein egg white lysozyme, a system that acts as a model for polymer-vector/protein-drug delivery systems. The complexation has been visualized and characterized using contact maps and hydrogen bonding analyses for five independent simulations of the complex, each running over 100 ns. Binding at physiological pH is, as expected, mediated by Coulombic attraction between the positively charged protein and negatively charged carboxylate groups on the polymer. However, we find that consideration of electrostatics alone is insufficient to explain the complexation behavior at low pH. Intracomplex hydrogen bonds, van der Waals interactions, as well as water-water interactions dictate that the polymer does not release the protein at pH 4.8 or indeed at pH 3.2 even though the Coulombic attractions are largely removed as carboxylate groups on the polymer become titrated. Experiments in aqueous solution carried out at pH 7.0, 4.5, and 3.0 confirm the veracity of the computed binding behavior. Overall, these combined simulation and experimental results illustrate that coulomb interactions need to be complemented with consideration of other entropic forces, mediated by van der Waals interactions and hydrogen bonding, to search for adequate descriptors to predict binding and release properties of polymer-protein complexes. Advances in computational power over the past decade make atomistic molecular dynamics simulations such as implemented here one of the few avenues currently available to elucidate the complexity of these interactions and provide insights toward finding adequate descriptors. Thus, there remains much room for improvement of design principles for efficient capture and release delivery systems.

Effect of Charged Group Spacer Length on Hydration State in Zwitterionic Poly(sulfobetaine) Brushes

Effect of alkyl chain spacer length between the charged groups (CSL) in zwitterionic poly(sulfobetaine) (PSB) brushes on the hydration state was investigated. PSB brushes with ethyl (PMAES), propyl (PMAPS), or butyl (PMABS) CSL were prepared by surface-initiated atom transfer radical polymerization on silicon wafers. Hydration states of the PSB brushes in aqueous solutions and/or humid vapor were investigated by contact angle measurement, infrared spectroscopy, AFM observation, and neutron reflectivity. The PSB brushes are swollen in humid air and deionized water due to the hydration of the charged groups leading to the reduction of hydrated PSB brushes/water interfacial free energy. The hydrated PSB brushes exhibit clear interface with low interfacial roughness due to networking of the PSB brush chains through association of the SBs. The hydrated PSB brushes produce diffusive swollen layer in the presence of NaCl because of the charge screening followed by SB dissociation by the bound ions. The ionic strength sensitivity in the hydration got more significant with increasing the CSL in SBs because of the augmentation in partial charge by charged group separation.

Interaction of Charged Patchy Protein Models with Like-Charged Polyelectrolyte Brushes

We study the adsorption of charged patchy particle models (CPPMs) on a thin film of a like-charged and dense polyelectrolyte (PE) brush (of 50 monomers per chain) by means of implicit-solvent, explicit-salt Langevin dynamics computer simulations. Our previously introduced set of CPPMs embraces well-defined one- and two-patched spherical globules, each of the same net charge and (nanometer) size, with mono- and multipole moments comparable to those of small globular proteins. We focus on electrostatic effects on the adsorption far away from the isoelectric point of typical proteins, i.e., where charge regulation plays no role. Despite the same net charge of the brush and globule, we observe large binding affinities up to tens of the thermal energy, k_BT, which are enhanced by decreasing salt concentration and increasing charge of the patch(es). Our analysis of the distance-resolved potentials of mean force together with a phenomenological description of all leading interaction contributions shows that the attraction is strongest at the brush surface, driven by multipolar, Born (self-energy), and counterion-release contributions, dominating locally over the monopolar and steric repulsions.

Smart Polymers with Special Wettability

Surface wettability plays a key role in addressing issues ranging from basic life activities to our daily life, and thus being able to control it is an attractive goal. Learning from nature, both of its structure and function, brings us much inspiration in designing smart polymers to tackle this major challenge. Life functions particularly depend on biomolecular recognition-induced interfacial properties from the aqueous phase onto either "soft" cell and tissue or "hard" inorganic bone and tooth surfaces. The driving force is noncovalent weak interactions rather than strong covalent combinations. An overview is provided of the weak interactions that perform vital actions in mediating biological processes, which serve as a basis for elaborating multi-component polymers with special wettabilities. The role of smart polymers from molecular recognitions to macroscopic properties are highlighted. The rationale is that highly selective weak interactions are capable of creating a dynamic synergetic communication in the building components of polymers. Biomolecules could selectively induce conformational transitions of polymer chains, and then drive a switching of physicochemical properties, e.g., roughness, stiffness and compositions, which are an integrated embodiment of macroscopic surface wettabilities.

Volume profile of α-chymotrypsin during adsorption and enzymatic reaction on a poly(acrylic acid) brush

The catalytic rate of α-chymotrypsin that is adsorbed on a poly(acrylic acid) brush can be enhanced by pressure.

Interactions between Polyelectrolyte Brushes and Hofmeister Ions: Chaotropes versus Kosmotropes

We have investigated the interactions between the positively charged poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) brushes and the Hofmeister anions and the interactions between the negatively charged poly(3-sulfopropyl methacrylate potassium) (PSPMA) brushes and the Hofmeister cations using a combination of quartz crystal microbalance with dissipation and spectroscopic ellipsometry. A V-shaped anion series is observed in terms of the ion-specific interactions between the PMETAC brushes and the Hofmeister anions. We have found that the chaotropic and kosmotropic anions interact with the PMETAC brushes in different manners. The ion-specific interactions between the PMETAC brushes and the chaotropic anions are dominated by the direct interactions between the anions and the positively charged quaternary ammonium group via ion pairing mediated by ionic hydration strength or polarizability, whereas the ion-specific interactions between the PMETAC brushes and the kosmotropic anions are dominated by the competition for water molecules between the anions and the brushes. The ion-specific interactions between the PMETAC brushes and the anions have significant influences on both the hydration and the conformation of the brushes. The cations exhibit weaker specific ion effects on the PSPMA brushes in comparison with the specific anion effects on the PMETAC brushes.