Effect of Protein Adsorption and Ionic Strength on the Equilibrium Partition Coefficient of Ionizable Macromolecules in Charged Nanopores

Molecular simulation study of feruloyl esterase adsorption on charged surfaces: effects of surface charge density and ionic strength

The surrounding conditions, such as surface charge density and ionic strength, play an important role in enzyme adsorption. The adsorption of a nonmodular type-A feruloyl esterase from Aspergillus niger (AnFaeA) on charged surfaces was investigated by parallel tempering Monte Carlo (PTMC) and all-atom molecular dynamics (AAMD) simulations at different surface charge densities (±0.05 and ±0.16 C·m(-2)) and ionic strengths (0.007 and 0.154 M). The adsorption energy, orientation, and conformational changes were analyzed. Simulation results show that whether AnFaeA can adsorb onto a charged surface is mainly controlled by electrostatic interactions between AnFaeA and the charged surface. The electrostatic interactions between AnFaeA and charged surfaces are weakened when the ionic strength increases. The positively charged surface at low surface charge density and high ionic strength conditions can maximize the utilization of the immobilized AnFaeA. The counterion layer plays a key role in the adsorption of AnFaeA on the negatively charged COOH-SAM. The native conformation of AnFaeA is well preserved under all of these conditions. The results of this work can be used for the controlled immobilization of AnFaeA.

Effect of electrostatic interactions on rejection of capsular and spherical particles from porous membranes: theory and experiment

HYPOTHESES

Particle rejection from porous membranes will increase when particle and membrane carry like charges. The influence of charge on particle rejection can be modeled by first solving the Poisson-Boltzmann equation for the electrostatic particle-pore wall interaction energy, enabling one to predict the cross sectional particle concentration in a pore. Rejection coefficients can then be predicted by combining the Boltzmann factor with a hydrodynamic lag coefficient.

EXPERIMENTS

Rejection experiments were conducted with three different spherical colloidal silica particles, a spherical virus (PRD1) and gold nanorods of two different aspect ratios (ratio of length to diameter). Track-etched polycarbonate microfiltration and ultrafiltration membranes having nearly parallel pores of cylindrical cross-section were used. Experiments were conducted under conditions where both particle and membrane carried a negative charge as well as under conditions where surface charges had minimal impact. Experiments were designed to cover a broad range of dimensionless particle sizes under conditions when convection dominated particle transport.

FINDINGS

Model predictions and experimental measurements demonstrate that particle rejection can be enhanced significantly when particle and pore carry like charges.

Protein diffusion through charged nanopores with different radii at low ionic strength

The diffusion of two similar molecular weight proteins, bovine serum albumin (BSA) and bovine haemoglobin (BHb), through nanoporous charged membranes with a wide range of pore radii is studied at low ionic strength. The effects of the solution pH and the membrane pore diameter on the pore permeability allow quantifying the electrostatic interaction between the charged pore and the protein. Because of the large screening Debye length, both surface and bulk diffusion occur simultaneously. By increasing the pore diameter, the permeability tends to the bulk self-diffusion coefficient for each protein. By decreasing the pore diameter, the charges on the pore surface electrostatically hinder the transport even at the isoelectric point of the protein. Surprisingly, even at pore sizes 100 times larger than the protein, the electrostatic hindrance still plays a major role in the transport. The experimental data are qualitatively explained using a two-region model for the membrane pore and approximated equations for the pH dependence of the protein and pore charges. The experimental and theoretical results should be useful for designing protein separation processes based on nanoporous charged membranes.

Attractive double-layer forces between neutral hydrophobic and neutral hydrophilic surfaces

The interaction between surface patches of proteins with different surface properties has a vital role to play driving conformational changes in proteins in different salt solutions. We demonstrate the existence of ion-specific attractive double-layer forces between neutral hydrophobic and hydrophilic surfaces in the presence of certain salt solutions. This is performed by solving a generalized Poisson-Boltzmann equation for two unequal surfaces. In the calculations, we utilize parametrized ion-surface potentials and dielectric-constant profiles deduced from recent non-primitive-model molecular dynamics simulations that partially account for molecular structure and hydration effects.

Polyelectrolyte layer-by-layer deposition in cylindrical nanopores

Layer-by-layer (LbL) deposition of polyelectrolytes within nanopores in terms of the pore size and the ionic strength was experimentally studied. Anodic aluminum oxide (AAO) membranes, which have aligned, cylindrical, nonintersecting pores, were used as a model nanoporous system. Furthermore, the AAO membranes were also employed as planar optical waveguides to enable in situ monitoring of the LbL process within the nanopores by optical waveguide spectroscopy (OWS). Structurally well-defined N,N-disubstituted hydrazine phosphorus-containing dendrimers of the fourth generation, with peripherally charged groups and diameters of approximately 7 nm, were used as the model polyelectrolytes. The pore diameter of the AAO was varied between 30-116 nm and the ionic strength was varied over 3 orders of magnitude. The dependence of the deposited layer thickness on ionic strength within the nanopores is found to be significantly stronger than LbL deposition on a planar surface. Furthermore, deposition within the nanopores can become inhibited even if the pore diameter is much larger than the diameter of the G4-polyelectrolyte, or if the screening length is insignificant relative to the dendrimer diameter at high ionic strengths. Our results will aid in the template preparation of polyelectrolyte multilayer nanotubes, and our experimental approach may be useful for investigating theories regarding the partitioning of nano-objects within nanopores where electrostatic interactions are dominant. Furthermore, we show that the enhanced ionic strength dependence of polyelectrolyte transport within the nanopores can be used to selectively deposit a LbL multilayer atop a nanoporous substrate.

The surface functionalization of 45S5 Bioglass-based glass-ceramic scaffolds and its impact on bioactivity

The first and foremost function of a tissue engineering scaffold is its role as a substrate for cell attachment, and their subsequent growth and proliferation. However, cells do not attach directly to the culture substrate; rather they bind to proteins that are adsorbed to the scaffold's surface. Like standard tissue culture plates, tissue engineering scaffolds can be chemically treated to couple proteins without losing the conformational functionality; a process called surface functionalization. In this work, novel highly porous 45S5 Bioglass-based scaffolds have been functionalized applying 3-AminoPropyl-TriethoxySilane (APTS) and glutaraldehyde (GA) without the use of organic solvents. The efficiency and stability of the surface modification was assessed by X-ray photoemission spectroscopy (XPS). The bioactivity of the functionalized scaffolds was investigated using simulated body fluid (SBF) and characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). It was found that the aqueous heat-treatment applied at 80 degrees C for 4 hrs during the surface functionalization procedure accelerated the structural transition of the crystalline Na2Ca2Si3O9 phase, present in the original scaffold structure as a result of the sintering process used for fabrication, to an amorphous phase during SBF immersion. The surface functionalized scaffolds exhibited an accelerated crystalline hydroxyapatite layer formation upon immersion in SBF caused by ion leaching and the increased surface roughness induced during the heat treatment step. The possible mechanisms behind this phenomenon are discussed.

Change of ζ Potential of Biocompatible Colloidal Oxide Particles upon Adsorption of Bovine Serum Albumin and Lysozyme

The amounts of negatively charged bovine serum albumin and positively charged lysozyme adsorbed on alumina, silica, titania, and zirconia particles (diameters 73 to 271 nm) in aqueous suspensions are measured. The adsorbed proteins change the zeta potentials and the isoelectric points (IEP) of the oxide particles. The added to adsorbed protein ratios at pH 7.5 are compared with the protein treated particle zeta potentials. It is found that the amounts of adsorbed proteins on the alumina, silica, and titania (but not on the zirconia) particle surfaces are highly correlated with the zeta potential. For the slightly less hydrophilic zirconia particles high amounts of protein adsorption are observed even under repulsive electrostatic conditions. One reason could be that the hydrophobic effect plays a more important role for zirconia than electrostatic interaction.

Volume exclusion effects in the ground-state dominance approximation for polyelectrolyte adsorption on charged interfaces

We consider the problem of polyelectrolyte molecules adsorbing on oppositely charged interfaces. For sufficiently long chains, the ground-state dominance approximation can be used which results in a (semi-)analytical solution of the self-consistent field equations (aSCF). Whereas existing aSCF theory assumes a low polyelectrolyte density, here the required electrostatic corrections for a high polymer density are implemented. Adsorbed polymer excludes volume for the solvent and small ions, a volume effect that also leads to a reduced dielectric permittivity and a resulting polarization term in the exchange potential. Calculations show the influence of volume exclusion on the polymer density profile.

A Modified Box Model Including Charge Regulation for Protein Adsorption in a Spherical Polyelectrolyte Brush

Recent experiments showed significant adsorption of bovine serum albumin (BSA) in spherical polyelectrolyte brushes (SPB) consisting of polyacrylic acid, even for pH values above the isoelectric point of the protein, when both protein and polyion are negatively charged. To describe these experimental findings theoretically, we have constructed a spherical box model for an annealed brush consisting of a weak polyelectrolyte that includes the adsorption of BSA. At equilibrium the chemical potential of BSA in solution equals that at each location in the brush, while the net force on the polyions (including osmotic, stretching, and excluded volume terms) is zero at each location. Protein adsorption is predicted above the isoelectric point and--in agreement with experimental data--is a strong function of ionic strength and pH. Adsorption of protein in the brush is possible because the pH in the brush is below the isoelectric point and protein reverses its charge from negative to positive when it adsorbs.

A Modified Poisson−Boltzmann Model Including Charge Regulation for the Adsorption of Ionizable Polyelectrolytes to Charged Interfaces, Applied to Lysozyme Adsorption on Silica

The equilibrium adsorption of polyelectrolytes with multiple types of ionizable groups is described using a modified Poisson-Boltzmann equation including charge regulation of both the polymer and the interface. A one-dimensional mean-field model is used in which the electrostatic potential is assumed constant in the lateral direction parallel to the surface. The electrostatic potential and ionization degrees of the different ionizable groups are calculated as function of the distance from the surface after which the electric and chemical contributions to the free energy are obtained. The various interactions between small ions, surface and polyelectrolyte are self-consistently considered in the model, such as the increase in charge of polyelectrolyte and surface upon adsorption as well as the displacement of small ions and the decrease of permittivity. These interactions may lead to complex dependencies of the adsorbed amount of polyelectrolyte on pH, ionic strength, and properties of the polymer (volume, permittivity, number, and type of ionizable groups) and of the surface (number of ionizable groups, pK, Stern capacity). For the adsorption of lysozyme on silica, the model qualitatively describes the gradual increase of adsorbed amount with pH up to a maximum value at pHc, which is below the iso-electric point, as well as the sharp decrease of adsorbed amount beyond pHc. With increasing ionic strength the adsorbed amount decreases (for pH > pHc), and pHc shifts to lower values.