Effect of Protein Surface Charge Distribution on Protein-Polyelectrolyte Complexation.

Charge anisotropy or the presence of charge patches at protein surfaces has long been thought to shift the coacervation curves of proteins and has been used to explain the ability of some proteins to coacervate on the "wrong side" of their isoelectric point. This work makes use of a panel of engineered superfolder green fluorescent protein mutants with varying surface charge distributions but equivalent net charge and a suite of strong and weak polyelectrolytes to explore this concept. A patchiness parameter, which assessed the charge correlation between points on the surface of the protein, was used to quantify the patchiness of the designed mutants. Complexation between the polyelectrolytes and proteins showed that the mutant with the largest patchiness parameter was the most likely to form complexes, while the smallest was the least likely to do so. The patchiness parameter was found to correlate well with the phase behavior of the protein-polymer mixtures, where both macrophase separation and the formation of soluble aggregates were promoted by increasing the patchiness depending on the polyelectrolyte with which the protein was mixed. Increasing total charge and increasing strength of the polyelectrolyte promote interactions for oppositely charged polyelectrolytes, while charge regulation is also key to interactions for similarly charged polyelectrolytes, which must interact selectively with oppositely charged patches.

Predicting Protein-Polymer Block Copolymer Self-Assembly from Protein Properties.

Protein-polymer bioconjugate self-assembly has attracted a great deal of attention as a method to fabricate protein nanomaterials in solution and the solid state. To identify protein properties that affect phase behavior in protein-polymer block copolymers, a library of 15 unique protein-b-poly(N-isopropylacrylamide) (PNIPAM) copolymers comprising 11 different proteins was compiled and analyzed. Many attributes of phase behavior are found to be similar among all studied bioconjugates regardless of protein properties, such as formation of micellar phases at high temperature and low concentration, lamellar ordering with increasing temperature, and disordering at high concentration, but several key protein-dependent trends are also observed. In particular, hexagonal phases are only observed for proteins within the molar mass range 20-36 kDa, where ordering quality is also significantly enhanced. While ordering is generally found to improve with increasing molecular weight outside of this range, most large bioconjugates exhibited weaker than predicted assembly, which is attributed to chain entanglement with increasing polymer molecular weight. Additionally, order-disorder transition boundaries are found to be largely uncorrelated to protein size and quality of ordering. However, the primary finding is that bioconjugate ordering can be accurately predicted using only protein molecular weight and percentage of residues contained within ß sheets. This model provides a basis for designing protein-PNIPAM bioconjugates that exhibit well-defined self-assembly and a modeling framework that can generalize to other bioconjugate chemistries.

Catalytic Biosensors from Complex Coacervate Core Micelle (C3M) Thin Films.

Enzymes have been applied to a variety of industrially and medically relevant chemistries as both catalysts and sensors. Incorporation of proteins and enzymes into complex coacervates has been demonstrated to improve the thermal, chemical, and temporal stability of enzymes in solution. In this work, a neutral-cationic block copolymer and an enzyme, alkaline phosphatase, are incorporated into complex coacervate core micelles (C3Ms) and coated onto a solid substrate to create a biocatalytic film from aqueous solution. The incorporation of photo-cross-linkable groups into the neutral block of the polymer allows the film to be cross-linked under ultraviolet light, rendering it insoluble. The morphology of the film is shown to depend most strongly on the protein loading within the film, while solvent annealing is shown to have a minimal effect. These films are then demonstrated as specific sensors for Zn2+ in solution in the presence of other metals, a model reaction for ion-selective heavy metal biosensing useful in environmental monitoring. They are shown to have low leaching and maintain sufficient activity and response for sensing for 1 month after aging under ambient conditions and at 40 °C and 50% relative humidity. The C3M immobilization method demonstrated can be applied to a wide variety of proteins with minimal chemical or genetic modification and could be used for immobilization of charged macromolecules in general to produce a wide variety of thin-film devices.

Membrane potential mediates the cellular binding of nanoparticles.

The use of nanoparticles for cellular therapeutic or sensing applications requires nanoparticles to bind, or adhere, to the cell surface. While nanoparticle parameters such as size, shape, charge, and composition are important factors in cellular binding, the cell itself must also be considered. All cells have an electrical potential across the plasma membrane driven by an ion gradient. Under standard conditions the ion gradient will result in a -10 to -100 mV potential across the membrane with a net negative charge on the cytosolic face. Using a combination of flow cytometry and fluorescence microscopy experiments and dissipative particle dynamics simulations, we have found that a decrease in membrane potential leads to decreased cellular binding of anionic nanoparticles. The decreased cellular binding of anionic nanoparticles is a general phenomenon, independent of depolarization method, nanoparticle composition, and cell type. Increased membrane potential reverses this trend resulting in increased binding of anionic nanoparticles. The cellular binding of cationic nanoparticles is minimally affected by membrane potential due to the interaction of cationic nanoparticles with cell surface proteins. The influence of membrane potential on the cellular binding of nanoparticles is especially important when considering the use of nanoparticles in the treatment or detection of diseases, such as cancer, in which the membrane potential is decreased.