Temperature-Induced Unfolding of Ribonuclease A Embedded in Spherical Polyelectrolyte Brushes

Understanding the Interaction of Polyelectrolyte Architectures with Proteins and Biosystems

The counterions neutralizing the charges on polyelectrolytes such as DNA or heparin may dissociate in water and greatly influence the interaction of such polyelectrolytes with biomolecules, particularly proteins. In this Review we give an overview of studies on the interaction of proteins with polyelectrolytes and how this knowledge can be used for medical applications. Counterion release was identified as the main driving force for the binding of proteins to polyelectrolytes: Patches of positive charge become multivalent counterions of the polyelectrolyte and lead to the release of counterions from the polyelectrolyte and a concomitant increase in entropy. This is shown from investigations on the interaction of proteins with natural and synthetic polyelectrolytes. Special emphasis is paid to sulfated dendritic polyglycerols (dPGS). The Review demonstrates that we are moving to a better understanding of charge-charge interactions in systems of biological relevance. Research along these lines will aid and promote the design of synthetic polyelectrolytes for medical applications.

Interaction of Proteins with Polyelectrolytes: Comparison of Theory to Experiment

We discuss recent investigations of the interaction of polyelectrolytes with proteins. In particular, we review our recent studies on the interaction of simple proteins such as human serum albumin (HSA) and lysozyme with linear polyelectrolytes, charged dendrimers, charged networks, and polyelectrolyte brushes. In all cases discussed here, we combined experimental work with molecular dynamics (MD) simulations and mean-field theories. In particular, isothermal titration calorimetry (ITC) has been employed to obtain the respective binding constants K_b and the Gibbs free energy of binding. MD simulations with explicit counterions but implicit water demonstrate that counterion release is the main driving force for the binding of proteins to strongly charged polyelectrolytes: patches of positive charges located on the surface of the protein become multivalent counterions of the polyelectrolyte, thereby releasing a number of counterions condensed on the polyelectrolyte. The binding Gibbs free energy due to counterion release is predicted to scale with the logarithm of the salt concentration in the system, which is verified by both simulations and experiment. In several cases, namely, for the interaction of proteins with linear polyelectrolytes and highly charged hydrophilic dendrimers, the binding constant could be calculated from simulations to very good approximation. This finding demonstrated that in these cases explicit hydration effects do not contribute to the Gibbs free energy of binding. The Gibbs free energy can also be used to predict the kinetics of protein uptake by microgels for a given system by applying dynamic density functional theory. The entire discussion demonstrates that the direct comparison of theory with experiments can lead to a full understanding of the interaction of proteins with charged polymers. Possible implications for applications, such as drug design, are discussed.

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.

Counterion-Specific Protein Adsorption on Polyelectrolyte Brushes

Protein adsorption is an important issue in biorelated fields. We have investigated the protein adsorption on the poly(ionic liquid) (PIL) brushes in the presence of different types of counterions. The protein adsorption is driven by a decrease in osmotic pressure within the brushes with an increase in entropy via the release of counterions. Our study demonstrates that counterion specificity has a significant influence on protein adsorption on the PIL brushes. There have been two different regimes for counterion-specific protein adsorption. When the released counterions cannot bind to the protein surface, the counterion-specific protein adsorption is dominated by the ion-specific counterion condensation within the PIL brushes. If the released counterions can bind to the protein surface, then counterion-specific protein adsorption is dominated by the ion-specific rebinding of released counterions on the protein surface. This work opens up a new opportunity for controlling protein adsorption on polyelectrolyte brushes.

Prevention of thermally induced aggregation of IgG antibodies by noncovalent interaction with poly(acrylate) derivatives

Prevention of thermal aggregation of antibodies in aqueous solutions was achieved by noncovalent association with hydrophobically modified poly(acrylate) copolymers. Using a polyclonal immunoglobin G (IgG) as a model system for antibodies, we have studied the mechanisms by which this multidomain protein interacts with polyanions when incubated at physiological pH and at temperatures below and above the protein unfolding/denaturation temperature, in salt-free solutions and in 0.1 M NaCl solutions. The polyanions selected were sodium poly(acrylates), random copolymers of sodium acrylate and N-n-octadecylacrylamide (3 mol %), and a random copolymer of sodium acrylate, N-n-octylacrylamide (25 mol %), and N-isopropylacrylamide (40 mol %). They were derived from two poly(acrylic acid) parent chains of Mw 5000 and 150000 g·mol(-1). The IgG/polyanion interactions were monitored by static and dynamic light scattering, fluorescence correlation spectroscopy, capillary zone electrophoresis, and high sensitivity differential scanning calorimetry. In salt-free solutions, the hydrophilic PAA chains form complexes with IgG upon thermal unfolding of the protein (1:1 w/w IgG/PAA), but they do not interact with native IgG. The complexes exhibit a remarkable protective effect against IgG aggregation and maintain low aggregation numbers (average degree of oligomerization <12 at a temperature up to 85 °C). These interactions are screened in 0.1 M NaCl and, consequently, PAAs lose their protective effect. Amphiphilic PAA derivatives (1:1 w/w IgG/polymer) are able to prevent thermal aggregation (preserving IgG monomers) or retard aggregation of IgG (formation of oligomers and slow growth), revealing the importance of both hydrophobic interactions and modulation of the Coulomb interactions with or without NaCl present. This study leads the way toward the design of new formulations of therapeutic proteins using noncovalent 1:1 polymer/protein association that are transient and require a markedly lower additive concentration compared to conventional osmolyte protecting agents. They do not modify IgG permanently, which is an asset for applications in therapeutic protein formulations since the in vivo efficacy of the protein should not be affected.

Supramolecular Structures Generated by Spherical Polyelectrolyte Brushes and their Application in Catalysis

We survey recent studies on composite particles made from spherical polyelectrolyte brushes (SPB) and catalytically active nanoparticles or enzymes. SPB consist of a solid core (diameter: ca. 100 nm) onto which long chains of anionic or cationic polyelectrolyte (PE) are densely grafted ("PE brush"). Immersed in water the PE layer affixed to the colloidal core will swell due to the enormous osmotic pressure of the confined counterions ("osmotic brush"). This confinement of the counterions can be used to generate metal nanoparticles on the surface of the SPB. Moreover, enzymes can be immobilized within the PE layer. In both cases, the resulting composite particles are stable against coagulation and can be easily handled and filtered off. The catalytic activity of both systems is largely preserved in case of the enzymes, in case of the metal nanoparticles it is even enhanced. Thus, the SPB present an excellent carrier system for applications in catalysis.

Responsive brush layers: from tailored gradients to reversibly assembled nanoparticles

We present a condensed overview of the recent developments of novel responsive thin polymer films from end-tethered chains (polymer brushes), which are different from conventional, uniform, and planar brush layers. For this discussion, we selected two types of recently introduced surface layers: binary brush layers with variable chemical composition forming a controllable gradient of composition and properties in a selected direction and brush layers either grafted directly to inorganic nanoparticles to form hybrid core-shell structures or combined with inorganic nanoparticles embedded into this layer. Unlike traditional brush layers, such a design brings a novel set of responsive surface properties allowing for capillary-driven microfluidic motion, combinatorial-like multiplexing response, reversible aggregation and dis-assembly of nanoparticles, fabrication of ultrahydrophobic coatings, and switchable mass transport across interfaces.

Review on the dynamics and micro-structure of pH-responsive nano-colloidal systems

This review presents an overview on the research on pH-responsive microgel particles in the last 10 years. Microgels are cross-linked latex particles that are swollen in a good solvent. Significant quantitative studies have been conducted to investigate the swelling behavior (microscopic) and rheological (macroscopic) properties of the pH-responsive microgel particles as a function of neutralization degree, ionic strength, and cross-linked density. Mono-dispersed, alkali-swellable microgels containing carboxylic acid lattices, whose properties display extreme pH sensitivity in water is considered in detail in terms of swelling behavior and rheological properties. Their stability in solution and ability to undergo reversible volume phase transitions in response to pH makes them ideal model systems for the development of a semi-empirical as well as theoretical approach for predicting the viscosity of dilute and concentrated hard and soft sphere systems. The review concludes with a discussion of some recent applications of pH-responsive microgel particles.

Structure and protein binding capacity of a planar PAA brush

We performed neutron reflectometry (NR) and total internal reflection fluorescence (TIRF) spectroscopy to characterize the structure and the protein binding capacity of a planar poly(acrylic acid) (PAA) brush at different temperatures. A PAA brush was prepared by spin-coating planar quartz or silicon wafers with a thin film of poly(styrene). Then, the diblock copolymer poly(styrene)-poly(acrylic acid) was deposited on these modified wafers using the Langmuir-Schäfer or Langmuir-Blodgett technique. PAA grafting densities of about 0.1 chains per nm2 were obtained. The NR experiments indicate a remarkable swelling of the PAA brush in contact with a buffer solution, when it is heated to 40 degrees C for several hours. The swollen brush structure remains upon cooling back to 20 degrees C suggesting a disentanglement of the initially formed PAA brush by the temporary heating. At pD = 6.7, the protein bovine serum albumin (BSA) with a negative net charge is strongly adsorbed to the swollen PAA brush. From the scattering length density profiles obtained from the NR curves, an almost homogeneous filling of the whole PAA brush space with BSA molecules can be deduced corresponding to an average BSA volume fraction of about 7-10% and an adsorbed protein mass of about 1.4 mg m-2. By analyzing the TIRF experiments, it is found that BSA adsorption is enhanced when increasing the temperature which represents an evidence for an entropic driving force for protein adsorption. However, the mechanism of BSA adsorption at a PAA brush appears to be different at 20 and 40 degrees C.

Incorporation of styrene enhances recognition of ribonuclease A by molecularly imprinted polymers

Ribonuclease A (RNase A) is an RNA-cleaving enzyme characterized by its high conformational stability and strong catalytic activity. This enzyme is ubiquitous in living organisms and is difficult to inactivate. In polymerase chain reaction (PCR) RNase activity is removed by adding inhibitors. Molecularly imprinted polymers (MIPs) with high selectivity, high stability, low cost and facile synthesis could prove useful in extraction of target molecules, such as RNase A, from reaction mixtures. In this investigation, MIPs were synthesized from the monomers styrene and polyethyleneglycol 400 dimethacrylate (PEG400DMA) in several different ratios. Styrene as a functional monomer gave MIPs with a higher affinity for RNase A than other functional monomers tested, according to both enzyme-linked immnuosorbent assay (ELISA) and isothermal titration calorimetry (ITC). The optimum volume ratio of styrene/PEG400DMA was 20/100 at 25 degrees C, and this ratio maximized the rebinding efficiency of RNase A to MIPs. Isothermal titration calorimetry was also used, and could be useful to design the composition of molecularly imprinted polymers for various target molecules.

Effects of rare earth ions on the conformational stability of anticoagulation factor II fromAgkistrodon acutus venom probed by fluorescent spectroscopy

Anticoagulation factor II (ACF II) isolated from the venom of Agkistrodon acutus is an activated coagulation factor X-binding protein in a Ca(2+)-dependent fashion with marked anticoagulant activity. The equilibrium unfolding of rare earth ions (RE(3+))-reconstituted ACF II in guanidine hydrochloride (GdnHCl) solution was studied by fluorescence. The GdnHCl-induced unfolding of RE(3+) (Nd(3+), Sm(3+), Eu(3+), Gd(3+))-reconstituted ACF II follows a three-state transition with a stable intermediate state. Substitutions of the RE(3+) ions for Ca(2+) in ACF II decrease the conformational stability of its native state but markedly increase the conformational stability of its intermediate state. The free energy change of RE(3+)-ACF II from the intermediate state to denatured state linearly increases with the increase of ionic potentials of bound metal ions (Ca(2+), Nd(3+), Sm(3+), Eu(3+), and Gd(3+)). The refolding of ACF II from the unfolded state to the intermediate state can be induced merely by adding 10 microM RE(3+) ions without changing the concentration of the denaturant. The kinetic results of the RE(3+)-induced refolding provide evidence indicating that the intermediate state of RE(3+)-ACF II consists of at least two refolding phases and that the refolding rate constant values of the faster phase decrease with the increase of the difference between the radii of Ca(2+) and RE(3+), but the refolding rate constant values of the slower phase are similar to each other. The results of this study indicate that the size of metal ion is the major factor responsible for the metal ion-induced conformational stabilization of the native ACF II, while the metal ionic potential plays a predominant role in stabilizing the conformation for the intermediate state.