Characterization of a planar poly(acrylic acid) brush as a materials coating for controlled protein immobilization

Kinetics of Protein Adsorption at a Poly(Acrylic Acid) Brush Studied by Surface Plasmon Resonance Spectroscopy

The kinetics of protein adsorption at a planar poly(acrylic acid) (PAA) brush has been studied by surface plasmon resonance spectroscopy. A PAA brush has been prepared by spin-coating a gold sensor chip with a thin poly(styrene) film. Then, the diblock copolymer poly(acrylic acid)-poly(styrene) was transferred onto the film using the Langmuir–Blodgett technique generating a PAA brush with a grafting density of about 0.1 nm-2. Hen egg white lysozyme and bovine serum albumin were used as model proteins, because they carry a positive and negative net charge at the applied pH-value of 7 and interact with the negatively charged PAA brush under electrostatic attraction and repulsion, respectively. It has been found that lysozyme is strongly interacting with a PAA brush. Initial rates of adsorption are constant over time suggesting a diffusion-controlled adsorption mechanism. In contrast, a continuously decreasing adsorption rate has been observed for BSA indicating a retarding influence of already adsorbed protein molecules on the adsorption kinetics right from the beginning. For a typical protein solution concentration of 0.1 mg mL-1, the adsorbed amount of lysozyme enters a plateau region after a few seconds only, whereas in the case of BSA adsorption is slower by a factor of 100. The observed differences in the adsorption kinetics of lysozyme and BSA are discussed in the light of recent proposed models of protein adsorption at a PAA brush.

Separation of peptides with polyionic nanosponges for MALDI-MS analysis

A polymer brush consisting of 70% poly(N-isopropylacrylamide) (PNIPAAM) and 30% polymethacrylic acid (PMAA) was synthesized from gold substrates with a "grafting from" AIBN-type free-radical initiator. Fractionation of two peptides, bradykinin and buccalin, was accomplished in less than 120 s by placing a 30 pM (pH approximately 6.2) droplet onto the polymer brush substrate. The eluant containing the anionic buccalin is pipetted away for MALDI analysis while the cationic bradykinin adsorbed to the swollen anionic brush and was subsequently released by adding a droplet of formic acid to the substrate. This caused the brush to collapse and release the bradykinin, much like squeezing a sponge; these nanosponge substrates exhibited very high loading capacity (>2.0 mg/mL) compared to plasma-polymer-modified MALDI substrates. Ellipsometric measurements showed that complementary peptides adsorb rapidly while those of the same charge do not, and MALDI-MS analysis of the two fractions showed separation of both peptides. The adsorption of bradykinin was monitored over time, and 85% of the peptide had been adsorbed to the nanosponge in 1 min from a 0.5 mg/mL aqueous solution.

Native-like structure of proteins at a planar poly(acrylic acid) brush

Applying ATR-FTIR (attenuated total reflection Fourier transform infrared) and TIRF (total internal reflection fluorescence) spectroscopy, we have studied the secondary structure and aggregation properties of different proteins which are adsorbed at a poly(acrylic acid) (PAA) brush that covers a macroscopically large, planar surface. The PAA brush has been prepared on the surface of an ATR silicon crystal or a quartz plate. The preparation includes the deposition of a thin poly(styrene) film by spin-coating and the transfer of the diblock copolymer poly(styrene)-poly(acrylic acid) onto the hydrophobic film using the Langmuir-Schafer technique. It has been found that the proteins hen egg white lysozyme, bovine serum albumin, bovine alpha-lactalbumin, and bovine insulin adsorb spontaneously at a PAA brush at neutral pD values, albeit to different degrees. The secondary structure of the proteins was estimated from a decomposition of the amide I'-band in the observed ATR-FTIR spectra. Generally, the fractions of secondary structure elements recovered in this way were almost identical to those found when the proteins are native in solution. In addition, the tendency of insulin to form amyloid fibrils has also been tested when the protein is adsorbed at a planar PAA brush. Insulin is known to form amyloid fibrils in solution at low pH values and elevated temperatures. The experiments performed in this study suggest that a PAA brush does not promote fibril formation of insulin. Rather, insulin that is adsorbed at a PAA brush seems to be excluded from fibril formation pathways even at pD = 2 and 60 degrees C, where fibril formation of insulin is triggered in solution. Overall, the results of this study demonstrate that a planar PAA brush may serve as a mild environment for immobilized proteins.

Surface-anchored poly(2-vinyl-4,4-dimethyl azlactone) brushes as templates for enzyme immobilization

We explored surface-anchored poly(2-vinyl-4,4-dimethyl azlactone) (PVDMA) brushes as potential templates for protein immobilization. The brushes were grown using atom transfer radical polymerization from surface-anchored initiators and characterized by a combination of ellipsometry, atomic force microscopy, and X-ray photoelectron spectroscopy. RNase A was immobilized as a model enzyme through the nucleophilic attack of azlactone by the amine groups in the lysines located in the protein. The surface density of RNase A increased linearly from 5 to 50 nm. For 50 nm thick poly(2-vinyl-4,4-dimethyl azlactone) brushes, 7.5 microg/cm2 of RNase A was bound. The kinetics and thermodynamics of RNase A immobilization, the activity relative to surface density, and the pH and temperature dependence were examined. A Langmuir-like model for binding kinetics indicates that the kinetics are controlled by the rate of adsorption of RNase A and has an adsorption rate constant, k(ads), of 2.8 x 10(-8) microg(-1) s(-1) cm3. A maximum relative activity of approximately 0.95, which is near the activity of free RNase A, was reached at 1.2 microg/cm2 (approximately 3.0 monolayers) of immobilized RNase A. The immobilized RNase A had a similar temperature and pH dependence as free RNase A, indicating no significant change in conformation. The PVDMA template was extended to other biotechnologically relevant enzymes, such as deoxyribonuclease I, glucose oxidase, glucoamylase, and trypsin, with relative activities higher than or comparable to those of enzymes immobilized by other means. PVDMA brushes offer an efficient route to immobilize proteins via the ring opening of azlactone without the need for activation or pretreatment while retaining high relative activities of the bound enzymes.

Adsorption of the protein bovine serum albumin in a planar poly(acrylic acid) brush layer as measured by optical reflectometry

The adsorption of bovine serum albumin (BSA) in a planar poly(acrylic acid) (PAA) brush layer has been studied by fixed-angle optical reflectometry. The influence of polymer length, grafting density, and salt concentration is studied as a function of pH. The results are compared with predictions of an analytical polyelectrolyte brush model, which incorporates charge regulation and excluded volume interactions. A maximum in adsorption is found near the point of zero charge (pzc) of the protein. At the maximum, BSA accumulates in a PAA brush to at least 30 vol %. Substantial adsorption continues above the pzc, that is, in the pH range where a net negatively charged protein adsorbs into a negatively charged brush layer, up to a critical pH value. This critical pH value decreases with increasing ionic strength. The adsorbed amount increases strongly with both increasing PAA chain length and increasing grafting density. Experimental data compare well with the analytical model without having to include a nonhomogeneous charge distribution on the protein surface. Instead, charge regulation, which implies that the protein adjusts its charge due to the negative electrostatic potential in the brush, plays an important role in the interpretation of the adsorbed amounts. Together with nonelectrostatic interactions, it explains the significant protein adsorption above the pzc.

Polymeric brushes as functional templates for immobilizing ribonuclease A: study of binding kinetics and activity

The ability to immobilize proteins with high binding capacities on surfaces while maintaining their activity is critical for protein microarrays and other biotechnological applications. We employed poly(acrylic acid) (PAA) brushes as templates to immobilize ribonuclease A (RNase A), which is commonly used to remove RNA from plasmid DNA preparations. The brushes are grown by surface-anchored atom-transfer radical polymerization (ATRP) initiators. RNase A was immobilized by both covalent esterification and a high binding capacity metal-ion complexation method to PAA brushes. The polymer brushes immobilized 30 times more enzyme compared to self-assembled monolayers. As the thickness of the brush increases, the surface density of the RNase A increases monotonically. The immobilization was investigated by ellipsometry, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The activity of the immobilized RNase A was determined using UV absorbance. As much as 11.0 microg/cm(2) of RNase A was bound to PAA brushes by metal-ion complexation compared to 5.8 microg/cm(2) by covalent immobilization which is 30 and 16 times the estimated mass bound in a monolayer. The calculated diffusion coefficient D was 0.63 x 10(-14) cm(2)/s for metal-ion complexation and 0.71 x 10(-14) cm(2)/s for covalent immobilization. Similar values of D indicate that the binding kinetics is similar, but the thermodynamic equilibrium coverage varies with the binding chemistry. Immobilization kinetics and thermodynamics were characterized by ellipsometry for both methods. A maximum relative activity of 0.70-0.80 was reached between five and nine monolayers of the immobilized enzyme. However, the relative activity for covalent immobilization was greater than that of metal-ion complexation. Covalent esterification resulted in similar temperature dependence as free enzyme, whereas metal-ion complexation showed no temperature dependence indicating a significant change in conformation.

The reorientation of poly(2-dimethylamino ethyl methacrylate) after environment stimuli improves hydrophilicity and resistance of protein adsorption

There is a dense poly(2-dimethylamino ethyl methacrylate) (PDMAEMA) layer on the surface of poly(acrylonitrile and 2-dimethylamino ethyl methacrylate) (PAN-DMAEMA) membrane. The contact angle measurement indicated that the hydrophilicity of membrane increases at higher pH value and ionic strength. The reorientation of PDMAEMA after environment stimuli results in further enrichment of ester groups on the membrane surface according to XPS analysis. The amount of adsorbed bovine serum albumin (BSA) on PAN-DMAEMA membrane is dramatically decreased at higher pH value and ionic strength. A viewpoint based on the minimizing electrostatic interactions between PDMAEMA groups after environment stimuli leading to the conformation switch of PDMAEMA chains from stretched to shrunk states, which results in higher surface enrichment of ester groups enhancing hydrophilic property of the PAN-DMAEMA membrane, was put forward to explain the resistance of protein adsorption on the membrane.

Reversible Immobilization and Direct Electron Transfer of Cytochrome c on a pH-Sensitive Polymer Interface

A pH-sensitive polymer interface has been used as a matrix for reversible immobilization of cytochrome c (Cyt c) on an Au surface through a dip-coating process. The pH-sensitive behavior of the polymer brush interface has been demonstrated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. The reversible immobilization and electron-transfer properties of Cyt c have been investigated by in situ UV/Vis spectrophotometry and CV. The results have shown that the poly(acrylic acid) (PAA) brush acted as an excellent adsorption matrix and a good accelerant for the direct electron transfer of Cyt c, which gave redox peaks with a formal potential of 40 mV versus Ag/AgCl in pH 7.6 phosphate buffer solution. The average surface coverage of Cyt c on the PAA film was about 1.7 x 10(-10) mol cm(-2), indicating a multilayer of Cyt c. The electron-transfer rate constant was calculated to be around 0.19 s(-1) according to the CV experiments. The interface was subjected to in situ attenuated total internal reflection Fourier-transform infrared (ATR-FTIR) spectroscopic analysis, in order to further confirm the immobilization of Cyt c on the surface. This polymer-protein system may have potential applications in the design of biosensors, protein separation, interfacial engineering, biomimetics, and so on.

Biocompatible polymer vesicles from biamphiphilic triblock copolymers and their interaction with bovine serum albumin

The self-assembly of the biamphiphilic triblock copolymer poly(ethylene oxide)-b-poly(caprolactone)-b-poly(acrylic acid) into polymer vesicles is studied. The vesicles provide both biocompatibility and biodegradability. Moreover, the biamphiphilic nature of the triblock copolymer provides different surface properties in the interior and in the outer interface of the vesicles. Preparation of the aggregates by direct dissolution of the copolymer in a solution of albumin does not alter the morphology of the aggregates, and thus, they have the potential to immobilize protein molecules. Since a part of the protein is encapsulated in the interior of the vesicles, they can be used as nanocontainers. A further fraction of the protein is bound to the outer interface, which is primarily composed of the poly(acrylic acid) tails. Immobilization of protein on the outer interface can stabilize the colloidal particles and also provide them with a biofunctional component.

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.

Stimuli-responsive polyelectrolyte block copolymer brushes synthesized from the Si wafer via atom-transfer radical polymerization

Surface-tethered oppositely charged weak polyelectrolyte block copolymer brushes composed of poly(2-vinyl pyridine) (P2VP) and poly(acrylic acid) (PAA) were grown from the Si wafer by atom-transfer radical polymerization. The P2VP-b-PAA brushes were prepared through hydrolysis of the second PtBA block to the corresponding acrylic acid. The P2VP-b-PAA brushes with different PAA block length were obtained. The P2VP-b-PAA brushes revealed a unique reversible wetting behavior with pH. The difference between the solubility parameters for P2VP and PAA, the changes of surface chemical composition and surface roughness, and the reversible wetting behavior illustrated that the surface rearrangement occurred during treatment of the P2VP-b-PAA brushes by aqueous solution with different pH value. The reversible properties of the P2VP-b-PAA brushes can be used to regulate the adsorption of the sulfonated PS nanoparticles.

Control of specific attachment of proteins by adsorption of polymer layers

This study concerns the design of protein-resistant polymer adsorbed layers for the control of surface binding of biospecific recognition entities. Polymer surface layers were prepared using the adsorption of poly(allylamine hydrochloride) (PAH), poly(l-lysine) (PL), and branched and linear polyethyleneimine (PEI) and further modified by the covalent attachment of biotin for specific avidin attachment. The adsorption of PAH, PL, and PEI on silicon substrates was studied as a function of pH and ionic strength using ellipsometry. Average dry layer thicknesses of approximately 10, approximately 5, approximately 9, and approximately 3 A (+/-1 A) were obtained when polymer adsorption occurred from solutions at pH 9.5 that contained 0.5 M NaCl for PAH, PL, branched PEI, and linear PEI, respectively. These polymers showed significant differences in their efficiency to suppress nonspecific avidin adsorption. At low ionic strength, avidin adsorption occurred on all polymer-coated surfaces at basic pH values, despite the same positive electrostatic charge for protein globules and the surface. Though the net electrostatic repulsion between avidin molecules and branched PEI was efficiently screened in a protein solution of pH 7 and 0.15 M NaCl, branched-PEI coatings of high molecular weight were unique in their ability to provide avidin-resistant surfaces as a result of steric hindrance from the branched architecture of adsorbed polymer chains. All polymers studied were effective in suppressing avidin adsorption at pH 3 as a result of protonation of the avidin surface functional groups at this pH. Branched-PEI-coated surfaces were also effective for the suppression of smaller positively charged proteins such as lysozyme and ribonuclease A at pH 7 and 0.15 M NaCl. They were also resistant to the adsorption of negatively charged proteins such as BSA and fibrinogen at pH 7 and 0.75 M NaCl. Furthermore, by using PEI-modified protein-repellent surfaces, selective binding of avidin was achieved to surface-bound silver nanoparticles, which should provide a promising application for the label-free detection of biological species using surface-enhanced Raman scattering (SERS).