Secondary structure of proteins adsorbed onto or embedded in polyelectrolyte multilayers

Multiple scale reorganization of electrostatic complexes of poly(styrenesulfonate) and lysozyme

We report on a SANS investigation into the potential for these structural reorganization of complexes composed of lysozyme and small PSS chains of opposite charge if the physicochemical conditions of the solutions are changed after their formation. Mixtures of solutions of lysozyme and PSS with high matter content and with an introduced charge ratio [-]/[+](intro) close to the electrostatic stoichiometry lead to suspensions that are macroscopically stable. They are composed at local scale of dense globular primary complexes of radius approximately 100 A; at a higher scale they are organized fractally with a dimension 2.1. We first show that the dilution of the solution of complexes, all other physicochemical parameters remaining constant, induces a macroscopic destabilization of the solutions but does not modify the structure of the complexes at submicronic scales. This suggests that the colloidal stability of the complexes can be explained by the interlocking of the fractal aggregates in a network at high concentration: dilution does not break the local aggregate structure, but it does destroy the network. We show, second, that the addition of salt does not change the almost frozen inner structure of the cores of the primary complexes, although it does encourage growth of the complexes; these coalesce into larger complexes as salt has partially screened the electrostatic repulsions between two primary complexes. These larger primary complexes remain aggregated with a fractal dimension of 2.1. Third, we show that the addition of PSS chains up to [-]/[+](intro) approximately 20, after the formation of the primary complex with a [-]/[+](intro) close to 1, only slightly changes the inner structure of the primary complexes. Moreover, in contrast to the synthesis achieved in the one-step mixing procedure where the proteins are unfolded for a range of [-]/[+](intro), the native conformation of the proteins is preserved inside the frozen core.

Effect of surface morphology and charge on the amount and conformation of fibrinogen adsorbed onto alginate/chitosan microcapsules

We report the influence of surface morphology and charge of alginate/chitosan (ACA) microcapsules on both the amount of adsorbed protein and its secondary structural changes during adsorption. Variations in surface morphology and charge were controlled by varying alginate molecular weight and chitosan concentration. Plasma fibrinogen (Fgn) was chosen to model this adsorption to foreign surfaces. The surface of ACA microcapsules exhibited a granular structure after incubating calcium alginate beads with chitosan solution to form membranes. The surface roughness of ACA microcapsule membranes decreased with decreasing alginate molecular weight and chitosan concentration. Zeta potential measurements showed that there was a net negative charge on the surface of ACA microcapsules which decreased with decreasing alginate molecular weight and chitosan concentration. The increase in both surface roughness and zeta potential resulted in an increase in the amount of Fgn adsorbed. Moreover, the higher the zeta potential was, the stronger the protein-surface interaction between fibrinogen and ACA microcapsules was. More protein molecules adsorbed spread and had a greater conformational change on rougher surfaces for more surfaces being available for protein to attach.

In situ ATR-FTIR investigation on the preparation and enantiospecificity of chiral polyelectrolyte multilayers

Chiral polyelectrolyte multilayers (PEMs) consisting of poly(l-lysine) (PLL), poly(N-(S)alkylated 4-vinylpyridinium iodide), or poly(ethyleneimine maltose) (PEI-m) as polycations and poly(styrenesulfonic acid) sodium salt (PSS) or poly(vinyl sulfate) as polyanions, as well as a nonchiral PEM composed of poly(ethyleneimine) (PEI) and PSS were deposited on silicon substrates and poly(tetrafluoroethylene) membranes using the layer-by-layer method. For these PEMs, enantiospecific interaction toward one enantiomer of either l/d-glutamic acid (l/d-GLU), l/d-tryptophan, or l/d-ascorbic acid (l/d-ASC), respectively, was studied under variation of the concentration, pH, and ionic strength. Both deposition and enantiospecific interaction were analyzed by attenuated total reflection Fourier transform infrared spectroscopy. Our results show a significant enantiospecific preference of d-GLU over l-GLU at PEMs containing PLL and of d-ASC over l-ASC at PEMs containing PEI-m. No such enantiospecific preference was found for nonchiral PEMs containing PEI. The enantiospecificity of PEMs of PLL/PSS toward l/d-GLU could be significantly influenced by the ionic strength and pH values, so that increasing attractive electrostatic interactions resulted in higher enantiospecificity.

In vitro behavior of layer-by-layer deposited molecular oligoelectrolyte films on Ti-6Al-4V surfaces

Layer-by-layer self-assembled films of molecular oligoelectrolytes were used to modify Ti-6Al-4V surfaces in order to test their ability as potential drug delivery system. With regard to medical application the in vitro behavior of the modified material was investigated. The Ti-6Al-4V (6% aluminium, 4% vanadium) material was treated in a layer-by-layer (LbL) process with 2, 4, 6 and 8 layers of molecular oligoelectrolytes 1 and 2 and thereby doped with a fluorescent reporter molecule 2. Human osteoblasts were cultured for a period up to 5 days on the modified material. Ti-6Al-4V surfaces without modification were used as control. In order to investigate the in vitro behavior of the coating as well as the influence of components of the coating on osteoblastic cells, respectively, cell proliferation, differentiation and attachment of hFOB cells were observed by means of cell number, osteoblastic gene expression and fluorescence microscopy. Degradation behavior of the OEM (oligoelectrolyte multilayer film) was examined using optical spectroscopy. Measurement data imply that the layer-by-layer coating was successfully assembled on the Ti surface and endures steam sterilization. The fluorescence signal in cell culture medium increased strictly linear with increasing pre-assembled number of layers on the surface. Proliferation rates of the cells in experimental groups did not differ significantly from each other (P >or= 0.783). Differentiation pattern was not significantly changed by the coating. The fluorescent reporter component of the film was absorbed by osteoblastic cells and was detected by fluorescence microscopy.

Tunable protein-resistance of polycation-terminated polyelectrolyte multilayers

The prevention of nonspecific protein adsorption is a crucial prerequisite for many biomedical and biotechnological applications. Therefore, the design of robust and versatile methods conferring optimal protein-resistance properties to surfaces has become a challenging issue. Here we report the unexpected case of polycation-ending polyelectrolyte multilayers (PEM) that efficiently prevented the adsorption of a negatively charged model protein, glucose oxidase (GOX). PEM films were based on two typical weak poyelectrolytes: poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). No chemical modification of the polyelectrolytes was required and tunable GOX adsorption was possible by simply changing the buildup pH conditions. Protein-resistance properties are attributed to high film hydration becoming the predominant factor over electrostatic interactions. We explain this effect by oscillations of the internal PAA ionization state throughout the buildup, which results in an excess of carboxylic acid groups within the film. This excess acts as a reservoir of potential carboxylate groups compensating the outer PAH positive charges. Partial results indicated that the system was also resistant to the adsorption of a positively charged protein, lysozyme. Control of the internal ionization of weak polyelectrolyte multilayers might open a route toward simple tuning of protein adsorption. These results should help to rationalize the design of biomaterials, biosensors, or protein separation devices.

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.

Factors Ruling Protein Adsorption

In this short review an introduction to the phenomenon of protein adsorption will be given. Protein adsorption often plays the central role in a wide variety of processes occurring in medicine, biochemistry, biotechnology and daily life. The main types of interactions ruling protein adsorption are summarized in this review and the principles of a few powerful experimental techniques to characterize protein adsorbates are outlined. Taking the silica/water interface as an example, the driving forces for protein adsorption are discussed on the basis of recent experimental results. Finally, the interactions of proteins with polyelectrolyte brushes are described. These brushes combine favourable properties of other interfaces and permit a controlled immobilization of protein molecules.

Internal structure of a thin film of mixed polymeric micelles on a solid/liquid interface

The adsorption of mixed micelles of poly(4-(2-amino hydrochloride-ethylthio)-butylene)- block-poly(ethylene oxide), PAETB 49- b-PEO 212 and poly(4-(2-sodium carboxylate-ethylthio)-butylene)- block-poly(ethylene oxide), PCETB 47- b-PEO 212 on solid/liquid interfaces has been studied with light, X-ray, and neutron reflectometry. The structure of the adsorbed layer can be described with a two-layer model consisting of an inner layer formed by the coacervate of the polyelectrolyte blocks PAETB 49 and PCETB 47 ( approximately 1 nm) and an outer layer of PEO 212 blocks ( approximately 6 nm). The micelles unfold upon adsorption forming a rather homogeneous flat layer that exposes its polyethylene oxide chains into the solution, thus rendering the surface antifouling after modification with the micelles.

Phospholipid bilayers as biomembrane-like barriers in layer-by-layer polyelectrolyte films

Dipalmitoylphosphatidylcholine (DPPC) bilayer was created on the surface of an exponentially growing poly(glutamic acid)/poly(lysine) (PGA/PLL) layer-by-layer polyelectrolyte film. The lipid bilayer decreased the surface roughness of the polyelectrolyte film. The layer-by-layer construction of the polyelectrolyte film could be continued on the top of the DPPC layer. The lipid bilayer, however, formed a barrier in the interior of the polyelectrolyte film, which blocked the diffusion (a prerequisite for exponential growth) of the polyelectrolytes. Thus, a new growth regime started in the upper part of the polyelectrolyte film, which was added to embed the DPPC bilayer. The structure and the dynamics of the DPPC bilayer on the polyelectrolyte film surface remained similar to that of its hydrated multibilayers, except that the phase transition became wider. In the case of embedded DPPC bilayers, in addition, the phase-transition temperature also decreased. This is the result of interactions with the nonconcerted movements of the barrier-separated lower and higher parts of the polyelectrolyte film. Gramicidin A (GRA) as a model of lipid-soluble peptides and proteins was successfully incorporated into such DPPC films. The DPPC films, either with or without GRA, were remarkably stable; as many heating-cooling cycles to measure phase transition could be carried out without visible alterations as wanted.

Cellulose Fiber−Enzyme Composites Fabricated through Layer-by-Layer Nanoassembly

Cellulose microfibers were coated with enzymes, laccase and urease, through layer-by-layer assembly by alternate adsorption with oppositely charged polycations. The formation of organized polyelectrolyte and enzyme multilayer films of 15-20 nm thickness was demonstrated by quartz crystal microbalance, zeta-potential analysis, and confocal laser scanning microscopy. These biocomposites retained enzymatic catalytic activity, which was proportional to the number of coated enzyme layers. For laccase-fiber composites, around 50% of its initial activity was retained after 2 weeks of storage at 4 degrees C. The synthesis of calcium carbonate microparticles on urease-fiber composites confirmed urease functionality and demonstrated its possible applications. This strategy could be employed to fabricate fiber-based composites with novel biological functions.

A QCM study of the immobilization of beta-galactosidase on polyelectrolyte surfaces: effect of the terminal polyion on enzymatic surface activity

This work describes the immobilization of beta-galactosidase onto polyelectrolyte multilayer assemblies of the polyanion poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) and the polycation poly(ethylenimine) (PEI) constructed by electrostatic self-assembly (ESA). A single layer of beta-galactosidase was deposited over a precursor film comprising up to five bilayers of the PEI/PAZO polyelectrolyte pair. The enzyme was deposited on both the polycationic (PEI) and the polyanionic (PAZO) surfaces. Quartz crystal microbalance with dissipation monitoring (QCM-D), single-wavelength ellipsometry, and UV-visible absorption spectroscopy revealed differences in both the amount of beta-galactosidase incorporated in each of the multilayer assemblies and the resulting enzyme packing density in the films. The enzymatic films were immersed in a reaction solution containing o-nitrophenyl-beta-d-galactopyranoside (ONPG), and absorbance measurements were used to monitor the concentration of o-nitrophenyl (ONP), the product of the beta-galactosidase catalyzed by hydrolysis of ONPG. Although our data indicate that comparable amounts of beta-galactosidase are incorporated onto both surfaces, enzymatic activity is substantially inhibited when the beta-galactosidase is immobilized on the polyanionic surface compared to the enzyme on the polycationic surface. The difference in catalytic activities reflects the different abilities of the two polyelectrolytes to screen the protein's active site from the substrate environment. In both assemblies, the protein interpenetrated the PEI/PAZO multilayer, disrupting the J-aggregated state of the PAZO chromophores. This work demonstrates that the charge, conformation, and composition of underlying polyelectrolyte cushions have a significant effect on the structure and function of an immobilized protein within functional nanoassemblies.

A Novel ATR-FTIR Approach for Characterisation and Identification of Ex Situ Immobilised Species

We demonstrate a novel method to analyse ex situ prepared protein chips by attenuated total reflection Fourier IR spectroscopy (ATR-FTIR), which circumvents tedious functionalisation steps of internal reflection elements (IREs), and simultaneously allows for complementary measurements by other analytical techniques. This concept is proven by utilising immobilised metal affinity capture (IMAC) chips containing about 10 mum thick films of copolymers coated with nitrilotriacetic acid (NTA) groups, which originally was manufactured for surface enhanced laser desorption ionisation (SELDI) spectrometry. Three immobilisation steps were analysed by ATR-FTIR spectroscopy: 1) NTA complexation with nickel(II) ions 2) binding of two histidine (His)-tagged synthetic peptides of 25 (25-His6) and 48 (48-His6) amino acids to the NTA-groups and 3) attachment of a ligand, mesyl amide, to the surface-bound 48-His6. Despite interference from H(2)O, both amide I and II were well resolved. Utilising peptide adsorption in the thick copolymer matrix yields a high saturation peptide concentration of approximately 100 mg mL(-1) and a dissociation constant of 116+/-11 muM, as determined by a detailed analysis of the Langmuir adsorption isotherm. The mesyl amide ligand was directly seen in the raw ATR-FTIR spectrum with specific peaks in the fingerprint region at 1172 and 1350 cm(-1). Several aspects of the fine structure of the amide I band of the peptide were analysed: influences from secondary structure, amino side chains and competing contamination product. We believe that this approach has great potential as a stand-alone or complementary analytical tool for determination of the chemical composition of functionalised surfaces. We emphasise further that with this approach no chemical treatment of IREs is needed; the chips can be regenerated and reused, and applied in other experimental set-ups.

Multilayered films fabricated from an oligoarginine-conjugated protein promote efficient surface-mediated protein transduction

The conjugation of cationic protein transduction domains to proteins results in an increase in the extent to which proteins are internalized by cells. This investigation sought to determine whether the conjugation of a protein transduction domain to a functional protein could be used to facilitate the incorporation of the protein into multilayered polyelectrolyte films and, subsequently, whether these films could be used to promote surface-mediated protein transduction. We demonstrate that it is possible to fabricate multilayered assemblies 80 nm thick using sodium polystyrene sulfonate (SPS) and bovine pancreatic ribonuclease (RNase A) conjugated to the cationic protein transduction domain nonaarginine (R(9)) using an entirely aqueous layer-by-layer process. We demonstrate further that the conjugation of R(9) to RNase A permits the assembly of multilayered films under conditions that do not allow for the incorporation of the unmodified protein. This result suggests that R(9) functions as a cationic anchor and serves to increase the strength of electrostatic interactions with SPS and facilitate layer-by-layer assembly. We also demonstrate that RNase A-R(9)/SPS films dissolve rapidly in physiologically relevant media and that macroscopic objects coated with these materials can be used to mediate high levels of protein transduction in mammalian cells. These results suggest the basis of general methods that could contribute to the design of materials that permit spatial and temporal control over the delivery of therapeutic proteins to cells and tissues.

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.

Immobilization of enamel matrix derivate protein onto polypeptide multilayers. Comparative in situ measurements using ellipsometry, quartz crystal microbalance with dissipation, and dual-polarization interferometry

The buildup of biodegradable poly(L-glutamic acid) (PGA) and poly(L-lysine) (PLL) multilayers on silica and titanium surfaces and the immobilization of enamel matrix derivate (EMD) protein was followed by utilizing in situ ellipsometry, quartz crystal microbalance with dissipation, and dual-polarization interferometry (DPI). The use of the relatively new DPI technique validated earlier published ellipsometry measurements of the PLL-PGA polypeptide films. The hydrophobic aggregating EMD protein was successfully immobilized both on top of and within the multilayer structures at pH 5.0. DPI measurements further indicated that the immobilization of EMD is influenced by the flow pattern during adsorption. The formed polypeptide-EMD multilayer films are of interest since it is known that EMD is able to trigger cell response and induce biomineralization. The multilayer films thus have potential to be useful as bioactive and biodegradable coatings for future dental implants.

A control over accessibility of immobilized enzymes through porous coating layer

We report immobilization of an enzyme by layer-by-layer (LbL) film deposition technique. All the enzyme layers, including the inner ones, contributed to the activity. We put-forwarded additional coating layers to protect the enzymes. To control the accessibility of the enzymes beneath the coating layer, pores have been introduced. Our results show controlled accessibility of immobilized enzymes in solid-state matrices.

The effect of the layer structure on the activity of immobilized enzymes in ultrathin films

The molecular engineering capability of the layer-by-layer (LbL) method for fabricating thin films has been exploited in order to immobilize glucose oxidase (GOD) in films with alternating layers of chitosan. Chitosan was proven a good scaffolding material, as GOD molecules preserved their catalytic activity towards glucose oxidation. Using electrochemical measurements, we showed that chitosan/GOD LbL films can be used to detect glucose with a limit of detection of 0.2 mmol l-1 and an activity of 40.5 microA mmol-1 L microg-1, which is highly sensitive when compared to other sensors in previous reports in the literature. The highest sensitivity of the LbL film was achieved when only the top layer contained GOD, thus indicating that GOD in inner layers did not contribute to glucose oxidation, probably because it hampers analyte diffusion and electron transport through the deposited layers. This may be explained by the dense packing of GOD molecules in the LbL films with chitosan, as inferred from estimates of the amount of GOD adsorbed per layer using a quartz crystal microbalance.

Partial poly(glutamic acid) <--> poly(aspartic acid) exchange in layer-by-layer polyelectrolyte films. Structural alterations in the three-component architectures

Layer-by-layer (LBL) polyelectrolyte films were constructed from poly(L-glutamic acid) (PGA) and poly(L-aspartic acid) (PAA) as polyanions, and from poly(L-lysine) (PLL) as the polycation. The terminating layer of the films was always PLL. According to attenuated total reflection Fourier transform infrared measurements, the PGA/PLL and PAA/PLL films, despite their chemical similarity, had largely different secondary structures. Extended beta-sheets dominated the PGA/PLL films, while alpha-helices and intramolecular beta-sheets dominated the PAA/PLL films. The secondary structure of the polyelectrolyte film affected the adsorption of human serum albumin (HSA) as well. HSA preserved its native secondary structure on the PGA/PLL film, but it became largely deformed on PAA/PLL films. Both PGA and PAA were able to extrude to a certain extent the other polyanion from the films, but the structural consequences were different. Adding PAA to a (PGA/PLL)5-PGA film resulted in a simple exchange and incorporation: PGA/PLL and PAA/PLL complexes coexisted with their unaltered secondary structures in the mixed film. The incorporation of PGA into a (PAA/PLL)5-PAA film was up to 50% and caused additional beta-structure increase in the secondary structure of the film. The proportions of the two polyanions were roughly the same on the surfaces and in the interiors of the films, indicating practically free diffusion for both polyanions. The abundance of PAA/PLL and PGA/PLL domains on the film surfaces was monitored by the analysis of the amide I region of the infrared spectrum of a reporter molecule, HSA, adsorbed onto the three-component polyelectrolyte films.

Structure and biosensor characteristics of complex between glucose oxidase and plasma-polymerized nanothin film

The structure and biosensor characteristics of complex between glucose oxidase (GOD) and plasma-polymerized nanothin film (PPF), in which the thickness is several nanometers, were investigated by atomic force microscopy (AFM) and electrochemical measurement. The GOD molecules were densely adsorbed onto the PPF surface treated by nitrogen plasma and the individual GOD molecules were observed. Subsequently, the GOD densely packed array on the PPF surface was subsequently treated by plasma polymerization (overcoating). AFM image showed that the thicker film gave the smoother surface, indicating that the GOD adsorbed on the surface was embedded more deeply in PPF. The amperometric biosensor characteristics of the GOD-PPF complex on a platinum electrode showed the current increment due to the enzymatic reaction with glucose addition, indicating that enzyme activity was retained although the enzyme has been exposed to the plasma gas related to diffusion of the substrate. This means that under mild exposure to organic plasma, the enzyme does not become seriously dysfunctional. Amperometric biosensor characteristics were strongly affected by monomer and thickness of PPF overcoating related with the diffusion of the substrate (glucose). Considering that the film deposition was performed using microfabrication-compatible organic plasma, our new method for protein architecture has a great potential of enabling high throughput production of bioelectronic devices.

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

The adsorption of two different proteins at a planar poly(acrylic acid) (PAA) brush was studied as a function of the ionic strength of the protein solutions applying total internal reflection fluorescence (TIRF) spectroscopy. Planar PAA brushes were prepared with a grafting density of 0.11 nm(-2) and were characterized using X-ray reflectometry. Hen egg-white lysozyme and bovine serum albumin (BSA) were used as model proteins, which have a net positive and negative charge at neutral pH-values, respectively. It has been found that both proteins adsorb strongly at a planar PAA brush at low ionic strength. Whereas lysozyme interacts with a PAA brush under electrostatic attraction at neutral pH-values, BSA binds under electrostatic repulsion at pH > 5. Even at pH = 8, significant amounts of BSA are adsorbed to a planar PAA brush. In addition, the reversibility of BSA adsorption has been characterized. Dilution of a BSA solution leads to an almost complete desorption of BSA from a PAA brush at short contact times. When the ionic strength of the protein solutions is increased to about 100-200 mM, a planar PAA brush appears largely protein-resistant, regardless of the protein net charge. The results of this study indicate that the salt-dependent protein affinity of a PAA brush represents a unique effect that must be explained by a novel protein-binding mechanism. On the basis of a recent model, it is suggested that a release of counterions is the most probable driving force for protein adsorption at a PAA brush. In a general view, this study characterizes a planar PAA brush as a new materials coating for the controlled immobilization of proteins whose use in biotechnological applications appears to be rewarding.

pH Dependence and Protein Selectivity of Poly(ethyleneimine)/Poly(acrylic acid) Multilayers Studied by in Situ ATR-FTIR Spectroscopy

The selective interaction between polyelectrolyte multilayers (PEM) consecutively adsorbed from poly(ethyleneimine) (PEI) and poly(acrylic acid) (PAC) and a binary mixture containing concanavalin A (COA) and lysozyme (LYZ) based on electrostatic interaction is reported. The composition and structure of the PEM and the uptake of proteins were analyzed by in situ attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy, and the morphology and thickness were characterized by atomic force microscopy (AFM) and ellipsometry. The PEM dissociation degree and charge state and the protein adsorption were shown to be highly dependent on the outermost layer type and the pH in solution. High protein uptake was obtained under electrostatically attractive conditions. This was used to bind selectively one protein from a binary mixture of LYZ/COA. In detail it could be demonstrated that six-layered PEM-6 at pH = 7.3 showed a preferential sorption of positively charged LYZ, while at PEM-5 and pH = 7.3 negatively charged COA could be selectively bound. No protein sorption from the binary mixture was observed at pH = 4.0 for both PEM, when COA, LYZ, and the outermost PEI layer of PEM-5 were positively charged or the outermost PAC layer of PEM-6 was neutral. Furthermore, from factor analysis of the spectral data the higher selectivity was found for PEM-5 compared to PEM-6. Increasing the ionic strength revealed a drastic decrease in the selectivity of both PEM. Evidence was found that the proteins were predominantly bound at the surface and to a minor extent in the bulk phase of PEM. These results suggest possible working regimes and application fields of PEI/PAC multilayer assemblies related to the preparative separation of binary and multicomponent protein mixtures (biofluids, food) as well as to the design of selective protein-resistant surfaces.

Fabrication of interdigitated micropatterns of self-assembled polymer nanofilms containing cell-adhesive materials

Micropatterns of different biomaterials with micro- and nanoscale features and defined spatial arrangement on a single substrate are useful tools for studying cellular-level interactions, and recent reports have highlighted the strong influence of scaffold compliance in determining cell behavior. In this paper, a simple yet versatile and precise patterning technique for the fabrication of interdigitated micropatterns of nanocomposite multilayer coatings on a single substrate is demonstrated through a combination of lithography and layer-by-layer (LbL) assembly processes, termed polymer surface micromachining (PSM). The first nanofilm pattern is constructed using lithography, followed by LbL multilayer assembly and lift-off, and the process is repeated with optical alignment to obtain interdigitated patterns on the same substrate. Thus, the method is analogous to surface micromachining, except that the deposition materials are polymers and biological materials that are used to produce multilayer nanocomposite structures. A key feature of the multilayers is the capability to tune properties such as stiffness by appropriate selection of materials, deposition conditions, and postdeposition treatments. Two- and four-component systems on glass coverslips are presented to demonstrate the versatility of the approach to construct precisely defined, homogeneous nanofilm patterns. In addition, an example of a complex system used as a testbed for in vitro cell adhesion and growth is provided: micropatterns of poly(sodium 4-styrenesulfonate)/poly-L-lysine hydrobromide (PSS/PLL) and secreted phospholipase A(2)/poly(ethyleneimine) (sPLA(2)/PEI) multilayers. The interdigitated square nanofilm array patterns were obtained on a single coverslip with poly(diallyldimethylammonium chloride) (PDDA) as a cell-repellent background. Cell culture experiments show that cortical neurons respond and bind specifically to the sPLA(2) micropatterns in competition with PLL micropatterns. The fabrication and the initial biological results on the nanofilm micropatterns support the usefulness of this technique for use in studies aimed at elucidating important biological structure-function relationships, but the applicability of the fabrication method is much broader and may impact electronics, photonics, and chemical microsystems.

A novel layer-by-layer approach for the fabrication of conducting polymer/RNA multilayer films for controlled release

Poly(anilineboronic acid) (PABA)/ribonucleic acid (RNA) multilayer films were prepared under neutral condition using a layer-by-layer deposition of PABA and RNA. RNA was used both as a polyelectrolyte for multilayer formation as well as dopant for PABA. Photoelastic modulated infrared reflection absorption spectroscopy measurements suggest that PABA interacts covalently with RNA through the formation of a boronate ester, a boron-nitrogen dative bond, as well as electrostatic interactions of anionic phosphates with cationic amines. The deposition procedure was monitored with UV-vis absorption spectroscopy, showing a linear dependence of absorbance with the number of PABA/RNA bilayers deposited. The multilayer films were further characterized using X-ray photoelectron spectroscopy and ellipsometry, which yielded a PABA/RNA bilayer thickness of approximately 10 nm. The PABA/RNA multilayer films are redox-active at neutral pH, consistent with the formation of a self-doped polymer. Electrochemical control of PABA under these conditions allows potential-induced controlled release of RNA from a multilayer at neutral pH, suggesting that this may serve as a novel method for controlled release of RNA under physiological conditions.

Polypeptide multilayer films

Research on polypeptide multilayer films, coatings, and microcapsules is located at the intersection of several disciplines: synthetic polymer chemistry and physics, biomaterials science, and nanoscale engineering. The past few years have witnessed considerable growth in each of these areas. Unexplored territory has been found at the borders, and new possibilities for technology development are taking form from technological advances in polypeptide production, sequencing of the human genome, and the nature of peptides themselves. Most envisioned applications of polypeptide multilayers have a biomedical bent. Prospects seem no less positive, however, in fields ranging from food technology to environmental science. This review of the present state of polypeptide multilayer film research covers key points of polypeptides as materials, means of polymer production and film preparation, film characterization methods, focal points of current research in basic science, and the outlook for a few specific applications. In addition, it discusses how the study of polypeptide multilayer films could help to clarify the physical basis of assembly and stability of polyelectrolyte multilayers, and mention is made of similarities to protein folding studies.

Polyelectrolyte-protein complexes: structure and conformation of each specie revealed by SANS

We report here the structure of complexes made of proteins (lysozyme, positively charged) and polyelectrolytes (PSSNa, negatively charged). We stay in conditions where the volume fractions of the components are of the same order and where PSS concentrations correspond to a semidilute regime. The final complexes structure is determined by SANS. We obtain three main types of structures: (i) For a protein excess and for long polyelectrolyte chains, the network preformed by PSS chains still exists but chains are partially shrunk due to cross-linking by lysozyme. Macroscopically, samples are gelled. (ii) For a protein excess and for short polyelectrolyte chains, PSS chains are locally shrunk and do not form a network anymore. Lysozyme and PSS chains are embedded in dense 3-D aggregates that arrange in a fractal network at a larger scale. Macroscopically, samples are liquid. (iii) For a polyelectrolyte excess and whatever the chain length, the internal structure of the lysozyme changes. After an initial strong electrostatic binding, lysozyme is progressively unfolded thanks to a hydrophobic contact with PSS. The two chainlike objects are finally organized in a homogeneous costructure. Macroscopically, samples are liquids.

Modeling Tubulin at Interfaces. Immobilization of Microtubules on Self-Assembled Monolayers

A theoretical study of protein docking to self-assembled monolayers using a new approach is presented. Docking experiments based on space complementarity implemented in FTDock software were performed for three different proteins: tubulin dimer, cytochrome c, and lysozyme. The proteins were adsorbed on alkanethiol surfaces with different terminating groups and 50,000 best orientations of each protein were analyzed. For all systems three filters based on different chemical and biological approaches were applied. Correctly docked proteins for the cytochrome c and lysozyme systems were found in a list of the first 12 results after applying the geometrical and grouping filter and in a list of the first 3 results after applying the biological filter. We have found that alkanethiol monolayers with odd and even numbers of -CH2- groups have similar properties in terms of interactions with the two proteins. Docking of the tubulin dimer revealed that the orientation favored from the applicational point of view can be found in a list of the first 14 results for monolayers with different terminating groups and that there may be a noticeable difference in tubulin dimer interactions with alkanethiol chains of various length. The results for tubulin dimer docking combined with microtubules ability of reversible assembly suggest that these biological structures may become good candidates to serve as templates for fabrication of nanowires and other nanoscale electronic devices. The new method of theoretical docking presented may be used as a fast and reliable tool complementing other theoretical and experimental techniques of exploring other protein-surface interfaces.

Perturbation of nanoscale structure of polypeptide multilayer thin films

Multilayer thin films formed by sequential deposition of oppositely charged polypeptides on a charged surface are known from previous studies to comprise a mixture of types of secondary structure. Here, study of the perturbation of polypeptide film structure by deposition of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate) (PSS) on the film surface has revealed differences in behavior attributable to physical properties of the peptides. The methods of analysis were circular dichroism spectroscopy (CD), ultraviolet spectroscopy (UVS), and quartz crystal microbalance (QCM). Films made of poly(L-lysine) (PLL) and poly(L-glutamic acid) (PLGA) with an average charge per monomer of about 1 were substantially more susceptible to perturbation of structure than films made of designed polypeptides with an average charge per monomer of about 0.5, despite preparation under identical conditions. PLL-PLGA films showed loss or gain of material and change in secondary structure content on perturbation, whether made of high molecular mass (ca. 90 kDa) or low molecular mass (ca. 14 kDa) polymers. By contrast, films made of very low molecular mass (ca. 3.5 kDa) designed polypeptides showed little change in secondary structure content. The data suggest that the penetrability of PSS or PAH into a film and therefore film density can depend substantially on the polypeptides of which it is made and the character of intermolecular interactions.

Multilayers of a Globular Protein and a Weak Polyacid: Role of Polyacid Ionization in Growth and Decomposition in Salt Solutions

Thin films obtained from a layer-by-layer deposition of a weak polycarboxylic acid and a positively charged globular protein were studied by in situ ATR-FTIR. The system was chicken egg lysozyme (Lys), bovine pancrease ribonuclease A (RNase), or bovine gamma-globulin (IgG) self-assembled with polycarboxylic acids. When the pH value was lowered below a critical point, the growth of films and their tolerance to decomposition by added sodium chloride improved dramatically. Stabilization of protein/polyacid films in salt solutions at lower pH values occurred due to the onset of nonelectrostatic interactions to intermolecular binding within protein/polyacid multilayers and was controlled by polyacid ionization within the film rather than the pH of the external solution. A fractional ionization of polyacid in the pH-stabilization region was lower with protein-containing films than for polyacid/linear polycation films, reflecting hindrance of the inter-association of protonated carboxylic groups by protein globules. Practical ramifications of the pH-stabilization effect might extend to areas of biotechnology and biomaterials.

Multivalent ion/polyelectrolyte exchange processes in exponentially growing multilayers

We show, in this paper that multivalent ferrocyanide anions can penetrate into exponentially growing (PGA/PAH)n multilayer films whatever the nature of the last deposited layer. These ions are not able to diffuse out of the film when it is brought in contact with a pure buffer solution. However, the contact of this film with a poly(allylamine) (PAH) or a poly(L-glutamic acid) (PGA) solution leads to the release of ferrocyanide ions from the multilayer. It is shown that the release of ferrocyanide anions, when the film is in contact with a PGA solution, is due to the diffusion of the PGA chains into the film so that an exchange between ferrocyanide ions and PGA chains takes place inside the film. On the other hand, PAH chains do not diffuse into PGA/PAH multilayers. When the film is then brought in contact with a PAH solution, the PAH chains from the solution are expected to strongly interact with the ferrocyanide ions and thus induce a diffusion mechanism of the multivalent anions out of the film, the film/solution interface playing the role of a sink for these ions. This work thus shows that interactions between multivalent ions and exponentially growing films are much more complex than expected at first sight and that polyelectrolyte multilayers must be seen as dynamic entities in which diffusion and exchange processes can take place.

Spatial distribution of protein molecules adsorbed at a polyelectrolyte multilayer

The spatial distribution of protein molecules interacting with a planar polyelectrolyte multilayer was determined using neutron reflectometry. Staphylococcal nuclease (SNase) was used as model protein that was adsorbed to the multilayer at 22 degrees C and 42 degrees C. At each temperature, the protein solution was adjusted to pD -values of 4.9 and 7.5 to vary the net charge of the protein molecules. The multilayer was built up on a silicon wafer by the deposition of poly(ethylene imine) (PEI), poly(styrene sulfonate) (PSS), and poly(allylamine hydrochloride) (PAH) in the order Si-PEI-PSS- (PAH-PSS)(5). Applying the contrast variation technique, two different neutron reflectivity curves were measured at each condition of temperature and pD -value. From the analysis of the curves, protein density profiles normal to the interface were recovered. Remarkably, it has been found that SNase is partially penetrating into the polyelectrolyte multilayer after adsorption at all conditions studied. The measured neutron reflectivities are consistent with a penetration depth of 50 A at pD=4.9 and 25 A at pD=7.5. Since SNase has an isoelectric point of pH=9.5, it carries a net positive charge at both pD -values and interacts with the PSS final layer under electrostatic attraction conditions. However, when increasing the temperature, the amount of adsorbed protein is increasing at both pD -values indicating the dominance of entropic driving forces for the protein adsorption. Interestingly, at pD=4.9 where the protein charge is relatively high, this temperature-induced mass increase of immobilized protein is more pronounced within the polyelectrolyte multilayer, whereas at pD=7.5, closer to the isoelectric point of SNase, raising the temperature has mainly the effect to accumulate protein molecules outside the polyelectrolyte multilayer at the water interface. It is suggested that the penetration of SNase into the polyelectrolyte multilayer is related to a complexation mechanism. The complexation is essentially entropic in nature due to the release of counterions.

Multilayered thin films that sustain the release of functional DNA under physiological conditions

The development of thin films and coatings that control the release of DNA from the surfaces of materials could have a significant impact on localized approaches to gene therapy. Here, we report multilayered polyelectrolyte assemblies that sustain the release of functional plasmid DNA from the surfaces of model substrates under physiological conditions. Multilayered assemblies consisting of alternating layers of plasmid DNA encoding for enhanced green fluorescent protein (EGFP) and a synthetic degradable polyamine were deposited on planar silicon and quartz substrates using a layer-by-layer fabrication process. Film growth was monitored by ellipsometry and UV spectrophotometry and correlated linearly with the number of polymer and plasmid layers deposited. In general, the thickness of deposited layers was found to be a function of both the pH and the ionic strength of the polyelectrolyte solutions used. Films up to 100 nm thick were investigated in this study. These assemblies erode gradually upon incubation in phosphate-buffered saline at 37 degrees C, as determined by ellipsometry and UV spectrophotometry, and sustain the release of incorporated plasmid into the incubation medium for a period of up to 30 h. Characterization of the released plasmid by agarose gel electrophoresis revealed that the DNA was released in a relaxed, open circular, rather than supercoiled, topology; subsequent cell transfection experiments demonstrated that the released plasmid is transcriptionally viable and promotes the expression of EGFP in the COS-7 cell line. These layered materials could represent an approach to the controlled administration of one or more functional DNA constructs from the surfaces of biomedical materials and devices.

The effect of polyelectrolyte chain length on layer-by-layer protein/polyelectrolyte assembly--an experimental study

The effect of polyelectrolyte chain length on the formation of multilayered assemblies of alternating globular proteins and linear polyanions prepared by the layer-by-layer electrostatic adsorption technique was investigated. The systems studied were albumin/sodium poly(styrenesulfonate), immunoglobulin G/sodium poly(styrenesulfonate), albumin/sodium dextran sulfate, and albumin/heparin. The formation of assemblies was followed using FTIR multiple internal reflection spectroscopy. While the amount of polyelectrolyte adsorbed on the first (primary) protein layer did not depend on its molecular weight, the effect of polyelectrolyte chain length was clearly observed in the following steps of alternating adsorption. Some short-chain polyanion molecules were removed from the surface when a next protein layer was adsorbed from solution. The short polyanion chains were not able to make a sufficient number of ion pairs for stable interaction with additional protein molecules and left the surface as soluble protein/polyanion complexes. The most pronounced effect could be seen with sodium poly(styrenesulfonate) of Mw up to ca. 2 x 10(4), but a detectable effect could be traced even up to Mw ca. 8 x 10(4). Such a pronounced effect, however, was not observed with dextran sulfate. The effect of molecular weight of heparin was clearly observed but all heparins tested, regardless of their molecular weight, effectively assembled with albumin to form multilayer.

Secondary Structure Analysis of Proteins Embedded in Spherical Polyelectrolyte Brushes by FT-IR Spectroscopy

The adsorption of bovine serum albumin (BSA), bovine beta-lactoglobulin, and bovine pancreatic ribonuclease A onto spherical polyelectrolyte brushes (SPB) is reported. The SPB consist of narrowly distributed poly(styrene) core particles (diameter approximately 100 nm) onto which linear chains of anionic polyelectrolytes are grafted. The polyelectrolyte shell consists of either the weak polyelectrolyte poly(acrylic acid) or the strong polyacid poly(styrenesulfonate). The SPB particles are dispersed in H(2)O at room temperature. The secondary structure of the proteins was investigated by Fourier transform infrared spectroscopy in transmission mode before and during adsorption to these colloidal brushes. The alpha-helix and beta-sheet content of the proteins was nearly fully retained in the adsorbed state for all systems. Only in the case of BSA interacting with poly(styrenesulfonic) brushes could a slight loss of alpha-helix structure be observed. As the interaction of SPB and proteins can be controlled by the ionic strength in the buffer, additional experiments were performed to release the adsorbed protein. The amount of released protein was quantified and was found to be strongly dependent on the kind of protein and brush used. The secondary structure of the released proteins could be analyzed as well. An almost full preservation of secondary structure was found. This demonstrates that SPB are well-suited to immobilize proteins. The SPB can be charged and decharged under retention of the secondary structure of the biomolecules.

Fibronectin adsorption onto polyelectrolyte multilayer films

The Layer-by-layer deposition of positively and negatively charged macromolecular species is an ideal method for constructing thin films incorporating biological molecules. We investigate the adsorption of fibronectin onto polyelectrolyte multilayer (PEM) films using optical waveguide lightmode spectroscopy (OWLS) and atomic force microscopy (AFM). PEM films are formed by adsorption onto Si(Ti)O2 from alternately introduced flowing solutions of anionic poly(sodium 4-styrenesulfonate) (PSS) and cationic poly(allylamine hydrochloride) (PAH). Using OWLS, we find the initial rate and overall extent offibronectin adsorption to be greatest on PEM films terminated with a PAH layer. The polarizability density of the adsorbed protein layer, as measured by its refractive index, is virtually identical on both PAH- and PSS-terminated films; the higher adsorbed density on the PAH-terminated film is due to an adsorbed layer of roughly twice the thickness. The binding of monoclonal antibodies specific to the protein's cell binding site is considerably enhanced to fibronectin adsorbed to the PSS layer, indicating a more accessible adsorbed layer. With increased salt concentration, we find thicker PEM films but considerably thinner adsorbed fibronectin layers, owing to increased electrostatic screening. Using AFM, we find adsorbed fibronectin layers to contain clusters; these are more numerous and symmetric on the PSS-terminated film. By considering the electrostatic binding of a segmental model fibronectin molecule, we propose a picture of fibronectin adsorbed primarily in an end-on-oriented monolayer on a PAH-terminated film and as clusters plus side-on-oriented isolated molecules onto a PSS-terminated film.

Mechanism of protein binding to spherical polyelectrolyte brushes studied in situ using two-photon excitation fluorescence fluctuation spectroscopy

We used two-photon excitation fluorescence fluctuation spectroscopy with photon counting histogram (PCH) analysis as a new tool to study the binding of globular proteins to colloidal particles in situ. Whereas fluorescence fluctuations are traditionally evaluated by calculating the autocorrelation function (fluorescence correlation spectroscopy), a complementary PCH analysis has been performed in this study which is advantageous when particle concentrations of a multicomponent system are of interest and the particles can be distinguished through particle brightness differences. The binding of two proteins, staphylococcal nuclease (SNase) and bovine serum albumin (BSA), to spherical polyelectrolyte brushes (SPB) was measured as a function of protein concentration and ionic strength of the solution at pH-values where SNase and BSA are positively and negatively charged, respectively. It has been found that SNase and BSA strongly bind to the SPB regardless of the protein charge. When the ionic strength of the solution is raised to 100 mM, the SPB become resistant to both proteins. These findings provide further evidence for a binding mechanism where the proteins are mainly driven to the SPB by the "counterion evaporation" force, while Coulomb interactions play a minor role. The results of this study characterize the potential of SPB as a new class of carrier particles for proteins whose use in biotechnological applications appears to be rewarding.

Incorporation of alkaline phosphatase into layer-by-layer polyelectrolyte films on the surface of affi-gel heparin beads: physicochemical characterization and evaluation of the enzyme stability

The preparation of functionalized beads in the micrometer size range that can be used to probe the action of immobilized biomolecules on cell cultures during controlled periods of time is of fundamental importance in cell biology. However, the preparation and characterization of such particles is tedious because of their fast sedimentation. It is hence difficult to prepare such beads in a reproducible manner. This highlights the need to prepare an important batch of functionnalized particles and to store them under conditions where the loss of biological activity is minimized. The aim of this paper was to immobilize alkaline phosphatase (AP) as a model enzyme on the surface of Affi-gel heparin beads functionnalized by means of a layer-by-layer (LBL) film made of poly-l-glutamic (PGA) acid and poly-l-lysine (PLL). The enzyme has been adsorbed either on the top of the LBL film or embedded under five polyelectrolyte layers. When embedded, the enzyme was not released in buffer and retained more than 30% of its initial activity after 3 months of storage at 4 degrees C. However, when the enzyme was adsorbed on top of the LBL film, about 80% of the adsorbed enzyme was released in the buffer after a few days of storage. Longer storage did not lead to any further desorption and the remaining enzyme displayed the same evolution of its activity with time as the embedded enzyme. The time evolution of the enzyme activity on the beads is compared with that in solution alone and in the presence of PGA and PLL separately.