Viscoelastic properties of adsorbed and cross-linked polypeptide and protein layers at a solid–liquid interface

The interaction between mucin and poly(amino acid)s with controlled cationic group content in bulk phase and in thin layers

The type and concentration of charged groups in polymers have a key role in mucoadhesive interactions. A series of cationic poly(amino acid)s with different charge densities was designed to unravel the correlation between chemical structure and mucin-polymer interactions. Colloidal interactions between the mucin protein and synthetic polyaspartamides were tested by dynamic light scattering, zeta potential measurements and turbidimetric titration as a function of polymer-to-mucin mass ratio. The mucoadhesive interactions displayed a strongly non-linear change with polymer composition. The attractive interactions between mucin and the polyaspartamides with at least 50 % cationic groups caused increased light scattering of dispersions due to the aggregation of mucin particles upon their charge reversal. Interactions were further analysed in a thin mucin layer to model life-like situations using a quartz crystal microbalance (QCM) in flow mode. Results pointed out that the fully cationic polyaspartamide is not necessarily superior to derivatives with lower cationic group content. The maximum of adsorbed mass of polymers on mucin was experienced at medium cationic group contents. This emphasizes the relevance of cationic polyaspartamides as mucoadhesive excipients due to their multiple functionalities and the possibility of fine-tuning their interactions with mucin via straightforward chemical steps.

Analysis of the Interaction between DNA Aptamers and Cytochrome C on the Surface of Lipid Films and on the MUA Monolayer: A QCM-D Study

We analyzed the possibility of the detection of cytochrome c (cyt c) being physically adsorbed on lipid films or covalently bounded to 11-mercapto-1-undecanoic acid (MUA) chemisorbed on the gold layer using quartz crystal microbalance with dissipation monitoring (QCM-D). The negatively charged lipid film composed of a mixture of zwitterionic DMPC and negatively charged DMPG phospholipids at a molar ratio of 1:1 allowed the formation of a stable cyt c layer. Addition of DNA aptamers specific to cyt c, however, resulted in removal of cyt c from the surface. The interaction of cyt c with the lipid film and its removal by DNA aptamers were accompanied by changes in viscoelastic properties evaluated using the Kelvin-Voigt model. Cyt c covalently bound to MUA also provided a stable protein layer already at its relatively low concentrations (0.5 μM). A decrease in the resonant frequency following the addition of gold nanowires (AuNWs) modified by DNA aptamers was observed. The interaction of aptamers with cyt c on the surface can be a combination of specific and non-specific interactions due to electrostatic forces between negatively charged DNA aptamers and positively charged cyt c.

Electroconvective Flow in Liquid Electrolytes Containing Oligomer Additives

Metal electrodeposition in batteries is fundamentally unstable and affected by different instabilities depending on operating conditions and electrolyte chemistry. Particularly, at high charging rates, a hydrodynamic instability loosely termed electroconvection sets in, which complicates all electrochemical processes by creating a nonuniform ion flux and preferential deposition at the electrode. Here, we isolate and study electroconvection by experimentally investigating how oligomer additives in liquid electrolytes interact with the hydrodynamic instability at a cation selective interface. From electrochemical measurements and direct visualization experiments, we find that electroconvection is delayed and suppressed at all voltages in the presence of oligomers. The underlying mechanism is revealed to involve formation of an oligomer ad-layer at the interface, which in response to perturbation is believed to exert an opposing body force on the surrounding fluid to preserve the ad-layer structure and in so doing suppresses electroconvection. Our results therefore reveal that in battery electrolytes without obvious sources of bulk elasticity, surface forces produced by adsorbed polymers can be used to advantage for suppressing instability.

Comparative Study of the Solid-Liquid Interfacial Adsorption of Proteins in Their Native and Amyloid Forms

The adhesive properties of amyloid fibers are thought to play a crucial role in various negative and positive aggregation processes, the study of which might help in their understanding and control. Amyloids have been prepared from two proteins, lysozyme and β-lactoglobulin, as well as an Exendin-4 derivative miniprotein (E5). Thermal treatment was applied to form amyloids and their structure was verified by thioflavin T (ThT), 8-Anilino-1-naphthalenesulfonic acid (ANS) dye tests and electronic circular dichroism spectroscopy (ECD). Adsorption properties of the native and amyloid forms of the three proteins were investigated and compared using the mass-sensitive quartz crystal microbalance (QCM) technique. Due to the possible electrostatic and hydrophobic interactions, similar adsorbed amounts were found for the native or amyloid forms, while the structures of the adsorbed layers differed significantly. Native proteins formed smooth and dense adsorption layers. On the contrary, a viscoelastic, highly loose layer was formed in the presence of the amyloid forms, shown by increased motional resistance values determined by the QCM technique and also indicated by atomic force microscopy (AFM) and wettability measurements. The elongated structure and increased hydrophobicity of amyloids might contribute to this kind of aggregation.

Affinity of Keratin Peptides for Cellulose and Lignin: A Fundamental Study toward Advanced Bio-Based Materials

Keratin is a potential raw material to meet the growing demand for bio-based materials with special properties. Keratin can be obtained from feathers, a by-product from the poultry industry. One approach for keratin valorization is to use the protein to improve the properties of already existing cellulose and lignin-based materials to meet the requirements for replacing fossil-based plastics. To ensure a successful combination of keratin with lignocellulosic building blocks, keratin must have an affinity to these substrates. Hence, we used quartz crystal microbalance with a dissipation monitoring (QCM-D) technique to get a detailed understanding of the adsorption of keratin peptides onto lignocellulosic substrates and how the morphology of the substrate, pH, ionic strength, and keratin properties affected the adsorption. Keratin was fractionated from feathers with a scalable and environmentally friendly deep eutectic solvent process. The keratin fraction used in the adsorption studies consisted of different sized keratin peptides (about 1-4 kDa), which had adopted a random coil conformation as observed by circular dichroism (CD). Measuring keratin adsorption to different lignocellulosic substrates by QCM-D revealed a significant affinity of keratin peptides for lignin, both as smooth films and in the form of nanoparticles but only a weak interaction between cellulose and keratin. Systematic evaluation of the effect of surface, media, and protein properties enabled us to obtain a deeper understanding of the driving force for adsorption. Both the structure and size of the keratin peptides appeared to play an important role in its adsorption. The keratin-lignin combination is an attractive option for advanced material applications. For improved adsorption on cellulose, modifications of either keratin or cellulose would be required.

Application of Multiharmonic QCM-D for Detection of Plasmin at Hydrophobic Surfaces Modified by β-Casein

Plasmin protease plays an important role in many processes in living systems, including milk. Monitoring plasmin activity is important for control of the nutritional quality of milk and other dairy products. We designed a biosensor to detect the proteolytic activity of plasmin, using multiharmonic quartz crystal microbalance with dissipation (QCM-D). The β-casein immobilized on the hydrophobic surface of 1-dodecanethiol on the AT-cut quartz crystal was used to monitor plasmin activity. We demonstrated detection of plasmin in a concentration range of 0.1–20 nM, with the limit of detection about 0.13 ± 0.01 nM. The analysis of viscoelastic properties of the β-casein layer showed rapid changes of shear elasticity modulus, μ, and coefficient of viscosity, η, at plasmin sub-nanomolar concentrations, followed by modest changes at nanomolar concentrations, indicating multilayer architecture β-casein. A comparative analysis of viscoelastic properties of β-casein layers following plasmin and trypsin cleavage showed that the higher effect of trypsin was due to larger potential cleavage sites of β-casein.

Analysis of trypsin activity at β-casein layers formed on hydrophobic surfaces using a multiharmonic acoustic method

Proteolysis of milk proteins, such as caseins, caused by milk proteases, can change the organoleptic and nutritional characteristics of milk, and therefore it is essential to monitor this enzymatic activity. We used trypsin as a model protease because of its role as a biomarker for pancreatitis. The aim of this work was to demonstrate the detection of proteolysis of β-casein by trypsin using a multiharmonic quartz crystal microbalance (QCM) biosensor. The β-casein layer was deposited from a 0.1 mg mL^-1 solution on a hydrophobic surface consisting of a self-assembled monolayer of 1-dodecanethiol on the gold electrode of the QCM. The addition of an increasing concentration of trypsin leads to the removal of the casein layer due to proteolysis, and correlates with an increase in the resonant frequency of the QCM. We investigated the effect of trypsin concentrations (0.3-20 nM) on the kinetics of the proteolysis of β-casein and demonstrated that the frequency increase is proportional to the protease concentration. Consequently, an inverse Michaelis-Menten model was used to estimate the Michaelis-Menten constant (K_M = 0.38 ± 0.02 nM) and the limit of detection (LOD = 0.16 ± 0.02 nM). The thickness, mass and viscoelastic properties of the protein adlayer after its formation and following the proteolytic cleavage were evaluated by means of multi-harmonic analysis. We found that β-casein is preferably adsorbed on the hydrophobic surfaces as an asymmetrical double layer, of which the innermost layer was found to be denser and thinner (about 2.37 nm) and the outermost layer was found to be less tightly bound and thicker (about 3.5 nm).

Changes of Viscoelastic Properties of Aptamer-Based Sensing Layers Following Interaction with Listeria innocua

A multiharmonic quartz crystal microbalance (QCM) has been applied to study the viscoelastic properties of the aptamer-based sensing layers at the surface of a QCM transducer covered by neutravidin following interaction with bacteria Listeria innocua. Addition of bacteria in the concentration range 5 × 103-106 CFU/mL resulted in a decrease of resonant frequency and in an increase of dissipation. The frequency decrease has been lower than one would expect considering the dimension of the bacteria. This can be caused by lower penetration depth of the acoustics wave (approximately 120 nm) in comparison with the thickness of the bacterial layer (approximately 500 nm). Addition of E. coli at the surface of neutravidin as well as aptamer layers did not result in significant changes in frequency and dissipation. Using the Kelvin-Voight model the analysis of the viscoelastic properties of the sensing layers was performed and several parameters such as penetration depth, Γ, viscosity coefficient, η, and shear modulus, μ, were determined following various modifications of QCM transducer. The penetration depth decreased following adsorption of the neutravidin layer, which is evidence of the formation of a rigid protein structure. This value did not change significantly following adsorption of aptamers and Listeria innocua. Viscosity coefficient was higher for the neutravidin layer in comparison with the naked QCM transducer in a buffer. However, a further increase of viscosity coefficient took place following attachment of aptamers suggesting their softer structure. The interaction of Listeria innocua with the aptamer layer resulted in slight decrease of viscosity coefficient. The shearing modulus increased for the neutravidin layer and decreased following aptamer adsorption, while a slight increase of µ was observed after the addition of Listeria innocua.

Enhancement of Probe Density in DNA Sensing by Tuning the Exponential Growth Regime of Polyelectrolyte Multilayers

Surface-based biosensing devices benefit from a dedicated design of the probe layer present at the transducing interface. The layer architecture, its physicochemical properties, and the embedding of the receptor sites affect the probability of binding the analyte. Here, the enhancement of the probe density at the sensing interface by tuning the exponential growth regime of polyelectrolyte multilayers (PEMs) is presented. PEMs were made of poly-l-lysine (PLL), with appended clickable dibenzocyclooctyne (DBCO) groups and oligo(ethylene glycol) chains, and poly(styrene sulfonate) (PSS). The DNA probe loading and target hybridization efficiencies of the PEMs were evaluated as a function of the PLL layer number and the growth regime by a quartz crystal microbalance (QCM). An amplification factor of 25 in the target DNA detection was found for a 33-layer exponentially grown PEM compared to a monolayer. A Voigt-based model showed that DNA probe binding to the DBCO groups is more efficient in the open, exponentially grown films, while the hybridization efficiencies appeared to be high for all layer architectures. These results show the potential of such engineered gel-like structures to increase the detection of bio-relevant analytes in biosensing systems.

Synergistic effect of lignin incorporation into polystyrene for producing sustainable superadsorbent

Lignin has gained intensive interest as an excellent raw material for the generation of advanced green products. Polystyrene (PS) is known for its worldwide application in water purification processes. To induce a sustainable PS, kraft lignin (KL) and polystyrene were polymerized via free radical polymerization in a facile aqueous emulsion process. KL enhanced surface area and porosity of PS. The physicochemical properties of induced KL–PS were analyzed, and the fate of lignin in KL–PS was discussed fundamentally. Wettability and surface energy analyses were implemented to monitor the surface properties of KL, PS and KL–PS. Incorporation of KL in PS (40 wt%) boosted its surface energy and oxygen content, which led to KL–PS with better compatibility than PS with copper ions in aqueous systems. A quartz crystal microbalance with dissipation (QCM-D) confirmed the noticeably higher adsorption performance of copper ion on KL–PS than on PS and KL. The sorption mechanism, which was revealed by FTIR studies, was primarily attributed to the coordination of Cu(ii) and hydroxyl group of KL–PS as well as the quadrupolar system of KL–PS.

Lignin has gained intensive interest as an excellent raw material for the generation of advanced green products.

Quartz Crystal Microbalances as Tools for Probing Protein-Membrane Interactions

Extensive studies on the spontaneous collapse of phospholipid vesicles into supported lipid bilayers (SLBs) have led to procedures which allow SLB formation on a wealth of substrates and lipid compositions. SLBs provide a widely accepted and versatile model system which mimics the natural cell membrane separating the extracellular and intracellular fluids of the living cell. The quartz crystal microbalance with dissipation monitoring (QCM-D) has been central in both the understanding of vesicle collapse into SLBs on various substrates but also in probing the kinetics and mechanisms of biomolecular interactions with SLBs in real time. We describe a robust procedure to form SLBs of zwitterionic and charged lipids on SiO₂ sensor crystals which subsequently can be exploited to probe the interaction between proteins and peptides with the SLB.

A new data processing routine facilitating the identification of surface adhered proteins from bacterial conditioning films via QCM-D/MALDI-ToF/MS

Conditioning films are an important factor in the initiation and development of microbial biofilms, which are the leading cause of chronic infections associated with medical devices. Here, we analyzed the protein content of conditioning films formed after exposure to supernatants of cultures of the human pathogen Pseudomonas aeruginosa PAO1. Adhesion of substances from the supernatant was monitored using quartz crystal microbalance with dissipation monitoring (QCM-D) sensor chips modified with the commonly used implant material titanium dioxide (TiO₂). Attached proteins were identified after on-chip digestion using matrix-assisted laser desorption/ionization (MALDI) time of flight (ToF) mass spectrometry (MS), and a new data processing tool consisting of an XML-database with theoretical tryptic peptides of every PAO1 protein and PHP scripts. Sub-databases containing only proteins, that we found in all replicates, were created and used for MS/MS precursor selection. The obtained MS/MS peaklists were then matched against theoretical fragmentations of the expected peptide sequences to verify protein identification. Using this approach we were able to identify 40 surface-associated proteins. In addition to extracellular proteins such as adhesins, a number of intra-cellular proteins were identified which may be involved in conditioning film formation, suggesting an as-yet unidentified role for these proteins, possibly after cell lysis. Graphical Abstract Flowchart of the method.

Cell and Protein Fouling Properties of Polymeric Mixtures Containing Supramolecular Poly(ethylene glycol) Additives

Fouling properties of new biomaterials are important for the performance of a material in a biological environment. Here, a set of three supramolecular polymeric additives consisting of ureidopyrimidinone (UPy)-functionalized poly(ethylene glycol) (UPyPEG) were formulated with UPy-modified polycaprolactone into thin supramolecular material films. The antifouling properties of these material films were determined by investigation of the relation of cell adhesion and protein adsorption on these materials films. The presence of the UPyPEG additives at the surface of the films was evident by an increased hydrophilicity. Adhesion of human epithelial and endothelial cells was strongly reduced for two of the UPyPEG-containing films. Analysis of adsorption of the first three proteins from the Vroman series, albumin, γ-globulin, and fibrinogen, using quartz crystal microbalance with dissipation in combination with viscoelastic modeling, revealed that the surfaces containing the UPyPEG additives had a limited effect on adsorption of these proteins. Despite a limited reduction of protein adsorption, UPyPEG-containing mixtures were non-cell-adhesive, which shows that non-cell-adhesive properties of supramolecular polymer surfaces are not always directly correlated to protein adsorption.

Polymer Brushes for Membrane Separations: A Review

The fundamentals and applications of polymer brush-modified membranes are reviewed. This new class of synthetic membranes is explored with an emphasis on tuning the membrane performance through polymer brush grafting. This work highlights the intriguing performance characteristics of polymer brush-modified membranes in a variety of separations. Polymer brushes are a versatile and effective means in designing membranes for applications in protein adsorption and purification, colloid stabilization, sensors, water purification, pervaporation of organic compounds, gas separations, and as stimuli responsive materials.

Predicting enzyme adsorption to lignin films by calculating enzyme surface hydrophobicity

The inhibitory action of lignin on cellulase cocktails is a major challenge to the biological saccharification of plant cell wall polysaccharides. Although the mechanism remains unclear, hydrophobic interactions between enzymes and lignin are hypothesized to drive adsorption. Here we evaluate the role of hydrophobic interactions in enzyme-lignin binding. The hydrophobicity of the enzyme surface was quantified using an estimation of the clustering of nonpolar atoms, identifying potential interaction sites. The adsorption of enzymes to lignin surfaces, measured using the quartz crystal microbalance, correlates to the hydrophobic cluster scores. Further, these results suggest a minimum hydrophobic cluster size for a protein to preferentially adsorb to lignin. The impact of electrostatic contribution was ruled out by comparing the isoelectric point (pI) values to the adsorption of proteins to lignin surfaces. These results demonstrate the ability to predict enzyme-lignin adsorption and could potentially be used to design improved cellulase cocktails, thus lowering the overall cost of biofuel production.

Amyloid aggregation and deposition of human islet amyloid polypeptide at membrane interfaces

Amyloid deposition of human islet amyloid polypeptide (hIAPP) within the islets of Langerhans is a pathological feature of type 2 diabetes mellitus. Substantial evidence indicates that the membrane-mediated aggregation and subsequent deposition of hIAPP are linked to dysfunction and death of pancreatic β-cells, but the molecular processes of hIAPP deposition are poorly understood. In this study, we examined the membrane-mediated aggregation and deposition of hIAPP at supported planar lipid bilayers with and without raft components (i.e. cholesterol and sphingomyelin) to provide insight into hIAPP-induced membrane dysfunction. The adsorption of hIAPP onto the bilayers was studied using a quartz crystal microbalance with dissipation monitoring, which showed enhanced accumulation of the peptide onto the bilayer containing raft components. Microscope observations demonstrated the growth of the aggregates formed from the membrane-adsorbed hIAPP. The examination of the membrane interfaces revealed that hIAPP aggregates retained the ability to associate with the membranes during the aggregation process, resulting in insertion of the aggregates into the bilayers. We also report the inhibitory effect of insulin on the hIAPP deposition. These findings demonstrate the aggregation of hIAPP at the membrane interfaces leading to amyloid deposits associated with the membrane and suggest a role for insulin in hIAPP deposition. A presumed mechanism regulating hIAPP deposition at the membrane interfaces is discussed.

Effect of peptide secondary structure on adsorption and adsorbed film properties on end-grafted polyethylene oxide layers

Poly-l-lysine (PLL), in α-helix or β-sheet configuration, was used as a model peptide for investigating the effect of secondary structures on adsorption events to poly(ethylene oxide) (PEO) modified surfaces formed using θ solvents. Circular dichroism results showed that the secondary structure of PLL persisted upon adsorption to Au and PEO modified Au surfaces. Quartz crystal microbalance with dissipation (QCM-D) was used to characterize the chemisorbed PEO layer in different solvents (θ and good solvents), as well as the sequential adsorption of PLL in different secondary structures (α-helix or β-sheet). QCM-D results suggest that chemisorption of PEO 750 and 2000 from θ solutions led to brushes 3.8 ± 0.1 and 4.5 ± 0.1 nm thick with layer viscosities of 9.2 ± 0.8 and 4.8 ± 0.5 cP, respectively. The average number of H2O per ethylene oxides, while in θ solvent, was determined as ~0.9 and ~1.2 for the PEO 750 and 2000 layers, respectively. Upon immersion in good solvent (as used for PLL adsorption experiments), the number of H2O per ethylene oxides increased to ~1.5 and ~2.0 for PEO 750 and 2000 films, respectively. PLL adsorbed masses for α-helix and β-sheet on Au sensors was 231 ± 5 and 1087 ± 14 ng cm(-2), with layer viscosities of 2.3 ± 0.1 and 1.2 ± 0.1 cP, respectively; suggesting that the α-helix layer was more rigid, despite a smaller adsorbed mass, than that of β-sheet layers. The PEO 750 layer reduced PLL adsorbed amounts to ~10 and 12% of that on Au for α-helices and β-sheets respectively. The PLL adsorbed mass to PEO 2000 layers dropped to ~12% and 4% of that on Au, for α-helix and β-sheet respectively. No significant differences existed for the viscosities of adsorbed α-helix and β-sheet PLL on PEO surfaces. These results provide new insights into the fundamental understanding of the effects of secondary structures of peptides and proteins on their surface adsorption.

Surface modification of poly(propylene carbonate) by layer-by-layer assembly and its hemocompatibility

Heparin and lysozyme were used to immobilize onto surface of poly(propylene carbonate) by layer-by-layer assembly to improve hemocompatibility.

Surface modification of a PVDF membrane by cross-linked collagen

PVDF membrane was modified by grafting cross-linked collagen after plasma treatment. The modified PVDF membrane has high OD value compared to original PVDF membrane, which means modified PVDF membrane has good hydrophilicity and biocompatibility.

Effects of sulfate groups on the adsorption and activity of cellulases on cellulose substrates

Pretreatment of lignocellulosic biomass with sulfuric acid may leave sulfate groups on its surface that may hinder its biochemical conversion. This study investigates the effects of sulfate groups on cellulase adsorption onto cellulose substrates and the enzymatic hydrolysis of these substrates. Substrates with different sulfate group densities were prepared from H2SO4- and HCl-hydrolyzed and partially and fully desulfated cellulose nanocrystals. Adsorption onto and hydrolysis of the substrates was analyzed by quartz crystal microbalance with dissipation monitoring (QCM-D). The surface roughness of the substrates, measured by atomic force microscopy, increased with decreasing sulfate group density, but their surface accessibilities, measured by QCM-D H2O/D2O exchange experiments, were similar. The adsorption of cellulose binding domains onto sulfated substrates decreased with increasing sulfate group density, but the adsorption of cellulases increased. The rate of hydrolysis of sulfated substrates decreased with increasing sulfate group density. The results indicated an inhibitory effect of sulfate groups on the enzymatic hydrolysis of cellulose, possibly due to nonproductive binding of the cellulases onto the substrates through electrostatic interactions instead of their cellulose binding domains.

Quartz crystal microbalances as tools for probing protein-membrane interactions

Extensive studies on the spontaneous collapse of phospholipid vesicles into supported lipid bilayers (SLBs) have led to procedures which allow SLB formation on a wealth of substrates and lipid compositions. SLBs provide a widely accepted and versatile model system which mimics the natural cell membrane separating the extracellular and intracellular fluids of the living cell. The quartz crystal microbalance with dissipation monitoring (QCM-D) has been central both in the understanding of vesicle collapse into SLBs on various substrates and in probing the kinetics and mechanisms of biomolecular interactions with SLBs in real time. We describe a robust procedure to form SLBs of zwitterionic and charged lipids on SiO(2) sensor crystals which subsequently can be exploited to probe the interaction between proteins and peptides with the SLB.

Surface and adsorption characteristics of three elastin-like polypeptide coatings with varying sequence lengths

The surface properties of a family of elastin-like polypeptides (ELPs), differing in molecular weight and sequence length, were investigated to understand how the nature of the polypeptide film might contribute to their thrombogenic profile. Physical adsorption of the ELPs onto Mylar increased surface wettability as the sequence length decreased while X-ray spectroscopy analysis showed an increasing amide content with sequence length. Chemical force microscopy analysis revealed that the ELP-coated surfaces displayed purely hydrophilic adhesion forces that increased as the ELP sequence length decreased. Adsorption isotherms performed using the quartz crystal microbalance with dissipation, showed that the surface coverage increased with ELP sequence length. The longer polypeptides (ELP2 and ELP4) also displayed higher specific dissipation values indicating that they established films with greater structural flexibility and associated water content than the shorter polypeptide, ELP1. Additionally, the stability of the ELP coating was lower with the shorter polypeptides. This study highlights the different surface properties of the ELP coatings as well as the dynamic nature of the ELP adsorbed layer wherein the conformational state may be an important factor contributing to their blood response.

Probing fundamental film parameters of immobilized enzymes--towards enhanced biosensor performance. Part I--QCM-D mass and rheological measurements

Enzyme immobilization is an ever-growing research-area for both analytical and industrial applications. Of critical importance in this area are the effects of immobilization procedures upon the functionality of the immobilized biomolecules. Both beneficial and detrimental effects can be conferred through the selection and tuning of the immobilization procedure. Quartz-crystal microbalance with dissipation (QCM-D) has been previously used to great effect in tracking alterations to thin films of biomolecules immobilized onto quartz transducers. In this study, we investigate the ability of QCM-D to track and monitor film parameters of a monolayer of laccase immobilized on a series of self-assembled monolayers (SAMs), differing in lateral density of binding residues on the SAM and height of the SAM from the quartz surface. Both mass gains and rheological parameters for these varying surfaces were measured and trends later compared to the apparent enzyme kinetics of the immobilized laccase films, assessed electroanalytically (Paper II in this two part study). For covalent attachment of proteins, both shear and viscosity were increased relative to physically adsorbed proteins. An increase in lateral density of protein-binding surface of the SAM components was shown to increase the shear/viscosity of the resultant film while an increase in distance from the electrode (through incorporation of lysine linkers) was shown to decrease the shear/viscosity while simultaneously increasing the wet mass gain of the films. Shear and viscosity may be indicative of both enzyme denaturation and increased lateral protein packing within the film structure hence it is assumed that less distortion occurs with the inclusion of linkers which allow for more optimal protein immobilization.

Analysing protein competition on self-assembled mono-layers studied with quartz crystal microbalance

The mechanisms by which proteins adsorb to surfaces of biomaterials have long been of interest. The present work started with the premise that small/hard and large/soft proteins will yield different sets of normalized frequency shift and dissipation signals when studied with a quartz crystal microbalance. The aim was to evaluate the usefulness of these raw data to study protein competition using protein incubations in sequence and from mixtures of albumin (BSA) and gamma-globulin (BGG) at various ratios. Increasing the concentration of BSA decreases the adsorption of subsequently incubated BGG. For BSA/BGG mixtures the dissipation is similar for all logarithmic molar ratios BGG/BSA below 1 but soon decreases when the molar ratio of BSA/BGG (and opposite for the normalized frequency shift) is above 1, indicating preferential binding of BGG. Modelling indicated that differences in the film shear modulus and viscosity depend more on the properties of the self-assembling mono-layers (SAMs) than on the proteins. Films high in BSA tentatively differ in film shear modulus and viscosity from that of films high in BGG but only on the hydrophobic surfaces. The results were encouraging as the raw data were deemed to be able to point at protein adsorption competition.

Evaluation of the specific interactions between glycodendrimeric porphyrins, free or incorporated into liposomes, and concanavalin A by fluorescence spectroscopy, surface pressure, and QCM-D measurements

In photodynamic therapy, the specificity of a photosensitizer and its penetration into tumor cells are crucial. We have analyzed the ability of newly synthesized meso-(tetraphenyl)porphyrins to be recognized by a model of mannose-specific proteins overexpressed at the surface of retinoblastoma cells. The specific interaction of porphyrin with Con A was studied by surface pressure measurements, fluorescence spectroscopy, dynamic light scattering, and QCM-D. The extent of porphyrins binding to Con A was highly dependent upon their chemical structure. Glycodendrimeric porphyrins showed the higher binding constant to Con A. The length of the spacer separating the sugar from the tetrapyrrolic ring appeared to be crucial in controlling the interaction of the compounds with the lectin in solution or immobilized onto a solid substrate. The methodology used proved to be efficient for the selection of potentially active compounds. The glycodendrimeric porphyrins, especially the derivative having the longer spacer, interacted more significantly with the lectin than the compound devoid of any sugar.

Viscoelastic modeling with interfacial slip of a protein monolayer electrode-adsorbed on an acoustic wave biosensor

Transverse-shear mode acoustic wave devices have been used as real-time, label-free detectors of conformational shifts in biomolecules on surfaces. However, material changes in the biochemical monolayers and coupling between the substrate and the surrounding liquid make it difficult to isolate the desired signal, so an understanding of these phenomena is required. An important step in this understanding is knowledge of the material properties of the linker layer that attaches a biochemically selective molecule to the gold surface, in our case, neutravidin. With the goal of obtaining material properties for a neutravidin monolayer, for use in future studies, neutravidin adsorption to the gold surface of an acoustic wave biosensor is described as a viscoelastic monolayer using one-dimensional modeling. Neutravidin is described as forming hydrated, viscoelastic monolayers, and slip is allowed at all interfaces. An impedance model is numerically fit to experimental values using a two-parameter minimization algorithm and values for the shear modulus of the neutravidin monolayer, in agreement with literature values for similar proteins, are obtained. Slip is found on the electrode surface prior to neutravidin adsorption. These results will be used for future modeling studies involving this protein as a linker protein.