Adsorption of lysozyme, beta-casein and their layer-by-layer formation on hydrophilic surfaces: Effect of ionic strength

Protein-based layer-by-layer films for biomedical applications

The surface engineering of biomaterials is crucial for their successful (bio)integration by the body, i.e. the colonization by the tissue-specific cell, and the prevention of fibrosis and/or bacterial colonization. Performed at room temperature in an aqueous medium, the layer-by-layer (LbL) coating method is based on the alternating deposition of macromolecules. Versatile and simple, this method allows the functionalization of surfaces with proteins, which play a crucial role in several biological mechanisms. Possessing intrinsic properties (cell adhesion, antibacterial, degradable, etc.), protein-based LbL films represent a powerful tool to control bacterial and mammalian cell fate. In this article, after a general introduction to the LbL technique, we will focus on protein-based LbL films addressing different biomedical issues/domains, such as bacterial infection, blood contacting surfaces, mammalian cell adhesion, drug and gene delivery, and bone and neural tissue engineering. We do not consider biosensing applications or electrochemical aspects using specific proteins such as enzymes.

Preparation of Polyacrylonitrile-Based Immobilized Copper-Ion Affinity Membranes for Protein Adsorption

A polyacrylonitrile (PAN)-based immobilized metal-ion affinity membrane (IMAM) was prepared with a high capacity for protein adsorption. PAN was selected as the substrate due to its excellent thermal and chemical stability. The cyano groups on the PAN membrane were substituted with carboxyl groups, followed by reactions with ethylenediamine (EDA) and ethylene glycol diglycidyl ether (EGDGE) to produce the terminal epoxy groups. The chelating agent iminodiacetic acid (IDA) was then bound to the modified PAN membrane and further chelated with copper ions. The immobilized copper ion amount of membrane was analyzed to obtain the optimal reaction conditions, which were 60 °C/3 h for EDA coupling and 60 °C/4 h for EGDGE grafting. Furthermore, under the use of minor IDA and copper ion concentrations, the immobilized copper ion capacity of the IMAM was 4.8 μmol/cm² (253.4 µmol/mL, or 1.47 μmol/mg). At a neutral pH, the cationic lysozyme exhibited a large adsorption capacity with the IMAM (1.96 μmol/mL), which was most likely multilayer binding, whereas the adsorption capacity for bovine serum albumin (BSA) and histidine-tagged green fluorescent protein (GFP-His₆) was 0.053 μmol/mL and 0.135 μmol/mL, respectively, with a monolayer adsorption arrangement. The protein desorption efficiency was greater than 95%, implying that the prepared IMAM could be reused for protein adsorption.

Transdermal delivery of celecoxib and α-linolenic acid from microemulsion-incorporated dissolving microneedles for enhanced osteoarthritis therapy

Dissolving microneedles, the promising vehicles for transdermal delivery, are incapable of directly loading hydrophobic components that limit the application of transdermal drug delivery. Microemulsion holds great potential in the solubilisation of water-insoluble drugs but is limited by the high epidermal retention. In this study, we fabricated microemulsion-incorporated dissolving microneedles co-loading celecoxib and α-linolenic acid (Cel-MEs@MNs) for enhancing osteoarthritis (OA) therapy via synergistic anti-inflammation and potent transdermal delivery. Cel-MEs@MNs composed of celecoxib & α-linolenic acid-coloaded microemulsion (Cel-MEs) and hyaluronic acid-based microneedles (MNs) can be completely dissolved in 90 s with a particle size of ∼30 nm. Each microneedle array with a hardness exceeding 30 N contained 57.9 ± 2.5 μg of celecoxib and 442.5 ± 24.2 μg of α-linolenic acid. The 8 h-cumulative transdermal of celecoxib from Cel-MEs@MNs was 89.2 ± 5.1 μg celecoxib/cm², which is 2.98-fold higher than that from Cel-MEs. Combinational celecoxib and α-linolenic acid with a weight ratio of 1/5 can synergistically induce M1-like macrophage to M2 repolarization, reduce M1-like macrophages-resulted chondrocytes apoptosis, and lower serum prostaglandin E-2 (PGE-2). Notably, Cel-MEs@MNs amplified such collaborated anti-inflammatory effects. More importantly, in the treatment of OA-bearing rat models, Cel-MEs@MNs with a half-dose of celecoxib and α-linolenic acid significantly shrunk the paw swelling, reduced inflammatory cytokines, and improved cartilage damage compared with the oral administration of celecoxib and α-linolenic acid, as well as transdermal administration of Cel-MEs. Such an integrational strategy of microemulsion-incorporated dissolving MNs offers the feasibility of combinational celecoxib and α-linolenic acid in transdermal permeation and boosted OA therapy.

Brownian Dynamics Simulations of Proteins in the Presence of Surfaces: Long-Range Electrostatics and Mean-Field Hydrodynamics

Simulations of macromolecular diffusion and adsorption in confined environments can offer valuable mechanistic insights into numerous biophysical processes. In order to model solutes at atomic detail on relevant time scales, Brownian dynamics simulations can be carried out with the approximation of rigid body solutes moving through a continuum solvent. This allows the precomputation of interaction potential grids for the solutes, thereby allowing the computationally efficient calculation of forces. However, hydrodynamic and long-range electrostatic interactions cannot be fully treated with grid-based approaches alone. Here, we develop a treatment of both hydrodynamic and electrostatic interactions to include the presence of surfaces by modeling grid-based and long-range interactions. We describe its application to simulate the self-association and many-molecule adsorption of the well-characterized protein hen egg-white lysozyme to mica-like and silica-like surfaces. We find that the computational model can recover a number of experimental observables of the adsorption process and provide insights into their determinants. The computational model is implemented in the Simulation of Diffusional Association (SDA) software package.

Adsorption and Conformation Behavior of Lysozyme on a Gold Surface Determined by QCM-D, MP-SPR, and FTIR

The physicochemical properties of protein layers at the solid–liquid interface are essential in many biological processes. This study aimed to link the structural analysis of adsorbed lysozyme at the water/gold surface at pH 7.5 in a wide range of concentrations. Particular attention was paid to the protein’s structural stability and the hydration of the protein layers formed at the interface. Complementary methods such as multi-parameter surface plasmon resonance (MP-SPR), quartz crystal microbalance with energy dissipation (QCM-D), and infrared spectroscopy (FTIR) were used for this purpose. The MP-SPR and QCM-D studies showed that, during the formation of a monolayer on the gold surface, the molecules’ orientation changes from side-on to end-on. In addition, bilayer formation is observed when adsorbing in the high-volume concentration range >500 ppm. The degree of hydration of the monolayer and bilayer varies depending on the degree of surface coverage. The hydration of the system decreases with filling the layer in both the monolayer and the bilayer. Hydration for the monolayer varies in the range of 50–70%, because the bilayer is much higher than 80%. The degree of hydration of the adsorption layer has a crucial influence on the protein layers’ viscoelastic properties. In general, an increase in the filling of a layer is characterized by a rise in its rigidity. The use of infrared spectroscopy allowed us to determine the changes taking place in the secondary structure of lysozyme due to its interaction with the gold surface. Upon adsorption, the content of II-structures corresponding to β-turn and random lysozyme structures increases, with a simultaneous decrease in the content of the β-sheet. The increase in the range of β-turn in the structure determines the lysozyme structure’s stability and prevents its aggregation.

Structure and sequence features of mussel adhesive protein lead to its salt-tolerant adhesion ability

Mussels can strongly adhere to hydrophilic minerals in sea habitats by secreting adhesive proteins. The adhesion ability of these proteins is often attributed to the presence of Dopa derived from posttranslational modification of Tyr, whereas the contribution of structural feature is overlooked. It remains largely unknown how adhesive proteins overcome the surface-bound water layer to establish underwater adhesion. Here, we use molecular dynamics simulations to probe the conformations of adhesive protein Pvfp-5β and its salt-tolerant underwater adhesion on superhydrophilic mica. Dopa and positively charged basic residues form pairs, in this intrinsically disordered protein, and these residue pairs can lead to firm surface binding. Our simulations further suggest that the unmodified Tyr shows similar functions on surface adhesion by forming pairing structure with a positively charged residue. We confirm the presence of these residue pairs and verify the strong binding ability of unmodified proteins using nuclear magnetic resonance spectroscopy and lap shear tests.

Mechanism of lysozyme adsorption onto gold surface determined by quartz crystal microbalance and surface plasmon resonance

In this study, the physicochemical characterization of lysozyme adsorbed on gold was investigated. Through the use of MP-SPR it was possible to establish that the orientation of molecules changes from side-on to between or end-on with increasing surface coverage. The data confirms that the process of adsorption is driven primarily by electrostatic interactions but also by hydrophobic forces. MP-SPR data was compared with the Random Sequential Adsorption model for a molecule with an ellipsoidal shape. Contact angle measurements showed that higher surface coverage also translates in more hydrophilic properties of obtained lysozyme layer. Comparison of CD and PM-IRRAS spectra in solution and adsorbed state respectively showed changes in the secondary structures of lysozyme. These changes are dependent on pH, but fundamentally they go in the direction of the increase of β-turn/random content with a simultaneous decrease in β-sheet fraction, which suggests that aggregation is not occurring. The combination of MP-SPR and QCM-D measurements allowed the estimation of the number of water molecules associated with the lysozymes films. It has been observed that hydration decreases from 70% in pH = 4 to 30% in pH = 11. This data indicates that hydration is driven mainly by the degree of protonation of lysozyme molecules.

Multifunctional Tannic Acid (TA) and Lysozyme (Lys) Films Built Layer by Layer for Potential Application on Implant Coating

A multifunctional (TA/Lys)_n film, featuring good antioxidant property, fast cell attachment at the initial stage, enhanced osteogenesis, and broad-spectrum antibacterial property, was constructed by the layer-by-layer (LBL) method. The building process was monitored by quartz crystal microbalance with dissipation (QCM-D); the physical properties, such as topography, stiffness in dry and liquid state, and conformation of Lys in the film, were thoroughly characterized. These physical properties were modulated by varying the salt concentration at which the film was constructed. The film not only allows for favorable cell attachment and proliferation of preosteoblasts Mc3t3-E1 but also provides antibacterial property against Gram-positive bacteria, S. aureus and M. lysodeikticus, and Gram-negative bacteria, E. coli. It also displays good antioxidant property, which plays a critical role on fast cell attachment at the initial stage.

Interactions of Casein and Polypeptides in Multilayer Films Studied by FTIR and Molecular Dynamics

Multilayer films containing α- and β-casein and polypeptides, poly-L-lysine (PLL), and poly-L-arginine (PLArg) were formed by the layer-by-layer technique and Fourier Transform InfraRed spectroscopy with Attenuated Total Reflection (FTIR-ATR) and FTIR/Grazing Angle analyzed their infrared spectra. We investigated the changes of conformations of casein and polypeptides in the complexes formed during the build-up of the films. To elucidate the differences in the mechanism of complex formation leading to various growths of (PLL/casein)_n and (PLArg/casein)_n films, we performed the molecular dynamics simulations of the systems consisting of short PLL and PLArg chains and the representative peptide chains—casein fragments, which consists of several aminoacid sequences. The results of the simulation indicated the preferential formation of hydrogen bonds of poly-L-arginine with phosphoserine and glutamic acid residues of caseins. FTIR spectra confirmed those, which revealed greater conformational changes during the formation of casein complex with poly-L-arginine than with poly-L-lysine resulting from stronger interactions, which was also reflected in the bigger growth of (PLArg/casein)_n films with the number of deposited layers.

Controlling adsorption and passivation properties of bovine serum albumin on silica surfaces by ionic strength modulation and cross-linking

Understanding the physicochemical factors that influence protein adsorption onto solid supports holds wide relevance for fundamental insights into protein structure and function as well as for applications such as surface passivation.

Interfacial Adsorption of Silk Fibroin Peptides and Their Interaction with Surfactants at the Solid-Water Interface

Regenerated silk fibroin (RSF) is a Food and Drug Administration-approved material and has been widely used in many biomedical and cosmetic applications. Because of the amphiphilic nature of the primary repeat amino acid sequence (e.g., AGAGAS), RSF peptides can significantly reduce the water surface tension and therefore have the potential to be used as a surface active component for many applications, particularly in the biomedical, cosmetic, pharmaceutical, and food industries. In this paper, the adsorption of RSF peptides separated into molecular fractions of 5-30, 30-300, and >300 kDa has been studied at the solid-water interface by neutron reflection and spectroscopic ellipsometry to assess its surface active behavior. A stable layer of RSF was found to be irreversibly adsorbed at the hydrophilic SiO2-water interface. Changes in solution concentration, pH, and ionic strength all had an impact on the final adsorbed amount found at the interface. There were no significant differences between the final adsorbed amounts or layer structure among the three RSF molecular fractions studied; however, >300 kDa RSF was more stable to changes in solution ionic strength. Adsorption of conventional anionic and cationic surfactants, sodium dodecyl sulfate (SDS) and dodecyl trimethylammonium bromide (C12TAB), to the preadsorbed 5-30 kDa RSF revealed penetration of the surfactant into the RSF layer, at concentrations below the critical micellar concentration (CMC). SDS was found in the preadsorbed RSF layer and gradually removed RSF from the surface with an increase in SDS concentration. At concentrations above the CMC, there is near complete removal of RSF by SDS at the interface. C12TAB adsorbed into the preadsorbed RSF layer with considerably less removal of RSF from the interface compared to SDS. At concentrations above the CMC, both C12Tab and RSF were found to coexist at the interface, forming a less thick layer but with a considerable amount of RSF still present.

Enhancing proliferation and osteogenic differentiation of HMSCs on casein/chitosan multilayer films

Creating a bioactive surface is important in tissue engineering. Inspired by the natural calcium binding property of casein (CA), multilayer films ((CA/CS)n) with chitosan (CS) as polycation were fabricated to enhance biomineralization, cell adhesion and differentiation. LBL self-assembly technique was used and the assembly process was intensively studied based on changes of UV absorbance, zeta potential and water contact angle. The increasing content of chitosan and casein with bilayers was further confirmed with XPS and TOF-SIMS analysis. To improve the biocompatibility, gelatin was surface grafted. In vitro mineralization test demonstrated that multilayer films had more hydroxyapatite crystal deposition. Human mesenchymal stem cells (HMSCs) were seeded onto these films. According to fluorescein diacetate (FDA) and cell cytoskeleton staining, MTT assay, expression of osteogenic marker genes, ALP activity, and calcium deposition quantification, it was found that these multilayer films significantly promoted HMSCs attachment, proliferation and osteogenic differentiation than TCPS control.

Bridging interactions of proteins with silica nanoparticles: the influence of pH, ionic strength and protein concentration

Charge-driven bridging of nanoparticles by macromolecules represents a promising route for engineering functional structures, but the strong electrostatic interactions involved when using conventional polyelectrolytes impart irreversible complexation and ill-defined structures. Recently it was found that the electrostatic interaction of silica nanoparticles with small globular proteins leads to aggregate structures that can be controlled by pH. Here we study the combined influence of pH and electrolyte concentration on the bridging aggregation of silica nanoparticles with lysozyme in dilute aqueous dispersions. We find that protein binding to the silica particles is determined by pH irrespective of the ionic strength. The hetero-aggregate structures formed by the silica particles with the protein were studied by small-angle X-ray scattering (SAXS) and the structure factor data were analyzed on the basis of a short-range square-well attractive pair potential (close to the sticky-hard-sphere limit). It is found that the electrolyte concentration has a strong influence on the stickiness near pH 5, where the weakly charged silica particles are bridged by the strongly charged protein. An even stronger influence of the electrolyte is found in the vicinity of the isoelectric point of the protein (pI = 10.7) and is attributed to shielding of the repulsion between the highly charged silica particles and hydrophobic interactions between the bridging protein molecules.

Controlling mixed-protein adsorption layers on colloidal alumina particles by tailoring carboxyl and hydroxyl surface group densities

We show that different ratios of bovine serum albumin (BSA) and lysozyme (LSZ) can be achieved in a mixed protein adsorption layer by tailoring the amounts of carboxyl (-COOH) and aluminum hydroxyl (AlOH) groups on colloidal alumina particles (d50 ≈ 180 nm). The particles are surface-functionalized with -COOH groups, and the resultant surface chemistry, including the remaining AlOH groups, is characterized and quantified using elemental analysis, ζ potential measurements, acid-base titration, IR spectroscopy, electron microscopy, nitrogen adsorption, and dynamic light scattering. BSA and LSZ are subsequently added to the particle suspensions, and protein adsorption is monitored by in situ ζ potential measurements while being quantified by UV spectroscopy and gel electrophoresis. A comparison of single-component and sequential protein adsorption reveals that BSA and LSZ have specific adsorption sites: BSA adsorbs primarily via AlOH groups, whereas LSZ adsorbs only via -COOH groups (1-2 -COOH groups on the particle surface is enough to bind one LSZ molecule). Tailoring such groups on the particle surface allows control of the composition of a mixed BSA and LSZ adsorption layer. The results provide further insight into how particle surface chemistry affects the composition of protein adsorption layers on colloidal particles and is valuable for the design of such particles for biotechnological and biomedical applications.

Kinetics of protein unfolding at interfaces

The conformation of protein molecules is determined by a balance of various forces, including van der Waals attraction, electrostatic interaction, hydrogen bonding, and conformational entropy. When protein molecules encounter an interface, they are often adsorbed on the interface. The conformation of an adsorbed protein molecule strongly depends on the interaction between the protein and the interface. Recent time-resolved investigations have revealed that protein conformation changes during the adsorption process due to the protein-protein interaction increasing with increasing interface coverage. External conditions also affect the protein conformation. This review considers recent dynamic observations of protein adsorption at various interfaces and their implications for the kinetics of protein unfolding at interfaces.

A comprehensive study of lysozyme adsorption using dual polarization interferometry and quartz crystal microbalance with dissipation

Protein adsorption plays a crucial role in biomaterial surface science as it is directly linked to the biocompatibility of artificial biomaterial devices. Here, elucidation of protein adsorption mechanism is effected using dual polarization interferometry and a quartz crystal microbalance to characterize lysozyme layer properties on a silica surface at different coverage values. Lysozyme is observed to adsorb from sparse monolayer to multilayer coverage. At low coverage an irreversibly adsorbed layer is formed with slight deformation consistent with side-on orientation. At higher coverage values dynamic re-orientation effects are observed which lead to monolayer surface coverages of 2-3 ng/mm² corresponding to edge-on or/and end-on orientations. These monolayer thickness values ranged between 3 and 4.5 nm with a protein density value of 0.60 g/mL and with 50 wt% solvent mass. Further increase of coverage results formation of a multilayer structure. Using the hydration content and other physical layer properties a tentative model lysozyme adsorption is proposed.

Adsorption of unstructured protein β-casein to hydrophobic and charged surfaces

In this Monte Carlo simulation study we use mesoscopic modeling to show that β-casein, an unstructured milk protein, adsorbs to surfaces not only due to direct electrostatic and hydrophobic interactions but also due to structural rearrangement and charge regulation due to proton uptake and release. β-casein acts as an amphiphilic chameleon, changing properties according to the chemical environment, and binding is observed to both positively and negatively charged surfaces. The binding mechanisms, however, are fundamentally different. A detailed, per-residue-level analysis shows that the adsorption process is controlled by a few very specific regions of the protein and that these change dramatically with pH. Caseins, being the most abundant proteins in milk, are crucial for the properties of fermented dairy products, such as nutrition, texture, and viscosity, but may also influence adhesion to packaging materials. The latter leads to product losses of about 10%, leading to economical and environmental problems.

Combined QCMD and AFM studies of lysozyme and poly-L-lysine-poly-galacturonic acid multilayers

A quartz crystal microbalance with dissipation monitoring (QCMD) has been used to monitor the adsorption and structure of lysozyme monolayers and multilayers, and poly-L-lysine (PLL)-polygalacturonic acid (PGalA) multilayers at a solid-liquid interface using freshly-cleaved mica as a substrate. QCMD measurements were complemented with atomic force microscopy (AFM). AFM images revealed that lysozyme formed incomplete monolayers and provided a basis for calculation of the thickness of the protein film. Comparative studies of adsorption onto standard and mica-coated quartz crystals showed higher areal mass adsorption and a longer-time adsorption process for mica-coated quartz crystals. Simultaneous AFM images and QCMD data were obtained for lysozyme, linear PLL-PGalA and 7 nm PLL dendrimer-PGalA multilayers. The layer-by-layer deposited multilayer films exhibited viscoelastic properties and their growth followed a non-linear regime, associated with the PLL diffusion in and out of the film formation for linear PLL-PGalA films. For the PLL 7 nm dendrimer-PGalA films the AFM images revealed marked changes in surface roughness during layer by layer deposition: these changes influence the interpretation of the QCMD data and provide additional information on the growth and structure of the multilayers.

Mechanism of hen egg white lysozyme adsorption on a charged solid surface

The mechanism of hen egg white lysozyme (HEWL) adsorption on a negatively charged, hydrophilic surface has been studied using atomistic molecular dynamics (MD) simulation. Sixteen 90 ns trajectories provide adequate data to allow a detailed description of the adsorption process to be formulated. Two distinct adsorption sites have been identified. The main one is located on the N,C-terminal protein face and comprises Arg128 (the crucial one), supplemented by Arg125, Arg5, and Lys1; the minor one is used accidentally and contains only Arg68. Adsorption of this protein is driven by electrostatics, where the orientation of the protein dipole moment defines the direction of protein movement. The diffusion range on the surface depends on protein side-chain penetration through the surface water layers. This is facilitated by the long-range electric field of the charged surface, which can align polar side chains to be perpendicular to the surface. A simulation of adsorption onto a neutral ionic surface shows no such surface water layer penetration. Therefore, protein flexibility is seen to be an important factor, and to adsorb the HEWL has to adjust its structure. Nevertheless, at a flat surface only a slight loss of α-helical content is required. The adsorbed HEWL molecule is oriented between side-on and end-on ways, where the angle between the protein long axis (which mostly approximates the dipole moment) and the surface varies between 45° and 90°. Simulations with targeted mutations confirm the picture that emerges from these studies. The active site is located on the opposite face to the main adsorption site; hence, the activity of the immobilized HEWL should not be affected by the surface interactions. Our results provide a detailed insight into the adsorption mechanism and protein mobility at the surface. This knowledge will aid the proper interpretation of experimental results and the design of new experiments and functional systems.