On the relationship between peptide adsorption resistance and surface contact angle: a combined experimental and simulation single-molecule study

Understanding the "Berg limit": the 65° contact angle as the universal adhesion threshold of biomatter

Surface phenomena in aqueous environments such as long-range hydrophobic attraction, macromolecular adhesion, and even biofouling are predominantly influenced by a fundamental parameter-the water contact angle. The minimal contact angle required for these and related phenomena to occur has been repeatedly reported to be around 65° and is commonly referred to as the "Berg limit." However, the universality of this specific threshold across diverse contexts has remained puzzling. In this perspective article, we aim to rationalize the reoccurrence of this enigmatic contact angle. We show that the relevant scenarios can be effectively conceptualized as three-phase problems involving the surface of interest, water, and a generic oil-like material that is representative of the nonpolar constituents within interacting entities. Our analysis reveals that attraction and adhesion emerge when substrates display an underwater oleophilic character, corresponding to a "hydrophobicity under oil", which occurs for contact angles above approximately 65°. This streamlined view provides valuable insights into macromolecular interactions and holds implications for technological applications.

Adsorbing DNA to Mica by Cations: Influence of Valency and Ion Type

Ion-mediated attraction between DNA and mica plays a crucial role in biotechnological applications and molecular imaging. Here, we combine molecular dynamics simulations and single-molecule atomic force microscopy experiments to characterize the detachment forces of single-stranded DNA at mica surfaces mediated by the metal cations Li+, Na+, K+, Cs+, Mg2+, and Ca2+. Ion-specific adsorption at the mica/water interface compensates (Li+ and Na+) or overcompensates (K+, Cs+, Mg2+, and Ca2+) the bare negative surface charge of mica. In addition, direct and water-mediated contacts are formed between the ions, the phosphate oxygens of DNA, and mica. The different contact types give rise to low- and high-force pathways and a broad distribution of detachment forces. Weakly hydrated ions, such as Cs+ and water-mediated contacts, lead to low detachment forces and high mobility of the DNA on the surface. Direct ion-DNA or ion-surface contacts lead to significantly higher forces. The comprehensive view gained from our combined approach allows us to highlight the most promising cations for imaging in physiological conditions: K+, which overcompensates the negative mica charge and induces long-ranged attractions. Mg2+ and Ca2+, which form a few specific and long-lived contacts to bind DNA with high affinity.

Surface Modification of PP and PBT Nonwoven Membranes for Enhanced Efficiency in Photocatalytic MB Dye Removal and Antibacterial Activity

In this study, we developed highly efficient nonwoven membranes by modifying the surface of polypropylene (PP) and poly(butylene terephthalate) (PBT) through photo-grafting polymerization. The nonwoven membrane surfaces of PP and PBT were grafted with poly(ethylene glycol) diacrylate (PEGDA) in the presence of benzophenone (BP) and metal salt. We immobilized tertiary amine groups as BP synergists on commercial nonwoven membranes to improve PP and PBT surfaces. In situ Ag, Au, and Au/Ag nanoparticle formation enhances the nonwoven membrane surface. SEM, FTIR, and EDX were used to analyze the surface. We evaluated modified nonwoven membranes for photocatalytic activity by degrading methylene blue (MB) under LED and sunlight. Additionally, we also tested modified membranes for antibacterial activity against E. coli. The results indicated that the modified membranes exhibited superior efficiency in removing MB from water. The PBT showed the highest efficiency in dye removal, and bimetallic nanoparticles were more effective than monometallic. Modified membranes exposed to sunlight had higher efficiency than those exposed to LED light, with the PBT/Au/Ag membrane showing the highest dye removal at 97% within 90 min. The modified membranes showed reuse potential, with dye removal efficiency decreasing from 97% in the first cycle to 85% in the fifth cycle.

Benzethonium chloride as a tungsten corrosion inhibitor in neutral and alkaline media for the post-chemical mechanical planarization application

The cleaning solution for the post-chemical mechanical planarization (post-CMP) process of tungsten in neutral-alkaline media requires corrosion inhibitors as an additive, especially for advanced devices where the device node size shrinks below 10 nm. In the present study, the corrosion inhibition performance of benzethonium chloride (BTC) is evaluated in neutral-alkaline conditions. The electrochemical impedance spectroscopy (EIS) analysis showed ∼ 90 % of corrosion inhibition efficiency with an optimum concentration of 0.01 wt% BTC at both pH 7 and 11. Langmuir adsorption isotherm, frontier molecular orbital theory, molecular simulation, contact angle, precipitation study, and X-ray photoelectron spectroscopy analysis were performed to identify the inhibition mechanism of the BTC molecule on the W surface. Based on the proposed mechanism, the electrostatic attraction between the positively charged N atom in the BTC molecule and the negatively charged W surface initiates the adsorption of the molecule. The high dipole moment and large molecular size enhance the physical adsorption of the molecule to the surface. In addition to this, the adsorption isotherm analysis shows that possible chemical interaction with a moderate value of Gibbs free energy change of adsorption exists between the W and BTC molecule. The excellent corrosion inhibition efficiency of BTC on W is confirmed by the frontier molecular orbital theory and molecular dynamic simulation analysis.

Characteristics and behaviors of microplastics undergoing photoaging and Advanced Oxidation Processes (AOPs) initiated aging

The fact that 94% of microplastics (MPs) ubiquitous in the environment are subject to natural weathering makes the aging study currently a research hotspot. This review summarized the physicochemical characteristics of MPs undergoing natural and artificial aging and evaluated current analytical methods used in aging studies. Besides, the differences in photoaging and aging induced by advanced oxidation processes (AOPs) were discussed, leading to a conclusion that AOPs composed of oxidant and ultraviolet (UV) irradiation can better facilitate the alteration of MPs compared to UV irradiation alone. In addition, the environmental behavior of aged MPs was outlined and their adsorption properties for organics and metals were highlighted as a result of combined effects of hydrophobic, π-π, diffusion, and hydrogen bond interaction. Furthermore, the mechanisms of photoaging and AOPs-initiated aging were analyzed, mainly the role of reactive oxygen species (ROS) and environmentally persistent free radicals (EPFRs). Finally, the applications of two-dimensional correlation spectroscopy (2D-COS) and three-dimensional fluorescence spectra using excitation emission matrix-parallel factor analysis (EEM-PARAFAC) were discussed for the aging process analysis. This overview plays an important role in explaining the aging characteristics of MPs and provides a theoretical foundation for further investigations into their toxicity and removal.

Hemocompatibility challenge of membrane oxygenator for artificial lung technology

The artificial lung (AL) technology is one of the membrane-based artificial organs that partly augments lung functions, i.e. blood oxygenation and CO₂ removal. It is generally employed as an extracorporeal membrane oxygenation (ECMO) device to treat acute and chronic lung-failure patients, and the recent outbreak of the COVID-19 pandemic has re-emphasized the importance of this technology. The principal component in AL is the polymeric membrane oxygenator that facilitates the O₂/CO₂ exchange with the blood. Despite the considerable improvement in anti-thrombogenic biomaterials in other applications (e.g., stents), AL research has not advanced at the same rate. This is partly because AL research requires interdisciplinary knowledge in biomaterials and membrane technology. Some of the promising biomaterials with reasonable hemocompatibility - such as emerging fluoropolymers of extremely low surface energy - must first be fabricated into membranes to exhibit effective gas exchange performance. As AL membranes must also demonstrate high hemocompatibility in tandem, it is essential to test the membranes using in-vitro hemocompatibility experiments before in-vivo test. Hence, it is vital to have a reliable in-vitro experimental protocol that can be reasonably correlated with the in-vivo results. However, current in-vitro AL studies are unsystematic to allow a consistent comparison with in-vivo results. More specifically, current literature on AL biomaterial in-vitro hemocompatibility data are not quantitatively comparable due to the use of unstandardized and unreliable protocols. Such a wide gap has been the main bottleneck in the improvement of AL research, preventing promising biomaterials from reaching clinical trials. This review summarizes the current state-of-the-art and status of AL technology from membrane researcher perspectives. Particularly, most of the reported in-vitro experiments to assess AL membrane hemocompatibility are compiled and critically compared to suggest the most reliable method suitable for AL biomaterial research. Also, a brief review of current approaches to improve AL hemocompatibility is summarized. STATEMENT OF SIGNIFICANCE: The importance of Artificial Lung (AL) technology has been re-emphasized in the time of the COVID-19 pandemic. The utmost bottleneck in the current AL technology is the poor hemocompatibility of the polymer membrane used for O₂/CO₂ gas exchange, limiting its use in the long-term. Unfortunately, most of the in-vitro AL experiments are unsystematic, irreproducible, and unreliable. There are no standardized in-vitro hemocompatibility characterization protocols for quantitative comparison between AL biomaterials. In this review, we tackled this bottleneck by compiling the scattered in-vitro data and suggesting the most suitable experimental protocol to obtain reliable and comparable hemocompatibility results. To the best of our knowledge, this is the first review paper focusing on the hemocompatibility challenge of AL technology.

Adsorption characteristics of peptides on ω-functionalized self-assembled monolayers: a molecular dynamics study

Molecular dynamics simulations were employed to investigate the adsorption behavior of a variety of amino-acid side-chain analogs (SCAs) and a β-hairpin (HP7) peptide on a series of liquid-like self-assembled monolayers (SAMs) with terminal functional groups of -OH, -OCH₃, -CH₃, and -CF₃. The relationships between the adsorption free energy of the SCAs and the interfacial properties of water on the SAMs were examined to determine the acute predictors of protein adsorption on the SAM surfaces. The structural changes of HP7 on the SAM surfaces were also investigated to understand the relationship between the surface nature and protein denaturation. It was found that the adsorption free energy of the SCAs was linearly related to the surface hydrophobicity, which was computed as the free energy of cavity formation near the SAM-water interfaces. In addition, the hydrophobic -CH₃ and -CF₃ SAMs produced substantial conformational changes in HP7 because of the strong hydrophobic attractions to the nonpolar side chains. The hydrophilic surface terminated by -OH also promoted structural changes in HP7 resulting from the formation of hydrogen bonds between the hydrophilic tail and HP7. Consequently, the moderate amphiphilic surface terminated by -OCH₃ avoided the denaturation of HP7 most efficiently, thus improving the biocompatibility of the surface. In conclusion, these results provide a deep understanding of protein adsorption for a wide range of polymeric surfaces, and they can potentially aid the design of appropriate biocompatible coatings for medical applications.

Tuning Contact Angles of Aqueous Droplets on Hydrophilic and Hydrophobic Surfaces by Surfactants

Adsorption of small amphiphilic molecules occurs in various biological and technological processes, sometimes desired while other times unwanted (e.g., contamination). Surface-active molecules preferentially bind to interfaces and affect their wetting properties. We use molecular dynamics simulations to study the adsorption of short-chained alcohols (simple surfactants) to the water–vapor interface and solid surfaces of various polarities. With a theoretical analysis, we derive an equation for the adsorption coefficient, which scales exponentially with the molecular surface area and the surface wetting coefficient and is in good agreement with the simulation results. We apply the outcomes to aqueous sessile droplets containing surfactants, where the competition of surfactant adsorptions to both interfaces alters the contact angle in a nontrivial way. The influence of surfactants is the strongest on very hydrophilic and hydrophobic surfaces, whereas droplets on moderately hydrophilic surfaces are less affected.

Hydrophobicity of Self-Assembled Monolayers of Alkanes: Fluorination, Density, Roughness, and Lennard-Jones Cutoffs

The interplay of fluorination and structure of alkane self-assembled monolayers and how these affect hydrophobicity are explored via molecular dynamics simulations, contact angle goniometry, and surface-enhanced infrared absorption spectroscopy. Wetting coefficients are found to grow linearly in the monolayer density for both alkane and perfluoroalkane monolayers. The larger contact angles of monolayers of perfluorinated alkanes are shown to be primarily caused by their larger molecular volume, which leads to a larger nearest-neighbor grafting distance and smaller tilt angle. Increasing the Lennard-Jones force cutoff in simulations is found to increase hydrophilicity. Specifically, wetting coefficients scale like the inverse square of the cutoff, and when extrapolated to the infinite cutoff limit, they yield contact angles that compare favorably to experimental values. Nanoscale roughness is also found to reliably increase monolayer hydrophobicity, mostly via the reduction of the entropic part of the work of adhesion. Analysis of depletion lengths shows that droplets on nanorough surfaces partially penetrate the surface, intermediate between Wenzel and Cassie-Baxter states.

Superamphiphilic TiO2 Composite Surface for Protein Antifouling

Unwanted protein adsorption deteriorates fouling processes and reduces analytical device performance. Wettability plays an important role in protein adsorption by affecting interactions between proteins and surfaces. However, the principles of protein adsorption are not completely understood, and surface coatings that exhibit resistance to protein adsorption and long-term stability still need to be developed. Here, a nanostructured superamphiphilic TiO2 composite (TiO2 /SiO2 ) coating that can effectively prevent nonspecific protein adsorption on water/solid interfaces is reported. The confined water on the superamphiphilic surface enables a low adhesion force and the formation of an energy barrier that plays a key role in preventing protein adsorption. This adaptive design protects the capillary wall from fouling in a harsh environment during the bioanalysis of capillary electrophoresis and is further extended to applications in multifunctional microfluidics for liquid transportation. This facile approach is not only perfectly applied in channels with complicated configurations but may also offer significant insights into the design of advanced superwetting materials to control biomolecule adhesion in biomedical devices, microfluidics, and biological assays.

Electrode Surface Potential-Driven Protein Adsorption and Desorption through Modulation of Electrostatic, van der Waals, and Hydration Interactions

When proteins in aqueous solutions are exposed to solid substrates, they adsorb due to the dynamic interplay of electrostatic, van der Waals, and hydration interactions and do so in a rather irreversible fashion, which makes protein recovery troublesome. Here, we use a gold electrode as the solid substrate and modulate the surface potential to systematically induce protein adsorption as well as partial desorption. We use different methods such as surface plasmon resonance, atomic force microscopy, and electrowetting and show that biasing the electrode to more negative potentials (by -0.4 V compared to the open-circuit potential at pH 6) results in an increased adsorption barrier of 6 kJ mol^-1 for the negatively charged protein β-lactoglobulin. Further, we clearly demonstrate that this is due to an increased double layer potential of -0.06 V and an increase in hydration repulsion. This indicates that an electric potential can directly influence surface interactions and thus induce partial β-lactoglobulin desorption. These observations can be the basis for biosensors as well as separation technologies that use only one trigger to steer protein ad- and desorption, which is low in energy requirement and does not generate large waste streams, as is the case for standard protein separation technologies.

Responsive Polyesters with Alkene and Carboxylic Acid Side-Groups for Tissue Engineering Applications

Main chain polyesters have been extensively used in the biomedical field. Despite their many advantages, including biocompatibility, biodegradability, and others, these materials are rather inert and lack specific functionalities which will endow them with additional biological and responsive properties. In this work, novel pH-responsive main chain polyesters have been prepared by a conventional condensation polymerization of a vinyl functionalized diol with a diacid chloride, followed by a photo-induced thiol-ene click reaction to attach functional carboxylic acid side-groups along the polymer chains. Two different mercaptocarboxylic acids were employed, allowing to vary the alkyl chain length of the polymer pendant groups. Moreover, the degree of modification, and as a result, the carboxylic acid content of the polymers, was easily tuned by varying the irradiation time during the click reaction. Both these parameters, were shown to strongly influence the responsive behavior of the polyesters, which presented adjustable pKα values and water solubilities. Finally, the difunctional polyesters bearing the alkene and carboxylic acid functionalities enabled the preparation of cross-linked polyester films by chemically linking the pendant vinyl bonds on the polymer side groups. The biocompatibility of the cross-linked polymers films was assessed in L929 fibroblast cultures and showed that the cell viability, proliferation, and attachment were greatly promoted on the polyester surface, bearing the shorter alkyl chain length side groups and the higher fraction of carboxylic acid functionalities.

Molecular Decoration of Ceramic Supports for Highly Effective Enzyme Immobilization-Material Approach

A highly effective method was developed to functionalize ceramic supports (Al₂O₃ powders and membranes) using newly synthesized spacer molecules. The functionalized materials were subsequently utilized for Candida antarctica lipase B enzyme immobilization. The objective is to systematically evaluate the impact of various spacer molecules grafted onto the alumina materials will affect both the immobilization of the enzymes and specific material surface properties, critical to enzymatic reactors performance. The enzyme loading was significantly improved for the supports modified with shorter spacer molecules, which possessed higher grafting effectiveness on the order of 90%. The specific enzyme activity was found to be much higher for samples functionalized with longer modifiers yielding excellent enantioselectivity >97%. However, the enantiomeric ratio of the immobilized lipase was slightly lower in the case of shorter spacer molecules.

Establishment of an in vitro thrombogenicity test system with cyclic olefin copolymer substrate for endothelial layer formation

In vitro thrombogenicity test systems require co-cultivation of endothelial cells and platelets under blood flow-like conditions. Here, a commercially available perfusion system is explored using plasma-treated cyclic olefin copolymer (COC) as a substrate for the endothelial cell layer. COC was characterized prior to endothelialization and co-cultivation with platelets under static or flow conditions. COC exhibits a low roughness and a moderate hydrophilicity. Flow promoted endothelial cell growth and prevented platelet adherence. These findings show the suitability of COC as substrate and the importance of blood flow-like conditions for the assessment of the thrombogenic risk of drugs or cardiovascular implant materials.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1557/s43579-021-00072-6.

Both Poly(ethylene glycol) and Poly(methyl ethylene phosphate) Guide Oriented Adsorption of Specific Proteins

Developing new functional biomaterials requires the ability to simultaneously repel unwanted and guide wanted protein adsorption. Here, we systematically interrogate the factors determining the protein adsorption by comparing the behaviors of different polymeric surfaces, poly(ethylene glycol) and a poly(phosphoester), and five different natural proteins. Interestingly we observe that, at densities comparable to those used in nanocarrier functionalization, the same proteins are either adsorbed (fibrinogen, human serum albumin, and transferrin) or repelled (immunoglobulin G and lysozyme) by both polymers. However, when adsorption takes place, the specific surface dictates the amount and orientation of each protein.

Dynamics and molecular interactions of single-stranded DNA in nucleic acid biosensors with varied surface properties

Electrochemical DNA biosensors utilizing self-assembled monolayers (SAMs) with inserted DNA probes are promising biosensor designs because of their ease of preparation, miniaturization, and tunability. However, much is still unknown about the interactions between biomolecules such as DNA and various surfaces. A fundamental question regarding these sensors concerns the nature of diffusion of target molecules taking place on sensor surfaces and whether it speeds up the molecular recognition process. Lack of understanding of molecular interaction and surface diffusion in addition to questions regarding the behavior of DNA probes immobilized on these surfaces currently limits the rational design of nucleic acid biosensors. Using all-atom unbiased molecular dynamics (MD) simulations we found that single-stranded DNA (ssDNA) behavior on SAMs is drastically altered by different surface chemistries, with ssDNA adopting very different orientations upon adsorption and surface diffusivity varying over an order of magnitude. Probe behavior varies equally broadly as probes are considerably more stable in certain SAMs than others, which affects the accessibility of probes to the target molecules and likely changes DNA hybridization kinetics in multiple ways. We also found that nearby probes can alter each other's orientations substantially, which highlights the importance of surface density control. Our results elucidate nucleic acid biosensor dynamics vital to rational design and offer insights that can aid in the design of surface properties and patterning for specific applications.

Interfacial Conformation of Hydrophilic Polyphosphoesters Affects Blood Protein Adsorption

Synthetic polymers are commonly used as protein repelling materials for a variety of biomedical applications. Despite their widespread use, the fundamental mechanism underlying protein repellence is often elusive. Such insights are essential for improving existing and developing new materials. Here, we investigate how subtle differences in the chemistry of hydrophilic polyphosphoesters influence the adsorption of the human blood proteins serum albumin and fibrinogen. Using thermodynamic measurements, surface-specific vibrational spectroscopy, and Brewster angle microscopy, we investigate protein adsorption, hydration, and steric repulsion properties of the polyphosphoester polymers. Whereas both surface hydration and polymer conformation of the polymers vary substantially as a consequence of the chemical differences in the polymer structure, the protein repellency ability of these hydrophilic materials appears to be dominated by steric repulsion.

Cross-linker effect on solute adsorption in swollen thermoresponsive polymer networks

Molecular dynamics study on the solute adsorption to thermoresponsive polymers estimating the cross-link impact on particle partitioning in swollen hydrogels.

Platelet-Activation Mechanisms and Vascular Remodeling

This overview article for the Comprehensive Physiology collection is focused on detailing platelets, how platelets respond to various stimuli, how platelets interact with their external biochemical environment, and the role of platelets in physiological and pathological processes. Specifically, we will discuss the four major functions of platelets: activation, adhesion, aggregation, and inflammation. We will extend this discussion to include various mechanisms that can induce these functional changes and a discussion of some of the salient receptors that are responsible for platelets interacting with their external environment. We will finish with a discussion of how platelets interact with their vascular environment, with a special focus on interactions with the extracellular matrix and endothelial cells, and finally how platelets can aid and possibly initiate the progression of various vascular diseases. Throughout this overview, we will highlight both the historical investigations into the role of platelets in health and disease as well as some of the more current work. Overall, the authors aim for the readers to gain an appreciation for the complexity of platelet functions and the multifaceted role of platelets in the vascular system. © 2017 American Physiological Society. Compr Physiol 8:1117-1156, 2018.

Angle-Dependent Atomic Force Microscopy Single-Chain Pulling of Adsorbed Macromolecules from Planar Surfaces Unveils the Signature of an Adsorption-Desorption Transition

The adsorption-desorption behavior of polymer chains is at the heart of macromolecular surface science and technology. With the current developments in atomic force microscopy (AFM), it has now become possible to address the desorption problem from the perspective of a single macromolecule. Here, we report on desorption of single polymer chains on planar surfaces by AFM-based single molecule force spectroscopy (SMFS) as a function of the pulling angle with respect to the surface-normal direction. SMFS experiments were performed in water with various substrates using different polymers covalently attached to the AFM probe tip. End-grafting at the AFM tip was achieved by surface-initiated polymerization using initiator functionalized tips. We found that the desorption force increases with a decreasing pulling angle, i.e., an enhanced adhesion of the polymer chain was observed. The magnitude of the desorption force shows a weak angular dependence at pulling angles close to the surface normal. A significant increase of the force is observed at shallower pulling from a certain pulling angle. This behavior carries the signature of an adsorption-desorption transition. The angular dependence of the normalized desorption force exhibits a universal behavior. We compared and interpreted our results using theoretical predictions for single-chain adsorption-desorption transitions.

Experimental characterization and simulation of amino acid and peptide interactions with inorganic materials

Inspired by nature, many applications and new materials benefit from the interplay of inorganic materials and biomolecules. A fundamental understanding of complex organic-inorganic interactions would improve the controlled production of nanomaterials and biosensors to the development of biocompatible implants for the human body. Although widely exploited in applications, the interaction of amino acids and peptides with most inorganic surfaces is not fully understood. To date, precisely characterizing complex surfaces of inorganic materials and analyzing surface-biomolecule interactions remain challenging both experimentally and computationally. This article reviews several approaches to characterizing biomolecule-surface interactions and illustrates the advantages and disadvantages of the methods presented. First, we explain how the adsorption mechanism of amino acids/peptides to inorganic surfaces can be determined and how thermodynamic and kinetic process constants can be obtained. Second, we demonstrate how this data can be used to develop models for peptide-surface interactions. The understanding and simulation of such interactions constitute a basis for developing molecules with high affinity binding domains in proteins for bioprocess engineering and future biomedical technologies.

Investigating the Role of Phosphorylation in the Binding of Silaffin Peptide R5 to Silica with Molecular Dynamics Simulations

Biomimetic silica formation, a process that is largely driven by proteins, has garnered considerable interest in recent years due to its role in the development of new biotechnologies. However, much remains unknown of the molecular-scale mechanisms underlying the binding of proteins to biomineral surfaces such as silica, or even of the key residue-level interactions between such proteins and surfaces. In this study, we employ molecular dynamics (MD) simulations to study the binding of R5-a 19-residue segment of a native silaffin peptide used for in vitro silica formation-to a silica surface. The metadynamics enhanced sampling method is used to converge the binding behavior of R5 on silica at both neutral (pH 7.5) and acidic (pH 5) conditions. The results show fundamental differences in the mechanism of binding between the two cases, providing unique insight into the pH-dependent ability of R5 and native silaffin to precipitate silica. We also study the effect of phosphorylation of serine residues in R5 on both the binding free energy to silica and the interfacial conformation of the peptide. Results indicate that phosphorylation drastically decreases the binding free energy and changes the structure of silica-adsorbed R5 through the introduction of charge and steric repulsion. New mechanistic insights from this work could inform rational design of new biomaterials and biotechnologies.

Single Molecule Study on Polymer-Nanoparticle Interactions: The Particle Shape Matters

The study on the nanoparticle-polymer interactions is very important for the design/preparation of high performance polymer nanocomposite. Here we present a method to quantify the polymer-particle interaction at single molecule level by using AFM-based single molecule force spectroscopy (SMFS). As a proof-of-concept study, we choose poly-l-lysine (PLL) as the polymer and several different types of polyoxometalates (POM) as the model particles to construct several different polymer nanocomposites and to reveal the binding mode and quantify the binding strength in these systems. Our results reveal that the shape of the nanoparticle and the binding geometry in the composite have significantly influenced the binding strength of the PLL/POM complexes. Our dynamic force spectroscopy studies indicate that the disk-like geometry facilitate the unbinding of PLL/AlMo₆ complexes in shearing mode, while the unzipping mode becomes dominate in spherical PLL-P₈W₄₈ system. We have also systematically investigated the effects of charge numbers, particle size, and ionic strength on the binding strength and binding mode of PLL/POM, respectively. Our results show that electrostatic interactions dominate the stability of PLL/POM complexes. These findings provide a way for tuning the mechanical properties of polyelectrolyte-nanoparticle composites.

Mixed Self-Assembled Aptamer and Newly Designed Zwitterionic Peptide as Antifouling Biosensing Interface for Electrochemical Detection of alpha-Fetoprotein

A sensitive and low-fouling aptasensor for alpha-fetoprotein (AFP) was developed based on mixed self-assembled aptamers and newly designed zwitterionic peptides, where densely immobilized peptides formed an antifouling layer to resist nonspecific protein adsorption, and sparsely attached aptamers acted as the recognizing layer to achieve target binding. The obtained biosensing interface responded to the target AFP with a strikingly selective and sensitive manner, exhibited excellent protein-resistant performance even in complex human serum solution, and showed promising feasibility for the quantitative analysis of AFP in real human serum samples.

Unraveling Hydrophobic Interactions at the Molecular Scale Using Force Spectroscopy and Molecular Dynamics Simulations

Interactions between hydrophobic moieties steer ubiquitous processes in aqueous media, including the self-organization of biologic matter. Recent decades have seen tremendous progress in understanding these for macroscopic hydrophobic interfaces. Yet, it is still a challenge to experimentally measure hydrophobic interactions (HIs) at the single-molecule scale and thus to compare with theory. Here, we present a combined experimental-simulation approach to directly measure and quantify the sequence dependence and additivity of HIs in peptide systems at the single-molecule scale. We combine dynamic single-molecule force spectroscopy on model peptides with fully atomistic, both equilibrium and nonequilibrium, molecular dynamics (MD) simulations of the same systems. Specifically, we mutate a flexible (GS)₅ peptide scaffold with increasing numbers of hydrophobic leucine monomers and measure the peptides' desorption from hydrophobic self-assembled monolayer surfaces. Based on the analysis of nonequilibrium work-trajectories, we measure an interaction free energy that scales linearly with 3.0-3.4 k_BT per leucine. In good agreement, simulations indicate a similar trend with 2.1 k_BT per leucine, while also providing a detailed molecular view into HIs. This approach potentially provides a roadmap for directly extracting qualitative and quantitative single-molecule interactions at solid/liquid interfaces in a wide range of fields, including interactions at biointerfaces and adhesive interactions in industrial applications.

Water-Mediated Interactions between Hydrophilic and Hydrophobic Surfaces

All surfaces in water experience at short separations hydration repulsion or hydrophobic attraction, depending on the surface polarity. These interactions dominate the more long-ranged electrostatic and van der Waals interactions and are ubiquitous in biological and colloidal systems. Despite their importance in all scenarios where the surface separation is in the nanometer range, the origin of these hydration interactions is still unclear. Using atomistic solvent-explicit molecular dynamics simulations, we analyze the interaction free energies of charge-neutral model surfaces with different elastic and water-binding properties. The surface polarity is shown to be the most important parameter that not only determines the hydration properties and thereby the water contact angle of a single surface but also the surface-surface interaction and whether two surfaces attract or repel. Elastic properties of the surfaces are less important. On the basis of surface contact angles and surface-surface binding affinities, we construct a universal interaction diagram featuring three different interaction regimes-hydration repulsion, cavitation-induced attraction-and for intermediate surface polarities-dry adhesion. On the basis of scaling arguments and perturbation theory, we establish simple combination rules that predict the interaction behavior for combinations of dissimilar surfaces.

Mechanism of Reversible Peptide-Bilayer Attachment: Combined Simulation and Experimental Single-Molecule Study

The binding of peptides and proteins to lipid membrane surfaces is of fundamental importance for many membrane-mediated cellular processes. Using closely matched molecular dynamics simulations and atomic force microscopy experiments, we study the force-induced desorption of single peptide chains from phospholipid bilayers to gain microscopic insight into the mechanism of reversible attachment. This approach allows quantification of desorption forces and decomposition of peptide-membrane interactions into energetic and entropic contributions. In both simulations and experiments, the desorption forces of peptides with charged and polar side chains are much smaller than those for hydrophobic peptides. The adsorption of charged/polar peptides to the membrane surface is disfavored by the energetic components, requires breaking of hydrogen bonds involving the peptides, and is favored only slightly by entropy. By contrast, the stronger adsorption of hydrophobic peptides is favored both by energy and by entropy and the desorption forces increase with increasing side-chain hydrophobicity. Interestingly, the calculated net adsorption free energies per residue correlate with experimental results of single residues, indicating that side-chain free energy contributions are largely additive. This observation can help in the design of peptides with tailored adsorption properties and in the estimation of membrane binding properties of peripheral membrane proteins.

Dynamics of Seeded Aβ40-Fibril Growth from Atomistic Molecular Dynamics Simulations: Kinetic Trapping and Reduced Water Mobility in the Locking Step

Filamentous β-amyloid aggregates are crucial for the pathology of Alzheimer's disease. Despite the tremendous biomedical importance, the molecular pathway of growth propagation is not completely understood and remains challenging to investigate by simulations due to the long time scales involved. Here, we apply extensive all-atom molecular dynamics simulations in explicit water to obtain free energy profiles and kinetic information from position-dependent diffusion profiles for three different Aβ9-40-growth processes: fibril elongation by single monomers at the structurally unequal filament tips and association of larger filament fragments. Our approach provides insight into the molecular steps of the kinetic pathway and allows close agreement with experimental binding free energies and macroscopic growth rates. Water plays a decisive role, and solvent entropy is identified as the main driving force for assembly. Fibril growth is disfavored energetically due to cancellation of direct peptide-peptide interactions and solvation effects. The kinetics of growth is consistent with the characteristic dock/lock mechanism, and docking is at least 2 orders of magnitude faster. During initial docking, interactions are mediated by transient non-native hydrogen bonds, which efficiently catch the incoming monomer or fragment already at separations of about 3 nm. In subsequent locking, the dynamics is much slower due to formation of kinetically trapped conformations caused by long-lived non-native hydrogen bonds. Fibril growth additionally requires collective motion of water molecules to create a dry binding interface. Fibril growth is further retarded due to reduced mobility of the involved hydration water, evident from a 2-fold reduction of the diffusion coefficient.

3D Au–SiO2 nanohybrids as a potential scaffold coating material for neuroengineering

We demonstrate 3D Au–SiO₂ hybrid nanoparticles render micro/nanotopography and provide a high density of stable adhesion cue domains facilitating strong adhesion, viability and guidance of the neurons.

Surface-water Interface Induces Conformational Changes Critical for Protein Adsorption: Implications for Monolayer Formation of EAS Hydrophobin

The class I hydrophobin EAS is part of a family of small, amphiphilic fungal proteins best known for their ability to self-assemble into stable monolayers that modify the hydrophobicity of a surface to facilitate further microbial growth. These proteins have attracted increasing attention for industrial and biomedical applications, with the aim of designing surfaces that have the potential to maintain their clean state by resisting non-specific protein binding. To gain a better understanding of this process, we have employed all-atom molecular dynamics to study initial stages of the spontaneous adsorption of monomeric EAS hydrophobin on fully hydroxylated silica, a commonly used industrial and biomedical substrate. Particular interest has been paid to the Cys3-Cys4 loop, which has been shown to exhibit disruptive behavior in solution, and the Cys7-Cys8 loop, which is believed to be involved in the aggregation of EAS hydrophobin at interfaces. Specific and water mediated interactions with the surface were also analyzed. We have identified two possible binding motifs, one which allows unfolding of the Cys7-Cys8 loop due to the surfactant-like behavior of the Cys3-Cys4 loop, and another which has limited unfolding due to the Cys3-Cys4 loop remaining disordered in solution. We have also identified intermittent interactions with water which mediate the protein adsorption to the surface, as well as longer lasting interactions which control the diffusion of water around the adsorption site. These results have shown that EAS behaves in a similar way at the air-water and surface-water interfaces, and have also highlighted the need for hydrophilic ligand functionalization of the silica surface in order to prevent the adsorption of EAS hydrophobin.

Single lipid extraction: the anchoring strength of cholesterol in liquid-ordered and liquid-disordered phases

Cholesterol is important for the formation of microdomains in supported lipid bilayers and is enriched in the liquid-ordered phase. To understand the interactions leading to this enrichment, we developed an AFM-based single-lipid-extraction (SLX) approach that enables us to determine the anchoring strength of cholesterol in the two phases of a phase-separated lipid membrane. As expected, the forces necessary for extracting a single cholesterol molecule from liquid-ordered phases are significantly higher than for extracting it from the liquid-disordered phases. Interestingly, application of the Bell model shows two energy barriers that correlate with the head and full length of the cholesterol molecule. The resulting lifetimes for complete extraction are 90 s and 11 s in the liquid-ordered and liquid-disordered phases, respectively. Molecular dynamics simulations of the very same experiment show similar force profiles and indicate that the stabilization of cholesterol in the liquid-ordered phase is mainly due to nonpolar contacts.

Parameterization of an interfacial force field for accurate representation of peptide adsorption free energy on high-density polyethylene

Interfacial force field (IFF) parameters for use with the CHARMM force field have been developed for interactions between peptides and high-density polyethylene (HDPE). Parameterization of the IFF was performed to achieve agreement between experimental and calculated adsorption free energies of small TGTG-X-GTGT host-guest peptides (T = threonine, G = glycine, and X = variable amino-acid residue) on HDPE, with ±0.5 kcal/mol agreement. This IFF parameter set consists of tuned nonbonded parameters (i.e., partial charges and Lennard-Jones parameters) for use with an in-house-modified CHARMM molecular dynamic program that enables the use of an independent set of force field parameters to control molecular behavior at a solid-liquid interface. The R correlation coefficient between the simulated and experimental peptide adsorption free energies increased from 0.00 for the standard CHARMM force field parameters to 0.88 for the tuned IFF parameters. Subsequent studies are planned to apply the tuned IFF parameter set for the simulation of protein adsorption behavior on an HDPE surface for comparison with experimental values of adsorbed protein orientation and conformation.

Tuning neuron adhesion and neurite guiding using functionalized AuNPs and backfill chemistry

Gold nanoparticles are used to investigate the dependence of neuron adhesion on the density of cell binding sites and particle backfill. Neurons viability and neurite development depend differently on cell attractive and cell repellant surface cues.

Detachment of semiflexible polymer chains from a substrate: a molecular dynamics investigation

Using Molecular Dynamics simulations, we study the force-induced detachment of a coarse-grained model polymer chain from an adhesive substrate. One of the chain ends is thereby pulled at constant speed off the attractive substrate and the resulting saw-tooth profile of the measured mean force ⟨f⟩ vs height D of the end-segment over the plane is analyzed for a broad variety of parameters. It is shown that the observed characteristic oscillations in the ⟨f⟩-D profile depend on the bending and not on the torsional stiffness of the detached chains. Allowing for the presence of hydrodynamic interactions (HI) in a setup with explicit solvent and dissipative particle dynamics-thermostat, rather than the case of Langevin thermostat, one finds that HI have little effect on the ⟨f⟩-D profile. Also the change of substrate affinity with respect to the solvent from solvophilic to solvophobic is found to play negligible role in the desorption process. In contrast, a changing ratio ε(s)(B)/ε(s)(A) of the binding energies of A- and B-segments in the detachment of an AB-copolymer from adhesive surface strongly changes the ⟨f⟩-D profile whereby the B-spikes vanish when ε(s)(B)/ε(s)(A)<0.15. Eventually, performing an atomistic simulation of (bio)-polymers, we demonstrate that the simulation results, derived from our coarse-grained model, comply favorably with those from the all-atom simulation.