Polypeptide friction and adhesion on hydrophobic and hydrophilic surfaces: a molecular dynamics case study

Using all-atomistic MD simulations including explicit water, the mobility and adhesion of a mildly hydrophobic single polypeptide chain adsorbed on hydrophobic and hydrophilic diamond surfaces is investigated by application of lateral and vertical pulling forces. Forced motion on the hydrophilic surface exhibits stick-slip due to breaking and reformation of hydrogen bonds; in contrast, on the hydrophobic surface, the motion is smooth. By carefully tuning the driving force magnitude, the linear-response regime is reached on a hydrophobic surface and equilibrium values for mobility and adhesive strength are obtained. On the hydrophilic surface, on the other hand, slow hydrogen-bond kinetics prevents equilibration and only upper bounds for adhesion force and mobility can be estimated. Whereas the desorption force is rather comparable on the two surfaces and differs at most by a factor of 2, the mobility on the hydrophilic surface is at least 30-fold reduced compared to the hydrophobic one. A simple model based on a single particle diffusing in a corrugated potential landscape suggests that cooperativity is rather limited and that the small mobility on a hydrophilic surface can be rationalized in terms of incoherently moving monomers. The experimentally well-known peptide mobility in bulk water is quantitatively reproduced in our simulations, which serves as a sensitive test on our methodology employed.     

The hydrophobic effect: molecular dynamics simulations of water confined between extended hydrophobic and hydrophilic surfaces

Structural and dynamic properties of water confined between two parallel, extended, either hydrophobic or hydrophilic crystalline surfaces of n-alkane C(36)H(74) or n-alcohol C(35)H(71)OH, are studied by molecular dynamics simulations. Electron density profiles, directly compared with corresponding experimental data from x-ray reflectivity measurements, reveal a uniform weak de-wetting characteristic for the extended hydrophobic surface, while the hydrophilic surface is weakly wetted. These microscopic data are consistent with macroscopic contact angle measurements. Specific water orientation is present at both surfaces. The ordering is characteristically different between the surfaces and of longer range at the hydrophilic surface. Furthermore, the dynamic properties of water are different at the two surfaces and different from the bulk behavior. In particular, at the hydrophobic surface, time-correlation functions reveal that water molecules have characteristic diffusive behavior and orientational ordering due to the lack of hydrogen bonding interactions with the surface. These observations suggest that the altered dynamical properties of water in contact with extended hydrophobic surfaces together with a partial drying of the surfaces are more indicative of the hydrophobic effect than structural ordering, which we suggest to be independent of surface topology.     

Fabrication of hydrophobic surfaces by coupling of Langmuir-Blodgett deposition and a self-assembled monolayer

A novel method coupling the Langmuir-Blodgett (LB) deposition of silica particles and the formation of a self-assembled monolayer (SAM) of alkylsilane is proposed for fabricating hydrophobic surfaces. The LB deposition and the SAM are supposed to confer the substrate surface roughness and low surface energy, respectively. By controlling the hydrophobic-hydrophilic balance of the silica particle surface through the adsorption of surfactant molecules, deposition of monolayers consisting of hexagonally close-packed arrays of particles on a glass substrate can then be successfully conducted in a Langmuir trough. LB particulate films with a particle layer number up to 5 were thereby prepared. A sintered and hydrophobically finished particulate film with roughness factor of 1.9 was finally fabricated by sintering and surface silanization. Effects of particle size and particle layer number on the wetting behavior of the particulate films were systematically studied by measuring static and dynamic water contact angles. The experimental results revealed that a static contact angle of about 130 degrees resulted from the particulate films regardless of the particle size and particle layer number. This is consistent with the predictions of both the Wenzel model and the Cassie and Baxter model in that roughness of a hydrophobic surface can increase its hydrophobicity and a switching of the dominant mode from Wenzel's to Cassie and Baxter's. In general, an advancing contact angle of about 150 degrees , a receding contact angle of about 110 degrees , and a contact angle hysteresis of about 40 degrees were exhibited by the particulate films fabricated.     

A molecular dynamics study of water near silicate surfaces

We have conducted a molecular dynamics simulation of water between uncharged silicate surfaces in order to understand, on a molecular basis, the structural and dynamical properties of vicinal water (i.e., the water between the surfaces). The results of our preliminary simulation indicate that the vicinal water differs substantially from pure bulk water in the transient orientation of molecular dipole moments and rate of relaxation of these moments. In contrast, no significant long-range differences between the radial distribution functions or hydrogen bonding patterns of vicinal and bulk water are evident.     

Interfacial water at hydrophobic and hydrophilic surfaces: depletion versus adsorption

The structure of the water-solid interface for widely varying surface properties is investigated with Monte Carlo simulations using the SPC/E water model. Of particular interest is the relation between the wetting coefficient as a measure of the hydrophobicity of the substrate and the density depletion close to the solid surface. The substrates are modeled as rigid ordered lattices of sites that interact with water molecules through an orientation-independent Lennard-Jones potential of varying strength. Hydrophilic character is obtained by addition of polar hydroxyl groups on the substrate surface, and the influence of density, spatial distribution, and angular orientation of the polar groups on the interfacial water structure is studied.     

Computer simulation of adhesion between hydrophilic and hydrophobic self-assembled monolayers in water

The grand canonical Monte Carlo technique and atomistic force fields are used to calculate the force-distance relations and free energies of adhesion between carboxyl and methyl terminated alkanethiolate self-assembled monolayers (SAMs) in water. Both symmetric and asymmetric confinements are considered, as formed by like and unlike SAMs, respectively. As the confinement is increased, water confined by the hydrophobic methyl terminated SAMs experiences capillary evaporation. As a consequence, the adhesion energy is determined by the direct interaction between bare SAMs. In the asymmetric system, an incomplete capillary evaporation is observed, with the number of water molecules dropped by more than an order of magnitude. The remaining water molecules are all adsorbed on the hydrophilic SAM, while the hydrophobic SAM is separated from the rest of the system by a thin vapor layer. The calculated free energies of adhesion are in acceptable agreement with experiment.     

Orientational dynamics of water trapped between two nanoscopic hydrophobic solutes: A molecular dynamics simulation study

We investigate thoroughly the effect of confinement and solute topology on the orientational dynamics of water molecule in the interplate region between two nanoscopic hydrophobic paraffinlike plates. Results are obtained from molecular dynamics simulations of aqueous solutions of paraffinlike plates in the isothermal-isobaric ensemble. An analysis of survival time auto correlation function shows that the residence time of the water molecule in the confined region between two model nanoscopic hydrophobic plates depends on solute surface topology (intermolecular distance within the paraffinlike plate). As expected, the extent of confinement also changes the residence time of water molecules considerably. Orientational dynamics was analyzed along three different directions, viz., dipole moment, HH, and perpendicular to molecular plane vectors. It has been demonstrated that the rotational dynamics of the confined water does not follow the Debye rotational diffusion model, and surface topology of the solute plate and the extent of confinement have considerable effect on the rotational dynamics of the confined water molecules.     

Toward molecular electronic circuitry: selective deposition of metals on patterned self-assembled monolayer surfaces

We have developed a simple, robust method by which to construct complex two-dimensional structures based on controlling interfacial chemistry. Our approach is to employ UV-photopatterning and the reaction of vapor-deposited metals with self-assembled monolayers. To demonstrate the method, we have selectively vapor-deposited Mg on a patterned -CH3/-COOH-terminated alkanethiolate surface. The deposited metal penetrates through the -CH3 SAM to the Au/S interface while reacting with and accumulating on top of the -COOH SAM. This work has important applications in molecular/organic electronics, sensing, and other technologies. Our method has many advantages: it is extensible to many different materials, easily parallelized, affords precise nanoscale placement, and is fully compatible with photolithography.     

Tuning nanoscopic water layers on hydrophobic and hydrophilic surfaces with laser light

The evolutional function of ordered interfacial water near solid surfaces was postulated by Szent-Gy?rgyi: "Life actually, may have started with building these water structures." Here we report their tunability with laser light on both hydrophobic and hydrophilic surfaces. On the former, the light caused their depletion--on the latter, an increase in fluidity--as measured by atomic force acoustic microscopy. Interfacial water layers play a key role in cellular recognition. Their tunability promises to revolutionize various fields in biomedical engineering and life sciences.     

Microscopic wetting of self-assembled monolayers with different surfaces: a combined molecular dynamics and quantum mechanics study

Molecular dynamics simulations are used to study the micronature of the organization of water molecules on the flat surface of well-ordered self-assembled monolayers (SAMs) of 18-carbon alkanethiolate chains bound to a silicon (111) substrate. Six different headgroups (-CH(3), -C═C, -OCH(3), -CN, -NH(2), -COOH) are used to tune the character of the surface from hydrophobic to hydrophilic, while the level of hydration is consistent on all six SAM surfaces. Quantum mechanics calculations are employed to optimize each alkyl chain of the different SAMs with one water molecule and to investigate changes in the configuration of each headgroup under hydration. We report the changes of the structure of the six SAMs with different surfaces in the presence of water, and the area of the wetted surface of each SAM, depending on the terminal group. Our results suggest that a corrugated and hydrophobic surface will be formed if the headgroups of SAM surface are not able to form H-bonds either with water molecules or between adjacent groups. In contrast, the formation of hydrogen bonds not only among polar heads but also between polar heads and water may enhance the SAM surface hydrophilicity and corrugation. We explicitly discuss the micromechanisms for the hydration of three hydrophilic SAM (CN-, NH(2)- and COOH-terminated) surfaces, which is helpful to superhydrophilic surface design of SAM in biomimetic materials.     

Adsorption of trypsin on hydrophilic and hydrophobic surfaces

The adsorption of trypsin onto polystyrene and silica surfaces was investigated by reflectometry, spectroscopic methods, and atomic force microscopy (AFM). The affinity of trypsin for the hydrophobic polystyrene surface was higher than that for the hydrophilic silica surface, but steady-state adsorbed amounts were about the same at both surfaces. The conformational characteristics of trypsin immobilized on silica and polystyrene nanospheres were analyzed in situ by circular dichroism and fluorescence spectroscopy. Upon adsorption the trypsin molecules underwent structural changes at the secondary and tertiary level, although the nature of the structural alterations was different for silica and polystyrene surfaces. AFM imaging of trypsin adsorbed on silica showed clustering of enzyme molecules. Rinsing the silica surface resulted in 20% desorption of the originally adsorbed enzyme molecules. Adsorption of trypsin on the surface of polystyrene was almost irreversible with respect to dilution. After adsorption on silica the enzymatic activity of trypsin was 10 times lower, and adsorbed on polystyrene the activity was completely suppressed. The trypsin molecules that were desorbed from the sorbent surfaces by dilution with buffer regained full enzymatic activity.     

Sliding of water droplets on hydrophobic surfaces with various hydrophilic region sizes

Four patterned surfaces with hydrophilic areas of different sizes were prepared using photolithography with a smooth octadecyltrimethoxysilane (ODS) hydrophobic coating. The hydrophilic area in the surfaces was aligned hexagonally with a constant area fraction. The sliding angle and contact angle hysteresis of the water droplets increased concomitantly with increasing pattern size. The increase of the contact line distortion between defects at the receding side plays an important role in this trend. The droplet sliding velocity also increased concomitantly with increasing pattern size. This trend was simulated by a simple flow model. The contribution of the interface between the ODS region and the hydrophilic area was deduced from this trend. This study demonstrated the different size dependency of the chemical surface defects for sliding behavior between the critical moment at which a droplet slides down and the period when a droplet is sliding.     

Shattering of Peptide ions on self-assembled monolayer surfaces

Time- and collision energy-resolved surface-induced dissociation (SID) of des-Arg(1)- and des-Arg(9)-bradykinin on a fluorinated self-assembled monolayer (SAM) surface was studied by use of a novel Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially equipped to perform SID experiments. Time-resolved fragmentation efficiency curves (TFECs) were modeled by an RRKM-based approach developed in our laboratory that utilizes a very flexible analytical expression for the internal energy deposition function capable of reproducing both single- and multiple-collision activation in the gas phase and excitation by collisions with a surface. Both experimental observations and modeling establish a very sharp transition in the dynamics of ion-surface interaction: the shattering transition. The experimental signature for this transition is the appearance of prompt (time-independent) fragmentation, which becomes dominant at high collision energies. Shattering opens a variety of dissociation pathways that are not accessible to slow collisional and thermal ion activation. This results in much better sequence coverage for the singly protonated peptides than dissociation patterns obtained with any of the slow activation methods. Modeling demonstrated that, for short reaction delays, dissociation of these peptides is solely determined by shattering. Internal energies required for shattering transition are approximately the same for des-Arg(1) and des-Arg(9)-bradykinin, resulting in the overlap of fragmentation efficiency curves obtained at short reaction delays. At longer delay times, parent ions depletion is mainly determined by a slow decay rate and fragmentation efficiency curves for des-Arg(1) and des-Arg(9)-bradykinin diverge. Dissociation thresholds of 1.17 and 1.09 eV and activation entropies of -22.2 and -23.3 cal/(mol K) were obtained for des-Arg(1) and des-Arg(9)-bradykinin from RRKM modeling of time-resolved data. Dissociation parameters for des-Arg(1)-bradykinin are in good agreement with parameters derived from thermal experiments. However, there is a significant discrepancy between the thermal data and dissociation parameters for des-Arg(9)-bradykinin obtained in this study. The difference is attributed to the differences in conformations that undergo thermal activation and activation by ion-surface collisions prior to dissociation.     

Adsorption of polyacrylic acid on hydrophobic and hydrophilic surfaces

In this paper the adsorption of polyacrylic acid (MW=5000) on the hydrophobic mercury surface and on the hydrophilic -Al₂O₃ surface at pH=3–4 in 0.55 M sodium chloride solution was investigated. Measurements of change of the double layer capacitance by phase selective a.c. voltammetry were used for determination of the adsorption of polyacrylic acid on the mercury electrode. The same method was used for the determination of the polyacrylic acid remaining in the solution after the adsorption on hydrophilic particles (-Al₂O₃ particles). The results obtained for adsorption of polyacrylic acid were compared to the results of the adsorption of humic substance of similar molecular weight under similar experimental conditions. The study has shown that polyacrylic acid in acidic solution is strongly adsorbed on the mercury surface, which is comparable to the adsorption of humic substance on the mercury surface. At the same time, the adsorption/deposition of polyacrylic acid on the -Al₂O₃ surface is weaker compared to humic acid, indicating at a smaller degree of interaction of polyacrylic acid with aluminium ions and with hydrophilic surface.     

Cationic electrolyte adsorption layers on hydrophilic and hydrophobic surfaces

The capillary electrokinetics method (measurements of streaming potential and current in original and hydrophobized fused quartz capillaries with radii of 5–7 μm) is employed to study the formation of adsorption layers upon contact with solutions containing a cationic polyelectrolyte, poly(diallyldimethylammonium chloride). It is shown that polyelectrolyte adsorption causes the charge reversal of both hydrophilic and hydrophobic surfaces, with a smaller amount of the substance being adsorbed on the hydrophobic than on the hydrophilic surface. The adsorption on both surfaces increases with the polymer solution concentration. The cationic polyelectrolyte adsorption on the pure quartz surface occurs mainly due to the electrostatic attraction, while, in the case of the hydrophobic surface, the contribution of hydrophobic interactions increases. The study of the layer deformability shows that, on the hydrophilic surfaces, the layer ages and its structure depends on the polymer solution concentration. On the modified surface, the deformation of even freshly formed layers is slight, which suggests that a denser layer is formed on the hydrophobic surface. In contrast to the hydrophilic surface, the polyelectrolyte is partly desorbed from the hydrophobic surface.     

Template-assisted patterning of nanoscale self-assembled monolayer arrays on surfaces

We report a general, simple, and inexpensive approach to pattern features of self-assembled monolayers (SAMs) on silicon and gold surfaces using porous anodic alumina films as templates. The SAM patterns, with feature sizes down to 30 nm and densities higher than 10(10)/cm(2), can be prepared over large areas (>5 cm(2)). The feature dimensions can be tuned by controlling the alumina template structure. These SAM patterns have been successfully used as resists for fabricating gold and silicon nanoparticle arrays on substrates by wet-chemical etching. In addition, we show that arrays of gold features can be patterned with 10-nm gaps between the dots.     

Molecular dynamics simulation study on adsorption and diffusion processes of a hydrophilic chain on a hydrophobic surface

Molecular dynamics simulations are applied to investigate the adsorption and diffusion processes of a single hydrophilic poly(vinyl alcohol) (PVA) chain with different chain lengths on a hydrophobic graphite surface. It is expected that the chain and the surface "dislike" each other because one is hydrophilic and the other is hydrophobic. But surprisingly, a short PVA chain is well adsorbed on the surface, accompanied by large changes in the chain configuration. With increasing degree of polymerization (N), the chain turns gradually from two-dimensional adsorption to possessing certain height in the direction perpendicular to the surface. Moreover, the adsorption energy increases and the diffusion coefficient decreases with increasing N. In particular, for N = 20 in equilibrium, the hydroxyls of this short chain are close to the graphite surface in the stable adsorption configuration. In addition, we change the effective dielectric constant to 76.0 to mimic good solvent condition. The chain configurations and the diffusion coefficients both vary in contrast to the foregoing results.     

Two-dimensional supercritical behavior of an ethanol monolayer: a molecular dynamics study

The two-dimensional (2D) supercritical behavior of an ethanol monolayer formed at the vapor/liquid interface of an ethanol solution has been investigated by a molecular dynamics (MD) calculations with a combination of the OPLS-UA and SPC/E potential models. A 100 A thick slab of ethanol solution was placed at the volume center of the rectangular unit cell by 10 A thick nonabsorbate water surfaces. With such an initial configuration, five independent 15 ns NVT constant MD calculations were carried out under 298.15 K, in which the initial ethanol mole fraction of the bulk solution layer was set to 0.010, 0.022, 0.045, 0.10, and 0.20, respectively. The 2D radial distribution function (rdf) of an adsorbed ethanol molecule showed that the ethanol monolayer could be regarded as a 2D fluid where the adsorbed ethanol molecule had an effective 2D diameter of 4.65 A. On the basis of the 2D rdf result, 2D cluster analysis was carried out from the perspective of the percolation theory. It is confirmed that the critical area occupation probability density, the critical exponents, and the fractal dimension of both nonpercolating and percolating clusters satisfied their nature of universality. Therefore, we concluded that an ethanol monolayer formed at the vapor/liquid interface of ethanol solution behaves as a 2D supercritical fluid at 298.15 K.     

Diffusion on a self-assembled monolayer: molecular modeling of a bound + mobile lubricant

The diffusion of tricresyl phosphate molecules on an octadecyltrichlorosilane self-assembled monolayer (SAM) was characterized using molecular dynamics simulations. The simulations predict that when placed on the top of a close-packed SAM, the molecules remain mobile on the surface with an isotropic diffusion activation energy of approximately 9 kJ/mol. In contrast, an anisotropic barrier that results from chain tilt within the SAM is predicted for diffusion into a defect created by reducing the alkane chain length within a cylinderical region of the surface. Once in the defect, the molecules become trapped by embedding part of the molecule into the side of the SAM.     

Multilayers of weak polyelectrolytes of low and high molecular mass assembled on polypropylene and self-assembled hydrophobic surfaces

Hydrophobic self-assembled octadecyltrichlorosilane (ODTS), ultrathin films of polypropylene, and ODTS modified with cationic dioctadecyldimethylammonium bromide are employed as substrates for deposition of multilayers of poly(allylamine hydrochloride) and poly(acrylic acid) from aqueous solution. The assembly of highly dissipative polyelectrolyte multilayers (PEMs) is demonstrated by quartz crystal microgravimetry. The initial rate of adsorption is faster and the adsorbed amount larger on the cationic surface, while the detailed structure of the PEMs, as determined by atomic force microscopy imaging, is related primarily to the molecular weight of the adsorbing polymers. A more extensive PEM adsorption on the hydrophobic surfaces takes place with increasing ionic strength of the background electrolyte solution. The water contact angle depends on the type of polymer adsorbed as the outermost layer, indicating that, despite the expected interdiffusion for the different polymer chains, there is a net macromolecular segregation to the free surface. Surface modification with the high molecular weight PEMs produces a more marked reduction of the hydrophilicity of the substrate.