Consequences of Hydrocarbon Contamination for Wettability and

Article pubs.acs.org/JPCC

Consequences of Hydrocarbon Contamination for Wettability and Protein Adsorption on Graphite Surfaces Christian Mücksch, Christina Rösch, Christine Müller−Renno, Christiane Ziegler, and Herbert M. Urbassek* Fachbereich Physik und Forschungszentrum OPTIMAS, University of Kaiserslautern, Erwin-Schrödinger-Straße, D-67663 Kaiserslautern, Germany ABSTRACT: We present results from molecular dynamics simulations and contact angle measurements on graphite showing that this surface is indeed intrinsically mildly hydrophilic contrary to the common belief. Hydrocarbon contamination, known to be the source for the usually observed hydrophobic property of graphite, also affects protein adsorption processes as shown in this study. In the computational part of this work ethane was used as a model hydrocarbon which acts on the surface by reducing the water− surface and protein−surface interactions. This contamination then results in higher water contact angles. The process of protein adsorption is studied for the example of insulin revealing a reduction in adsorption strength despite the surface being more hydrophobic when contaminated. Although the proteins did not denature on the contaminated surfaces, further processes, such as the displacement of hydrocarbons by the protein, may occur on a longer time scale. In conclusion, we argue that proteins adsorb faster on pure than on contaminated surfaces with respect to the molecular dynamics time scale of ns.

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INTRODUCTION Protein adsorption is a phenomenon that occurs on nearly all kinds of surfaces and affects various research and technology areas such as implantology,1 contact lenses,2 and ship propulsion,3 to name a few. Yet, it remains a challenging topic for study both experimentally and computationally.4 A typical model surface due to its hydrophobic characteristic is graphite, which has been extensively used both in experiment and simulation; in its pyrolytic form it is even used as an implant material.5 In addition, the graphene−water interface is perhaps one of the simplest solid−water interfacial systems when viewed from a chemical perspective,6 which makes it an ideal candidate for studying wettability and protein adsorption. It is generally believed that graphite is intrinsically hydrophobic, but it was shown that this feature is induced by onsetting hydrocarbon contamination,7,8 typically alkanes, alkenes, and so on, present in ambient air. Particularly, recent experiments using 1-octadecene vapor proved that indeed hydrocarbon contamination on graphene is responsible for altered wetting behavior.7 Furthermore, a computational ab initio study by Wu and Aluru9 provided parameters for a hydrophilic graphite surface and showed that by introducing ethane contaminations, the contact angle of water droplets will rise to a hydrophobic regime. In this study we take this concept a step further and study the consequences for protein adsorption in silico. The question of how protein adsorption is affected by surface contaminations has not been raised by now so our study may shed new light on these kind of interfacial processes. In addition, we provide experimental contact angle data that follows similar trends as Kozbial et al.8 © 2015 American Chemical Society

but shows of course due to regional differences in the airborne hydrocarbon concentrations somewhat varying water contact angles (WCA). We use insulin as a model protein to perform molecular dynamics (MD) simulations of protein adsorption on pure and contaminated graphite surfaces. Insulin adsorption on various surfaces used in the administration process is a very relevant matter regarding tight glycaemic control for adult long-stay critically ill patients and neonates with hyperglycaemia.10

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METHODS Contact Angle Measurements. The highly ordered pyrolytic graphite (HOPG) surfaces were grade SPI-1 (mosaic spread of 0.4° ± 0.1°) and grade SPI-3 (mosaic spread of 3.5° ± 1.5°) by SPI Supplies, West Chester, Pennsylvania, U.S.A., with sizes of 10 × 10 × 1 mm3 and grade ZYA (mosaic spread ≤ 0.4°) with sizes of 12 × 12 × 2 mm3, supplied by Momentive, Strongsville, Ohio, U.S.A. To obtain a clean HOPG surface a piece of tape (3 M, Scotch brand) was stuck to the surface and carefully pressed onto it. Removal of the tape cleaves a thin layer of HOPG and creates a fresh surface, which was used directly or stored under ambient conditions between 5 min and 3 days before performing the wettability measurements. The wettability of the HOPG surfaces was characterized by sessile-drop contact angle measurements with a Contact Angle Measurement Instrument G2 (Krü s s, Hamburg, Received: March 27, 2015 Revised: May 8, 2015 Published: May 8, 2015 12496

DOI: 10.1021/acs.jpcc.5b02948 J. Phys. Chem. C 2015, 119, 12496−12501

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The Journal of Physical Chemistry C

were harmonically restrained to prevent desorption while the carbon atoms of the graphite surface were kept fixed as done in all other simulations. Protein Adsorption Simulations. For studying protein adsorption on the different contaminated surfaces insulin was taken as a model protein due to its high availability and medical relevance. The protein taken from the protein database with PDB ID 4INS17 at pH 7.0 was placed in two different starting orientations differing by a 90° rotation around an axis parallel to the surface at 6 Å to the top surface atoms, that is, either the pure graphite atoms or the ethane molecules in case of contamination, see Figure 2.

Germany) using ultrapure water (18.2 MΩ·cm, Milli-Q-A10system, Millipore, Massachusetts, U.S.A.) under ambient conditions. The relative humidity during the measurements was 28−52 %. No trend of contact angles with humidity within this range was observed. The temperature ranged between 20.0 and 24.5 °C. Following the deposition of 2 mL of ultrapure water onto the sample surface, a circle was fitted to the side view of the droplet to determine the contact angle (software DCAII Krüss, Hamburg, Germany). The given contact angles present average values of a minimum of 8 droplets measured at least on two different days. Molecular Dynamics Simulations. NAMD 2.911 was used for carrying out all simulations together with the CHARMM27 force field12 and the TIP3P13 water model, which is a CHARMM modified version in NAMD. Throughout all contact angle and adsorption simulations the temperature was set to 310 K and the cutoff for the van der Waals interactions to 12 Å. Contact Angle Simulations. Several surfaces, ranging from 200 to 460 nm2 with two layers of (0001)-graphite were prepared in order to account for varying droplet sizes including 2000−10000 water molecules. The Lennard-Jones parameters for the carbon atoms were set according to the results based on the ab initio data by Wu and Aluru9 for carbon−water interactions in order to represent a clean noncontaminated surface. Here, the mixing rules for the CHARMM force field were applied to derive the carbon−carbon parameters based on the oxygen−oxygen parameters from CHARMM-modified TIP3P. To study hydrocarbon contamination on graphite ethane molecules were placed in two different surface coverage number densities in analogy to Wu and Aluru,9 namely, n = 1.0 nm−2, as seen in Figure 1 with a water droplet and n = 4.0 nm−2. The

Figure 2. Two different initial orientations of insulin above the graphite surface (indicated by 2 gray lines). Orientation 2 is obtained from orientation 1 by a 90° rotation around an axis parallel to the surface. Nonpolar residues are shown in blue, neutral residues in green, and polar residues in red.

The protein−surface systems were solvated in a periodic box with approximately 42000 water molecules and salt concentrations of 0.1 M NaCl. Long range electrostatics were handled using Particle Mesh Ewald (PME).18 To account for long time scale dynamics needed for adsorption studies, the method of dual-accelerated MD19−21 is used as proposed by our previous work.22 The simulations are done with a 2 fs time step enabled by the SHAKE23 algorithm to ensure rigid hydrogen atoms and 1.5 × 107 MD steps in total. Analysis of the Simulation Results. The droplet fitting procedure was done as proposed by Werder et al.24 and statistics were obtained by averaging over the last 50 ps of the trajectory, where we are sure that the droplet shape is stable and equilibrium is reached. To compare the contact angle results obtained by MD simulations to our experimental contact angles, the modified Young’s equation,25 which relates the microscopic contact angle θ to the macroscopic contact angle θ∞, is used τ 1 cos θ = cos θ∞ − γLV rB (1)

Figure 1. Water droplet consisting of ≈9400 TIP3P water molecules on graphite with an ethane surface coverage number density of n = 1.0 nm−2.

clean surface will be denoted by n = 0.0 nm−2 in the following. Force field parameters for ethane were taken from the CHARMM additive force field files.14,15 In the case of the contaminated surfaces, the ethane molecules that were placed on graphite using Packmol16 were first minimized for 8000 steps and equilibrated for 400 ps at low temperature of 100 K to allow for slow surface diffusion. To obtain sufficiently sampled and stable water droplets, all setups were simulated for 6 ns. Here, the ethane molecules

Here rB denotes the droplet base radius, τ is the line tension, and γLV is the surface tension of the liquid−vapor interface. The linear relation between the cosine of the microscopic contact angle and the droplet base radius is then used for obtaining the macroscopic contact angle for 1/rB → 0. To further characterize the surface hydration, we determine the radial pair distribution function between water, in our case, 12497


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The Journal of Physical Chemistry C the oxygen atom due to its dominating mass in the water molecule, and graphite, which gives us the probability of finding a water molecule at a distance r to the graphite surface:26 gsurf − O (r ) = w

⟨ρO (r )⟩ w

⟨ρO ⟩vol w

(2)

Here ⟨ρOw(r)⟩ denotes the particle density of water oxygen atoms in a spherical shell at r + dr around the surface averaged over the last 1 ns of the MD trajectory. The normalization term ⟨ρOw⟩vol denotes the mean density of O atoms in the total water volume. All adsorption snapshots were rendered using VMD27 and Tachyon.28

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Figure 4. Simulation results of the dependence of the contact angle θ on the inverse droplet base radius rB. Three different surfaces are studied, characterized by the number density n of adsorbed ethane. The extrapolation to 1/rB → 0 provides the macroscopic droplet contact angle, as displayed in Table 1.

RESULTS AND DISCUSSION Influence of Hydrocarbon Contamination on Wettability of Graphite. When HOPG is freshly cleaved we obtain water contact angles for different grades (see Figure 3) in the

Table 1. Simulation Results of the Macroscopic Contact Angles Obtained from Figure 4, As Well As Surface Interaction Energies Normalized By Division Through The Total Number of Water Molecules surface coverage number density (nm−2)

macroscopic contact angle (°)

total surface interaction energy per water molecule (kcal/mol)

0.0 1.0 4.0

55.18 ± 1.44 77.28 ± 0.88 105.15 ± 1.36

−0.2618 ± 0.0047 −0.1816 ± 0.0048 −0.0543 ± 0.0024

Jones parameters for water−graphite interactions developed by Wu and Aluru we observed different contact angles for the clean graphite surface. Table 1 shows a macroscopic contact angle of about 55° for the pure surface, which is 19° larger than that by Wu and Aluru. One of the main reasons hereto is probably the different water model of CHARMM-modifiedTIP3P used in our study within NAMD. Despite these differences on the computational level we find almost perfect agreement to our experimental results which emphasizes the mildly hydrophilic nature of pure graphite. To further analyze the effect of increasing contamination on the first hydration layer of graphite, we plot the radial pair distribution function between the surface and the oxygen atoms in water in Figure 5. We clearly see an increasing depletion of water with surface contamination. Due to the ethane layer less interaction sites between water and graphite remain, that is, water molecules cannot obtain positions where ethane molecules are residing. But one of the main contributions here is the lowered interaction energy between water and the surface by introduction of ethane molecules on the graphite surface. This property is shown in Table 1 where the total surface interaction energy per water molecule (van der Waals + electrostatic energy) was averaged over the last 50 ps of the trajectory, in this case for droplet sizes of ≈4000 water molecules. Here, a clear trend toward increasing contamination can be observed. Influence of Hydrocarbon Contamination on Protein Adsorption on Graphite. Figure 6 demonstrates that the surface contamination of graphite clearly leads to a reduction in the interaction energy between the protein and the surface. This is in principle the same observation as the lowered interaction energy between water and surface. Furthermore, as

Figure 3. Experimental data of the evolution of the water contact angle with time after exposure to ambient air. Data obtained for different HOPG grades and manufacturers/suppliers.

range of 55.9° ± 3.8° (grade SPI-3) to 56.7° ± 1.6° (grade ZYA); this demonstrates that graphite can be considered as a mildly hydrophilic surface. Figure 3 shows that the contact angle considerably increases after exposure to air on a time scale of minutes. After 3 h of air exposure, the contact angles were in the range of 80° and after 3 days of about 90°. Such large contact angles are in the range of what is usually described in the literature.29−32 This finding is in line with earlier experimental work7,8 that identified onsetting hydrocarbon contamination on the surface as the source for increased hydrophobicity. Although we see similar trends regarding wettability a quantitative comparison shows small deviations which can be traced back to different laboratory conditions regarding airborne hydrocarbon concentrations. To look into this phenomenon a MD scheme for three different surface contaminations by ethane was adapted from Wu and Aluru.9 Our MD results for the contact angles of water droplets of various sizes on pure and contaminated graphite are displayed in Figure 4, and the extrapolated macroscopic contact angles for direct comparison to the experiment are assembled in Table 1. Interestingly, although we employed the Lennard12498


Article


Figure 5. Radial pair distribution function between the oxygen atoms of the TIP3P water molecules and the graphite surface.

Figure 7. Evolution of the radius of gyration of insulin (component parallel to the graphite surface).

protein depend on several factors. First of all, due to the lowered interaction energy on the ethane-contaminated surface, the adsorption to the surface might be considered more reversible; this is related to our observation that the protein denatures less on contaminated surfaces than on clean ones. We note that the interaction between noncharged pure graphite and protein is only determined by van der Waals forces and this interaction is altered by hydrocarbons on the surface. The electrostatic interactions between protein and hydrocarbons are negligible small. Second, it has been reported that also the surface crystallinity has a significant effect on the structural rearrangement of adsorbed proteins where formation of parallel strands on pure graphite34 or preferred diffusion on polyethylene surfaces35 could be observed. This effect of preferred orientation guided by the materials crystal structure is of course not maintained after contamination. Although we do not observe protein denaturation on the hydrophobic contaminated surfaces in our model system despite the advanced sampling it is possible that on longer time scales these processes might occur. One possible explanation for the usually observed denatured proteins on HOPG36 may be the displacement of hydrocarbons by the proteins in analogy to the Vroman effect37 for protein−protein displacement, but this process is expected to occur on longer time scales. Interestingly, a test simulation with free, that is, not restrained ethane molecules, revealed that most of the ethane molecules will stick to the surface or its periodic replica and arrange themselves in clusters and clearing some adsorption sites. The protein structure in this scenario stayed intact during our standard simulation time; this fact justifies our approach to restrain the ethane molecules. However, we observed a beginning displacement of ethane in proximity to the protein when ethane molecules were not restrained, indicating a higher surface affinity of the protein. We conclude that denaturing on contaminated surfaces may become possible on longer time scales, but eventually adsorption processes are faster on pure graphite surfaces.

Figure 6. Total interaction energies (van der Waals and electrostatic energies) between insulin and surface atoms, i.e., graphite and ethane in case of contamination. Two different protein orientations on three contamination degrees are considered.

discussed in previous works22,33 strong early stage interactions due to a larger interaction area reduce the flexibility of the protein to spread out on the surface. Orientation 2 (compare Figure 2) therefore shows less surface interaction due to its greater starting surface contact area. To quantify this and to look for structural rearrangements upon adsorption, the radius of gyration with its component parallel to the surface can give an estimate of the protein’s spreading on the surface; it is shown in Figure 7. Due to the protein’s small size only little changes can be observed. But we clearly see that for a surface contamination of 1.0 nm−2 the radius of gyration is not much altered in the course of the simulation. The snapshots shown in Figure 8 show the reason hereto: the protein tries to maximize its contact with graphite and so squeezes itself between the ethane molecules on top of the graphite surface. Furthermore, protein orientation 1 initially has less surface contact and, hence, a higher flexibility in spreading; it therefore becomes completely denatured. This is seen in the final adsorption snapshots in Figure 8 and also in the secondary structure analysis in Table 2. Additionally, the secondary structure analysis also reveals a transformation of a 310-helix toward an α -helix in case of adsorption on the contaminated surfaces. Insulin adsorbs both on the pure and on the hydrocarbon contaminated surface but the induced structural changes in the

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CONCLUSIONS Our water contact angle experiments and molecular dynamics simulations indicate that graphite is indeed an intrinsically mild hydrophilic surface with contact angles of ≈56°. It then becomes hydrophobic due to onsetting airborne hydrocarbon 12499


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Figure 8. Snapshots of insulin after 1.5 × 107 steps of dual accelerated MD for two initial orientations of insulin above the surface and various surface contaminations. The secondary structure of these molecules is analyzed in Table 2.

Table 2. Secondary Structure of Insulin before Adsorption and at the End of the Simulation for Two Initial Orientations of Insulin above the Surface and Various Surface Contaminations Calculated with dssp38 orientation 1 α-helix 310-helix β-sheet

orientation 2

initial minimized structure (%)

0.0 nm−2 (%)

1.0 nm−2 (%)

4.0 nm−2 (%)

0.0 nm−2 (%)

1.0 nm−2 (%)

4.0 nm−2 (%)

52.38 14.29 0

0 0 0

66.67 0 0

66.67 0 0

42.86 0 0

66.67 0 0

57.14 0 0

contamination within minutes after freshly cleaving the surface. These findings are in line with previous studies that proved hydrocarbons to be responsible for the hydrophobic behavior of graphite.7,8 We saw similar trends regarding increasing contact angles as Kozbial et al. The MD simulations carried out in this study were able to reproduce the experimental WCA of the clean surface quite nicely with ab initio parameters from Wu and Aluru.9 They also showed increasing WCA on graphite when introducing ethane contaminations which can be understood by lowering the total surface interaction energy. Future MD studies of various contamination levels could directly provide the corresponding contamination degree to experimental contact angles seen in Figure 3. Also, one could incorporate other alkanes, alkenes, and so on, present in ambient air in this model as well. The MD simulations of insulin adsorption show lowered surface interaction energy on contaminated surfaces causing the protein to denature less than on clean surfaces. This might appear surprising since it is well-known that hydrophobic surfaces induce protein unfolding.39 It may be a consequence of the so-called Vroman effect,37 which probably operates on longer time scales of μs, but to clarify this issue, future studies using accelerated sampling methods have to be carried out. Both hydrocarbons and proteins compete for adsorption sites, so it is probable that the less mobile but more strongly bound proteins could displace the initially adsorbed hydrocarbons from the surface. The more weakly bound hydrocarbons may then leave the surface or cluster together and so clearing adsorption sites, allowing the proteins in solution to adsorb to surface parts with less contamination, as was seen in our simulations.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support by Deutsche Forschungsgemeinschaft within Projects Zi 487/17-1, Ur 32/26-1, and SFB 926. Furthermore, we appreciate the computational resources provided by the compute cluster “Elwetritsch” of the University of Kaiserslautern as well as the permission to use the contact angle measuring device of the Institute for Surface and Thin Film Analysis (IFOS), Kaiserslautern.

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Consequences of Hydrocarbon Contamination for Wettability and

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