Molecular Dynamics Simulation Study of Adsorption of Bioinspired

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Molecular Dynamics Simulation Study of Adsorption of Bioinspired Oligomers on Alumina Surfaces In-Chul Yeh, Joseph L. Lenhart, Joshua A. Orlicki, and Berend Christopher Rinderspacher J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b04473 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 23, 2019

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The Journal of Physical Chemistry

Molecular Dynamics Simulation Study of Adsorption of Bioinspired Oligomers on Alumina Surfaces In-Chul Yeh, Joseph L. Lenhart, Joshua A. Orlicki, and B. Christopher Rinderspacher* Polymers Branch, Materials & Manufacturing Science Division, U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, United States [email protected]

ABSTRACT. The adsorption of small oligomers on a model metal oxide surface was studied with atomistically detailed molecular dynamics simulations. The oligomers consisted of two different repeat units: a maleimide, which contains a catechol functional group as in the dopamine residue found in marine adhesive proteins, and a methyl acrylate. A hydroxylated alumina surface was used as the model metal oxide surface. Adsorption interactions were investigated in aqueous as well as anhydrous conditions. In anhydrous conditions, the model oligomers displayed strong adsorption interactions with the surface. However, in aqueous conditions, the adsorption interactions were significantly weakened because of the competition with the water molecules for adsorption sites near the surface. Catechol functional groups in the model oligomers were found to play an important role in adsorption interactions with the alumina surface via hydrogen bonds. However, diverse adsorption properties were observed depending on compositions and sequences

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of two different repeat units and self-aggregations, indicating that the hydrogen bonding capability of catechol groups is not the sole factor determining adsorption properties.

Introduction Marine adhesive proteins (MAPs) found in aquatic organisms such as mussels or barnacles, which possess a remarkable ability to adhere to various surfaces in aqueous environments,1-4 have sparked significant interest in the design of biomimetic adhesive polymers5 to mitigate synthetic adhesives’ susceptibility to extreme humidity. The 3,4dihydroxy-l-phenylalanine (DOPA) containing catechol moiety is considered to be one of the key functional groups for the adhesive properties.4, 6-7 It is found usually flanked by cationic amino acid residues in the adhesive proteins mfp-38 and mfp-59 of marine mussels. The catechol functional group contains a benzene ring with a pair of ortho-substituted hydroxyl groups, which may contribute to adhesion with hydrophobic and hydrophilic interactions, respectively,10-11 in addition to chemisorption.12-13 Many synthetic bioadhesives containing catechol moieties have been designed and studied experimentally.14-20 However, the full characterization of the atomistic details of solvated phenolic compounds adsorbing to solid substrates is still challenging due to many complex competing interactions among the phenolic compounds, the substrate, and water molecules. Atomistically detailed molecular dynamics simulations complement the experimental efforts to understand the structural and dynamical details of such interactions and were used in recent experimental and theoretical studies.21-25 Alumina or aluminum oxide (Al2O3) finds use in many technological applications due to its high melting temperature and very high hardness.26 It is also a useful model for the native


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oxide surface that forms on components made of aluminum. Interactions that control adsorption of organic molecules to the alumina surface is of importance in both practical and fundamental perspectives. Recently, Yeh et al.27 studied adsorption properties of catechol and other related phenolic compounds near alumina surfaces in both aqueous and anhydrous conditions with atomistic molecular dynamics (MD) simulations. Catechol with two neighboring hydroxyl groups displayed the strongest adsorption on alumina surfaces with frequent and persistent hydrogen bonding interactions. Catechol/L-DOPA is not the only functional group affecting adhesive interactions of MAPs. Instead, MAPs also contain various amino acid residues including hydrophobic and cationic residues. The roles of the various amino acids in the adhesive behavior of MAPs are poorly understood. Copolymers and oligomers composed of different repeat units, which contain key functional groups of MAPs, can be used to elucidate the roles of composition and sequence of different functional groups of MAPs in adsorption properties and to design adhesive polymers. As a first step in understanding the adsorption processes of polymers with catechol moieties, we investigated the adsorption behavior of prototype oligomers near the alumina surface under both aqueous and anhydrous conditions with extensive MD simulations. The prototype dimers and tetramers contained different number and sequences of two different repeat units, one containing catechol and the other containing acrylate. Methods Description of simulated systems The prototypical oligomers were built with two different repeat units, a methyl acrylate (abbreviated as A) and a maleimide containing catechol functional group (abbreviated as C), as


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illustrated in Figure 1 (a). Recently, Bartucci et al.28 synthesized and characterized block copolymers containing these repeat units. We prepared monomers, dimers, and tetramers with different combinations of up to four A or C units as summarized in Table 1. Alumina slabs with different terminations and hydroxylation of the surfaces introduce different charge distributions and display different stabilities and adsorption interaction characteristics.29 Yeh et al.27 studied adsorption of catechol and related phenolic compounds with two different oxygen-terminated alumina surfaces, one hydrogenated and the other non-hydrogenated, with MD simulations. However, an alumina surface with non-hydrogenated oxygens is expected to be unstable27, 30-31 and was not used in this study. Instead, a slab of alumina with a thickness of 12 Å containing a hydroxylated surface was prepared by replicating the unit cell of the crystalline α-Al2O3, cutting along the oxygen-terminated (0001) surface, and fully hydrogenating the exposed oxygen atoms as

described previously27 and illustrated in Figure 1 (b). Alternative hydroxylated configurations such as an hydroxylated Al-terminated surface can be generated with the exchange of surface hydrogens by the adsorbates and may play an important role in adsorption interactions.29 However, in this study, only non-reactive interactions of oligomers and water with alumina surfaces, which can be described by classical force fields, were considered, and the resultant adsorption interactions of the model

oligomers with the fully hydroxylated alumina surface in aqueous and anhydrous conditions were investigated. There are 3 possible pairwise interaction modes involving oligomers; first, between oligomers and the alumina surface; second, between oligomers and water; and finally, between oligomers. Table 1 summarizes the 27 simulations performed in this study. Free energy or potential-of-mean force (PMF)32-34 profiles were estimated by a series of umbrella sampling35 MD simulations, where the center-of-mass distance of a single oligomer from the surface was biased at various points with a harmonic potential. Simulations of multiple oligomers without any bias were also performed for comparison. The adsorption interactions of a single oligomer with the


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alumina surface were probed with umbrella sampling simulations in vacuum (simulation sets 1-5) and in aqueous conditions (simulation sets 6-10). In simulations 11-13, the adsorption behavior of multiple hydrated oligomers enclosed between two hydroxylated alumina surfaces were studied. In simulation sets 14-16, the pair interaction of oligomers in the bulk aqueous solution was investigated with umbrella sampling simulations to understand self-aggregation behaviors of oligomers in more detail. In simulation 17, the adsorption interaction of multiple AC oligomers with the alumina surface in vacuum was investigated. Finally, free energy profiles of select tetramers near the alumina surface were estimated with umbrella sampling in simulation sets 1827. Force fields The CLAYFF force field,36 which treats most interatomic interactions in crystalline materials as nonbonded, and the simple point charge (SPC) model of water37 were used to describe alumina and water, respectively. The PCFF force field38 was used to describe the prototype oligomers, but the partial charges on atomic sites were assigned with those corresponding to the Condensed-phase Optimized Molecular Potentials for Atomic Simulation Studies (COMPASS) force field.39 The nonbonded van der Waals interactions between the atoms subjected to the CLAYFF force field were described by a 12-6 Lennard-Jones (LJ) function

𝑈(𝑟𝑖𝑗) = 4𝜖𝑖𝑗

𝜎𝑖𝑗 12

[( ) 𝑟𝑖𝑗

―

𝜎𝑖𝑗 6

( ) ], 𝑟𝑖𝑗

(1)

where the rij is the distance between the two atoms designated by i and j, and the parameter σij and the potential well depth ϵij are given by applying the Lorentz–Berthelot rule40 1

𝜎𝑖𝑗 = 2(𝜎𝑖 + 𝜎𝑗)

(2)


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and 𝜖𝑖𝑗 = 𝜖𝑖𝜖𝑗 ,

(3)

to CLAYFF van der Waals parameters σi and ϵi for atom i. The minimum potential energy in the 12-6 LJ function in eq 1 is -ϵij obtained at rij = 21/6σij, which will be used in the adjustment of van der Waals parameter σi in the force field mixing procedure described below. The van der Waals interactions between atoms i and j at the distance rij in PCFF was described by a different 9-6 LJ potential energy function 𝜎′𝑖𝑗 9

𝜎′𝑖𝑗 6

[ ( ) ( ) ].

𝑈(𝑟𝑖𝑗) = 𝜖′𝑖𝑗 2

𝑟𝑖𝑗

―3

(4)

𝑟𝑖𝑗

The prime symbol signifies van der Waals parameters that can be used for the 9-6 LJ function. The parameters 𝜎′𝑖𝑗 and 𝜖′𝑖𝑗 were set by applying the following sixth power mixing rules39 suitable for the PCFF force field

𝜎′𝑖𝑗 =

[ ((𝜎′𝑖) 1 2

6

)]

6

+ (𝜎′𝑗)

1 6

(5)

and

𝜖′𝑖𝑗 =

3 3 2 𝜖′𝑖𝜖′𝑗 (𝜎′𝑖) (𝜎′𝑗)

(𝜎′𝑖)6 + (𝜎′𝑗)6

,

(6)

where 𝜎′𝑖 and 𝜖′𝑖 are PCFF van der Waals parameters for atom i. It is to be noted that the minimum potential energy -𝜖′𝑖𝑗 is obtained at 𝑟𝑖𝑗 = 𝜎′𝑖𝑗 in the 9-6 LJ function described by eq 4. For the mixed interaction between atoms i and j described by CLAYFF and PCFF force fields, respectively, the 9-6 LJ function and the sixth power mixing rules in eqs 4-6 were used,


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but the van der Waals parameter σi,CLAYFF of CLAYFF atom i was modified by the following relation so that the potential energy minimum with the PCFF 9-6 LJ function occurs at the same distance as with the CLAYFF force field. 𝜎′𝑖,CLAYFF = 21/6𝜎𝑖,CLAYFF

(7)

The same mixing scheme was used by Heinz et al.41 in simulations of surfaces and interfaces of face-centered cubic metals and by Yeh et al.27 in recent simulations of adsorption of phenolic compounds on alumina surfaces. Simulation details MD simulations were performed using the LAMMPS program42 with a time step of 1 fs. Each system was prepared in a simulation cell of a rectangular prism with the alumina surface normal to the z-axis and subjected to the three-dimensional periodic boundary conditions as illustrated in Figure 1 (b). The cut-off distance of 12 Å was used for LJ interactions as in the previous simulation study.27 The electrostatic interactions were calculated with the particleparticle-particle mesh (PPPM) method43 using a real space cut-off distance of 12 Å and the Ewald correction term for the slab geometry.44-45 Lateral dimensions of the simulation cell, 47.59 and 41.21 Å in x and y directions, respectively, were chosen to match those of the alumina slab. Dimensions of the simulation cell in the z direction were chosen to have a vacuum gap with thickness of at least 100 Å along the z direction as illustrated in Figure 1 (b). Simulated systems are neutral but polarized along the z direction due to the polar alumina slab. For such systems, the large vacuum gap in conjunction with the Ewald correction term for the slab geometry removes the long-range interactions between periodic images in the z direction and essentially implements two-dimensional periodic boundary conditions.44-45 Simulations were performed


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under constant volume at the room temperature of 298.15 K (NVT ensemble). However, local densities in simulated systems were optimized spontaneously without intervention through selfadjustment of the location of the interface with the vacuum along the z direction during the course of the simulation, and the bulk-like region was formed beyond the z distance of 12 Å from the alumina surface.27 Evaporation of water molecules from the aqueous solution to the vacuum, which is expected to be very rare, was allowed, but the complete migration across the periodic z boundary was prevented with a reflecting wall at the boundary. The temperature was maintained by a Nose-Hoover thermostat with a relaxation time constant of 0.1 ps. Separate thermostats were used for oligomers and alumina in simulations of a single oligomer without water to avoid the artifacts of the thermostat for an inhomogeneous system in the absence of solvent.46-47 In umbrella sampling simulations, biased harmonic potentials with a force constant of 1 kcal/mol/Å2 were applied at 21 planar z distances from the alumina surface ranging from 0 to 20 Å at 1 Å intervals. The average z position of oxygen atoms of hydroxyl groups attached to the surface was defined as the location of the surface (z = 0 Å). Each biased run lasted 10 ns, and the last 8 ns were used for analysis. Each non-biased MD simulation lasted 20 ns, and the last 19 ns were used for analysis. Free energy or PMF profiles32 were obtained with the weighted histogram analysis method (WHAM).35, 48 Free energy profiles of the quantities of interest, such as the closest z distance of non-hydrogen atoms, other than the biased quantity (z COM) were also calculated with WHAM.34, 48-49 Free energy profiles from the non-biased MD simulations w(z) were estimated from the number density distribution along the z direction n(z) by the following relation. 𝑤(𝑧) = ― 𝑅𝑇log 𝑛(𝑧) + 𝐶,

(8)


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where R, T, and C represent the gas constant, temperature, and an undetermined constant, respectively. Results and Discussions Adsorption properties of monomer and dimer in vacuum First, we investigated adsorption properties of model oligomers having one or two repeat units of A or C near the alumina surface in the absence of water with umbrella sampling simulations described earlier and summarized as simulations 1-5 in Table 1. From the visual analysis of the trajectory, we confirmed that there was significant conformational sampling due to thermal motion. Figure 2 shows the free energy profiles of single monomers and dimers (A, C, AA, AC, and CC) near the alumina surface in vacuum. Table 2 summarizes the minimum free energy and the corresponding distance from the surface. The free energy was significantly lowered near the surface compared to the reference value far from the surface, which was set to zero, indicating significant preference to adsorption to the surface in the absence of water. The number of atomic sites in each oligomer, which can have non-bonded interactions with the surface, was correlated with the amount of the free energy decrease near the surface. This indicates that non-bonded interactions between an oligomer and the surface seem to be the dominant interaction in the absence of water. Consistent with this additive interaction model, the dimers were bound roughly twice as strongly compared to their respective monomers, and AC splits the difference between AA and CC. Conformations where an oligomer lies parallel to the surface with most of its atomic sites located near the surface were found to be the most favorable, as indicated by the predominant free energy minimum near 3.5 Å from the surface and illustrated by the representative snapshot of AC shown in Figure 2 (b). Similar parallel


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orientations of catechol molecules relative to the alumina surface in the absence of water were also observed in the previous simulation study by Yeh et al.27 This preference for parallel orientations of small oligomers may be weakened as oligomer length increases and intramolecular interactions become increasingly important. Adsorption properties of monomer in aqueous condition To understand the influence of water in the adsorption interaction between an oligomer and the alumina surface, we calculated free energy profiles of monomers, A and C, near the alumina surface in aqueous condition as shown in Figure 3 from simulations 6 and 7 described earlier and summarized in Table 1. Compared to the corresponding profiles in Figure 2 obtained without water, the free energy profiles of COM z distance in Figure 3 (a) feature global minima located more distant from the surface at 6.6 and 5.9 Å, respectively, for A and C, with significantly lower free energies of adsorption. However, it is possible to have the adsorption interaction at close distances between parts of the monomers and the surface even though the COM z position is distant from the surface. To examine the specific interactions of the monomers with the alumina surface in more detail, we calculated the free energy profiles of the z distance of the non-hydrogen atom closest to the alumina surface as shown in Figure 3 (b). For monomer A, the global minima in free energy profiles of the z distances of the closest atom and COM are both located significantly far from the surface at z = 5.8 Å and 6.6 Å, respectively. However, for monomer C, the global minimum in the free energy profile of the z distance of the closest non-hydrogen atom is located at z = 2.5 Å from the surface, even though the minimum in z COM was located at 5.9 Å away from the surface, indicating that the monomer C does not lie flat on the surface in aqueous solution. Similar differences between free energy minima in closest z distance and z COM profiles were previously observed in catechol and related phenolic


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compounds.27 To identify the origin of the specific interactions near the surface, we show more detailed free energy profiles in Figure 3 (c) and (d) for monomers A and C, respectively. Figure 3 (c) shows the distance profiles of methyl carbon and carbonyl oxygen atoms in monomer A along with that of the non-hydrogen atom closest to the alumina surface. It shows that the local minimum nearest to the surface in the profile of the overall closest non-hydrogen atom matches that of the carbonyl oxygen, while the global minimum farther from the surface (~6 Å) corresponds to that of the methyl carbon. Figure 3 (d) shows free energy profiles in monomer C of the center of benzene ring and the hydroxyl oxygen closest to the surface along with the overall closest non-hydrogen atom. The global minimum of the center of benzene ring in C is far from the surface at z = 4.9 Å even though there is a small local minimum near the surface at z = 3.4 Å, which corresponds to the benzene ring parallel to the surface. The z distance 2.5 Å of the closest hydroxyl oxygen at the global minimum coincides with that in the profile of the overall closest distance as shown in Figure 3 (d). This confirms that the free energy minimum of the overall closest z distance of monomer C results mostly from the contribution of the hydroxyl group in C, which strongly interacts with the hydroxyl group on the alumina surface via hydrogen bonding. It was also found previously27 that the density profile of oxygen atoms in water is structured with high number densities observed at z distances of 2.6 Å, 5.2 Å, and 6.4 Å from the surface. Free energy profiles shown in Figure 3 also display multiple minima in general, which were not apparent in the profiles in vacuum shown in Figure 2 at similar distances, indicating that the solvent water competes for surface area as well as influences the long range ordering of A and C. Figure 3 (e) and (f) show two-dimensional (2D) free energy profiles with respect to the z distance of the closest non-hydrogen atom and the COM z distance for monomers A and C, respectively. The z distance of the closest non-hydrogen atom was closely correlated


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with the COM z distance in monomer A as shown in Figure 3 (e). Figure 3 (f) shows that the closest z distance of 2.5 Å can be correlated with the COM z distance of 6.5 Å in monomer C as described earlier. These observations were illustrated with typical snapshots of monomers A and C near the alumina surface shown in Figure 3 (g) and (h), respectively. Adsorption properties of dimers in aqueous condition We also investigated interactions of dimers having two repeat units of A or C near the alumina surface in the presence of water with free energy profiles. Figure 4 (a) and (b) show free energy profiles of z distances of the COM and the closest non-hydrogen atom, respectively, of dimers, AA, AC, and CC. For AC and CC, z distance free energy profiles of the center of the closest benzene ring and the closest hydroxyl oxygen are also shown in Figure 4 (c) and (d), respectively. In free energy profiles of the COM z distance in Figure 4 (a), CC exhibits the lowest free energy of adsorption with a broad minimum near z = 7.4 Å as summarized in Table 2. CC also displays the most favorable free energy of adsorption at z = 2.5 Å near the surface in the z distance free energy profile of the closest atom shown in Figure 4 (b). The secondary deep wells in Figure 4 (b) located between z distances of 5 and 6 Å suggest that water interactions may play a crucial role here, because they correspond to the distance of a layer of water molecules next to the adsorbed water layer near the alumina surface. Figure 4 (c) shows that the most favorable locations of the benzene ring center in AC and CC are at z = 7.0 and 6.4 Å from the surface, respectively, while the closer z distance of 4.7 Å is also favorable in CC. Figure 4 (d) shows that the hydroxyl oxygen atom is preferentially located at z = 2.5 Å in both AC and CC. However, the free energy minimum is significantly lower in CC than in AC. Figure 5 shows 2D free energy profiles with respect to z distances of the methyl carbon in A and the center of the benzene ring in C along with selective snapshots. The 2D profile of AA in Figure 5 (a) indicates


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that a configuration where both methyl carbons of A units are located about 6 Å from the surface simultaneously as shown in the associated snapshot is the most probable. This is consistent with the profiles shown in Figure 4 (a) and (b) and indicates relatively weak adsorption interactions with the surface. In the 2D profile of AC in Figure 5 (b), a configuration where z distances of the methyl carbon of A and the center of the phenyl ring in C are about 10 and 6 Å from the surface, respectively, is the most probable. This indicates a significant adsorption interaction of C while A is farther away from the surface. The 2D profile of CC (Figure 5 (c)) displays multiple minima with different combinations of z distances of the centers of the phenyl rings in each C. As noted before, the catechol ring lies flat on the surface at a distance of about 3 Å, while the ring is tilted with respect to the surface at a distance of about 4.8 Å when hydroxyl groups are forming hydrogen bonds with the alumina surface. Four of the minima with a combination of phenyl ring distances of 3 and 4.8 Å therefore correspond to configurations in which both C are bound to the surface, whereas other minima farther away from the surface were formed due to the interactions with layers of water molecules near the surface. The configuration in which both C units have significant adsorption interactions through hydrogen bonding simultaneously at a phenyl ring distance of about 4.8 Å is illustrated in the associated snapshot. Our results with a single oligomer of dimers in aqueous solutions indicate that the number of C containing the catechol is correlated with the strength of the absorption interactions with the surface. Furthermore, the number of surface-bound states increases combinatorially. Adsorption properties with multiple oligomers in aqueous solution and in vacuum As summarized in Table 1 (simulations 11-13), we performed MD simulations of multiple instances of A, C, and AC in aqueous solutions enclosed between alumina surfaces. Even though they were randomly distributed initially, significant degrees of molecular


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aggregation were found as illustrated in the snapshots of the last configurations shown in Figure 6. Free energy profiles were estimated from the number density distribution collected from the last 19 ns of 20 ns MD simulations using Eq. (8). Figure 6 shows free energy profiles of the z distances of COM and the closest non-hydrogen atom. Corresponding free energy profiles obtained with biased simulations of a single oligomer were also compared in Figure 6. Free energy distributions obtained with MD simulations of multiple oligomers and biased simulations of a single oligomer were very similar for A and C oligomersbecause of lower degree of aggregation, as is evident from the snapshots. C shows a greater propensity than A to bind to the surface as evidenced by the deeper wells at roughly 3 and 6 Å in Figure 6 (d) and the deeper well at ca. 7 Å in Figure 6 (c). However, significant differences between free energy profiles with multiple oligomers and a single oligomer were found for AC even though the main features of the free energy profiles such as z distances of major minima remained the same. In particular in Figure 6 (f), the order of global and second lowest minimum switches to favoring surface proximity going to multiple oligomers from a single oligomer. The free energy at the global minimum in Figure 6 (e) is ca. 0.5 kcal/mol higher with multiple oligomers, indicating the weakened adsorption interaction with the surface due to competition from the cohesive interaction among AC oligomers in the aggregated state as illustrated in the final snapshot. We also performed simulations at lower concentrations of AC and C, and similar aggregation was observed (data not shown). In order to understand the cohesive interactions between the oligomers in aqueous solution in more detail, we performed umbrella sampling simulations of two oligomers of A, C, and AC in bulk aqueous solutions (simulation sets 14-16 in Table 1). In each set of umbrella sampling simulations, the COM radial distance between the two oligomers was restrained at 21


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points from 0 to 20 Å at 1 Å intervals with a harmonic potential with a force constant of 1 kcal/mol. Free energy profiles of pair interactions are shown in Figure 7 (a). Selective snapshots are shown in Figure 7 (b) and (c). AC-AC interactions were strongest with a minimum free energy of about 1.3 kcal/mol followed by C-C and A-A interactions. These are consistent with stronger pair interactions with increased number of atoms in each molecule. Distances corresponding to free energy minima also increased with the number of atoms in each oligomer. Free energy profiles for C-C and AC-AC shown in Figure 7 (a) did not show any specific local minima, which could result from specific intermolecular interactions such as hydrogen bonding, except for a small local minimum at 4 Å in the profile for C-C interaction related with a coplanar interaction shown with a snapshot in Figure 7 (c). Any possible specific hydrogen bond interactions in C and AC may have been weakened by the presence of water molecules, which can act as strong hydrogen bonding donors and acceptors. These results are consistent with a higher degree of aggregation of AC shown in Figure 6. Figure 8 shows the snapshot from a MD simulation of 45 AC dimers in vacuum (simulation 17 in Table 1). The simulation started with the final configuration of the MD simulation of 45 AC dimers with water, but water molecules were removed to achieve an anhydrous condition. As can be seen from the snapshot, the highly aggregated structure transformed quickly to a surface-bound configuration. Adsorption properties of select tetramers in aqueous condition We estimated the free energy profiles of single tetramers listed in Table 1 near the alumina surface (simulation sets 18-27). The minimum free energy values and the corresponding z distances with the tetramers are listed in Table 2 along with those obtained for monomers and


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dimers described earlier. Despite the larger size of the tetramers, the visual inspection of the trajectory confirmed frequent and large conformational changes of the tetramers away from the initial conformation near the alumina surface, which may be attributed to thermal motion and interactions with competing water molecules. Figure 9 shows free energy profiles of tetramers with two or more C units. Figure 9 (a) and (b) show free energy profiles of z distances of COM and closest atom, respectively, of C4 along with C and CC. C4 has the most favorable free energy of adsorption, -1.70 kcal/mol as indicated in Figure 9 (a) and Table 2. Figure 9 (c) and (d) show free energy profiles with tetramers containing two C units. Free energy profiles in Figure 9 (c) show significant dependencies on the sequences of A and C units despite having the same composition of A and C. AACC shows the most favorable free energy of adsorption of -1.65 kcal/mol in Figure 9 (c) among the tetramers having two 2 C units. Figure 9 (e) and (f) show free energy profiles of the tetramers having three C units. Among the tetramers having three C units, the most favorable free energy of adsorption was about -1.34 kcal/mol with CACC. Overall, among the tetramers, the most favorable free energy of adsorption was -1.70 kcal/mol obtained with C4, which has the most units of C. The second most favorable free energy of adsorption, 1.65 kcal/mol, was obtained with AACC rather than CACC, despite AACC containing only two C units. The global free energy minima for the closest z distance for C4, AACC, CAAC, and CACC are virtually indistinguishable whereas AC3 falls short. Again, these results show that the number of C units is not the sole factor determining the adsorption properties of the oligomers. Instead, different sequences of repeat units seem to lead to significant differences in adsorption properties of the oligomers as shown in Figure 9. However, as indicated by the range of confidence intervals for the minimum free energies shown in Table 2, more extensive sampling and careful analysis may be needed for definitive quantitative comparisons.


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Figure 10 shows 2D free energy profiles of the tetramers containing two C units with respect to z distances of the centers of benzene rings in the catechol groups and indicates various local minimum configurations. Compared to the free energy map of CC in Figure 5 (c), the free energy maps of AACC and CAAC in Figure 10 (a) and (c), respectively, display deeper and more pronounced free energy minima near the surface, which indicate that the AA group may enhance the binding of the catechol group to the alumina by protecting the catechol group from the interaction with water molecules. This enhancement of the binding due to the AA group is also supported by the summary in Table 2 and the comparisons of the free energy profiles in Figures 4 and 9. ACCA in Figure 10 (b) retains the same qualitative map as found in Figure 5 (c), but wells are less localized. ACAC in Figure 10 (d) shows the least resemblance to Figure 5 (c) with shallow wells concentrated on the exterior catechol binding to the surface. Figure 11 shows various configurations of AACC near the free energy minimum and illustrates variety of modes of adsorption interactions between AACC and the alumina surface. Summary and Conclusion Molecular dynamics simulations have been performed on prototype oligomers analogous to marine adhesive proteins. Different lengths and sequences of two different repeat units of maleimide containing the catechol functional group and/or methyl acrylate were used to understand the underlying interactions governing adhesive properties with a hydroxylated alumina surface as a model surface. We observed adsorption interactions between the model oligomers and the alumina surface in aqueous as well as anhydrous conditions. Catechol functional groups play an important role in adsorption interactions with the alumina surface with hydrogen bonds. However, diverse adsorption properties were observed depending on


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composition and sequence of the two different monomers, even given the same number of catechol groups. Higher binding free energies were observed for oligomers containing catechol at chain ends. Furthermore, having the consecutive A group as in AACC and CAAC is preferable even to adding another terminal C as in CACC let alone extending to ACCC. Water molecules compete with hydroxyl groups of catechol groups for adsorption with the alumina surface. In addition, aggregations among the model oligomers in aqueous solution hinder adhesive properties by limiting the availability of hydroxyl groups to adsorption interactions with the surface. Insights gained in this study can be used to design bioinspired adhesive polymers and antifouling coatings and surfaces.50-52


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Figure 1. Schematics of model oligomers and the simulation set-up. (a) Illustrations of oligomers consisting of two repeat units, maleimide containing catechol functional group (C) and methyl acrylate (A). (b) The schematic of the simulation cell of aqueous solution containing a single oligomer of dimer AC next to the alumina slab with planar vacuum interfaces perpendicular to the z axis.


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Figure 2. Free energy profiles of monomer and dimer in anhydrous conditions. (a) Free energy profile as a function of center-of-mass z-distance from the surface. (b) Side and top views of a snapshot of AC near the surface.


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Figure 3. Free energy profiles with monomers in aqueous conditions. Free energy profiles are shown with respect to the z distance of the center of mass (COM) in (a) and the z distance closest to the surface in (b)-(d). (b) compares the profiles of the non-hydrogen atoms closest to the surface in A and C. (c) and (d) show profiles of the overall closest z distances in A and C, respectively, along with those of the subgroups. (e) and (f) show 2D free energy profiles with respect to z distances of COM and the closest atom for A and C, respectively. The snapshots near free energy minima are shown in (g) and (h). Water molecules were present in the simulations but omitted for clarity in the snapshots in this and subsequent figures.


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Figure 4. Free energy profiles of dimers. (a) z center of mass, (b) closest z distance, (c) z distance of the center of benzene ring in C, (d) z distance of the oxygen atom of the closest hydroxyl groups to the alumina surface.


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Figure 5. 2D free energy profiles of dimers (a) AA, (b) AC, and (c) CC with respect to z distances of the acrylate methyl group and the benzene ring in the catechol group and selective snapshots from simulations with corresponding z distances in 2D profiles indicated with arrows.


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Figure 6. Free energy profiles obtained from MD simulations of A, C, and AC prototype oligomers in aqueous solutions. (a), (c), and (e) represent free energy profiles of the z center-ofmass distance from the left surface of A, C, and AC, respectively. (b), (d), and (f) show profiles of the z distance of non-hydrogen atom closest to the surface in A, C, and AC, respectively. Solid and dashed lines represent free energy profiles estimated from MD simulations with multiple molecules and biased umbrella sampling simulations with a single molecule, respectively. Snapshots of final configurations with A, C, and AC are also shown in the left side.


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Figure 7. Free energy profiles of pair interactions of oligomers AC, A, and C in aqueous solutions and snapshots. (a) Free energy profiles of pair interactions of AC, A, and C are shown in solid, dashed, and dashed-dot lines, respectively. Snapshots of pair interactions of (b) AC and (c) C at specified COM distances.


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Figure 8. The final snapshot of a simulation of 45 oligomers of AC near the alumina surface in vacuum.


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Figure 9. Free energy profiles of prototype oligomers with different number of C. Panels in top, middle, and bottom show profiles for oligomers of C, tetramers with 2 C units, and tetramers with 3 C units, respectively. Left and right panels show profiles of z distances of COM and the nonhydrogen atom closest to the surface, respectively.


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Figure 10. 2D Free energy profiles with respect to z distances of the centers of benzene rings in catechol functional groups in polymers containing two As and two Cs. (a), (b), (c), and (d) show the profiles of oligomers AACC, ACCA, CAAC, and ACAC, respectively.


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Figure 11. Snapshots of AACC sampled from a simulation biasing the COM z distance to 8 Å close to the minimum free energy distance.


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Table 1. Summary of simulations performed in this study. Simulation Type # and type of oligomer 1-5 6-10 11-13 14-16 17 18-27

USa US MDb US MD US

1 (A,C,AA,AC,CC) 1 (A,C,AA,AC,CC) 45 (A,C,AC) 2 (A,C,AC) 45 (AC) 1 (A4, CA3, ACAA, AACC, ACCA, CAAC, ACAC, AC3, CACC, C4)

# of Water

# of Alumina slab

None 3000 3000 3000 None 3000

1 1 2 0 1 1

a

US signifies that the potential of mean force (PMF) was calculated by umbrella sampling (US).

b

MD signifies that the PMF was calculated with MD simulations without umbrella sampling.


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Table 2. Summary of minimum free energy and corresponding distance from the surface in free energy profiles of a single oligomer estimated by the biased umbrella sampling simulations.

a

Condition

Minimum Free Distance at energy minimum free (kcal/mol) energy (Å)

A

Vacuum

-7.91 (0.13)

3.5 (0.0)

C

Vacuum

-21.31 (1.05)

3.3 (0.0)

AA

Vacuum

-14.17 (0.12)

3.5 (0.0)

AC

Vacuum

-24.88 (0.88)

3.5 (0.0)

CC

Vacuum

-33.58 (1.92)

3.6 (0.0)

A

Aqueous

-0.48 (0.21)

6.6 (0.3)

C

Aqueous

-0.89 (0.33)

5.9 (0.7)

AA

Aqueous

-0.56 (0.24)

7.7 (0.2)

AC

Aqueous

-0.82 (0.17)

7.3 (0.2)

CC

Aqueous

-1.02 (0.22)

7.4 (0.2)

A4

Aqueous

-0.29 (0.61)

7.9 (0.4)

CA3

Aqueous

-0.79 (0.32)

6.9 (1.9)

ACAA

Aqueous

-0.77 (0.50)

7.0 (1.0)

AACC

Aqueous

-1.65 (0.89)

7.7 (0.6)

ACCA

Aqueous

-0.94 (0.27)

8.5 (1.1)

CAAC

Aqueous

-1.40 (0.24)

7.2 (1.1)

ACAC

Aqueous

-0.99 (0.20)

7.2 (0.7)

AC3

Aqueous

-1.25 (0.42)

8.7 (0.5)

CACC

Aqueous

-1.34 (0.76)

7.8 (1.9)

C4 Aqueous -1.70 (0.14) 8.3 (0.4) Minimum free energy and distance at minimum free energy were estimated from the free

energy profiles calculated from the last 8-ns trajectory after a 2-ns equilibration. Errors in parentheses were 1.96 times standard error, corresponding to 95% confidence interval, estimated from the analysis of four 2-ns blocks of the 8-ns trajectory.


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AUTHOR INFORMATION Corresponding Author *Tel.: +1 410 306 2811; fax: +1 410 306 0676; Email: [email protected]

ACKNOWLEDGMENT This work was supported in part by an appointment to the Postgraduate Research Participation Program at the U.S. Army Research Laboratory (ARL) administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and ARL. The DoD HPC Modernization Office supported this project by supplying supercomputer time. We thank Drs. Matthew A. Bartucci and Daniel Knorr for helpful discussions and comments on our work. REFERENCES 1. Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B., Mussel-Inspired Surface Chemistry for Multifunctional Coatings. Science 2007, 318 (5849), 426-430. 2. Lee, H.; Lee, B. P.; Messersmith, P. B., A reversible wet/dry adhesive inspired by mussels and geckos. Nature 2007, 448 (7151), 338-341. 3. Shao, H.; Stewart, R. J., Biomimetic Underwater Adhesives with Environmentally Triggered Setting Mechanisms. Adv. Mater. 2010, 22 (6), 729-733. 4. Dalsin, J. L.; Messersmith, P. B., Bioinspired antifouling polymers. Mater. Today 2005, 8 (9), 38-46. 5. Meredith, H. J.; Jenkins, C. L.; Wilker, J. J., Enhancing the Adhesion of a Biomimetic Polymer Yields Performance Rivaling Commercial Glues. Adv. Funct. Mater. 2014, 24 (21), 3259-3267. 6. Waite, J. H.; Tanzer, M. L., Polyphenolic Substance of Mytilus edulis: Novel Adhesive Containing L-Dopa and Hydroxyproline. Science 1981, 212 (4498), 1038-1040.


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Molecular Dynamics Simulation Study of Adsorption of Bioinspired

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