Interaction Between Silicon–Carbide Nanotube and Cholesterol

Article pubs.acs.org/JPCC

Interaction Between Silicon−Carbide Nanotube and Cholesterol Domain. A Molecular Dynamics Simulation Study. Przemysław Raczyński,*,† Krzysztof Górny,† Jannis Samios,‡ and Zygmunt Gburski† †

Institute of Physics, University of Silesia, Uniwersytecka 4, 40−007, Katowice, Poland Laboratory of Physical & Theoretical Chemistry, Department of Chemistry, University of Athens, GR-15771 Athens, Greece

‡

S Supporting Information *

ABSTRACT: The influence of the single walled silicon−carbide nanotube on the cholesterol domain localized on a surface of the endothelial protein 1LQV has been investigated using NPT and NVT molecular dynamics computer simulation method. Our study reveals that the silicon−carbide nanotube can pull cholesterol molecules out of the domain; i.e., it is able to reduce effectively the domain placed on the protein surface. The capability of silicon−carbide nanotube to remove cholesterol molecules from protein surface has been observed during an extraction process simulated by means of steered molecular dynamics. Single walled silicon−carbide nanotube was compared with homogeneous carbon nanotube in terms of their efficiency in cholesterol extraction in the same conditions.

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sciences. In our previous studies,6,7 using the molecular dynamics (MD) simulation technique, we investigated the influence of a single walled carbon nanotube (SWCNT) on the structure of a cholesterol molecular cluster (domain) developed on the surface of the endothelial protein 1LQV in water environment. In those works, we choose to study carbon nanotube (CNT) since it is known to be hydrophobic, and we assumed that in the biosystems, where water is an essential component, the rather nonpolar cholesterol molecules should show preferential affinity to hydrophobic CNT. It should be also noted that CNTs exhibit interesting properties for a number of technological applications and drug delivery systems,5,8,9 and their interactions with the phospholipid membranes were extensively studied.3,10,11 In the present work, we would like to extend our recent studies, by investigating heterogeneous CNT, namely silicon− carbide nanotube (SiCNT) to study its impact on a cholesterol domain located around a chosen extra cellular protein in water environment. In fact, the synthesis of SiCNT is a difficult task due the large difference between sp2 and sp3 hybridization bond structures with a value of 1.25 eV per Si−C bond pair.12 Despite this, there have been some successful attempts to synthesize silicon multiwall SiCNTs with graphitic wall morphology.13,14 Recent studies have shown that SiCNTs might have some advantages when compared to CNTs. They exhibit higher reactivity, which might facilitate aimed side wall functionalization. Ab initio computational studies15 have shown that the

INTRODUCTION Cholesterol is a special type of lipid, known as a sterol due to its molecular structure made of a steroid and alcohol. The steroid component consists of four linked hydrocarbon rings forming its bulk part with a single hydrocarbon tail at one end and a hydroxyl polar group (alcohol component) at the other. Cholesterol is present in all mammalian (including human) cell membranes (biomembranes) with amounts varying from approximately 20% to about 50%; however, it is absent in intracellular as well as prokaryote membranes. The role of cholesterol in biosystems was thoroughly studied.1−3 The so far performed experimental and computational studies4,5 have shown that cholesterol is one of the most important lipid molecules in biomembranes, due to its functional ability to modulate their physical properties. It is well-known, for instance, that the fluidity of the cell membrane is regulated by cholesterol concentration. The stiffening of the bilayer is a result of cholesterol appearance in the gaps between phospholipids. Increased fluidity of the bilayer, on the other hand, is a consequence of the bending of hydrocarbon tails in phospholipid molecules which takes place when cholesterol is present at a very low concentration. Consequently, cholesterol is essential for cell viability with the maintenance of the appropriate cell membrane structure being its key function. Cholesterol also circulates with the bloodstream as a component of lipoproteins and can be found in the lymphatic fluid of the human body. Although cholesterol is essential for the functioning of cell membranes, its excess may lead to the formation of plaques in blood vessels.1 Therefore, the search for the new methods to remove the excess of cholesterol molecules, which are the precursors of plaque deposition in an early phase of atherosclerosis disease, is a vital subject of molecular biomedical © 2014 American Chemical Society

Received: June 4, 2014 Revised: November 17, 2014 Published: November 25, 2014 30115

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parameters for carbon and silicon−carbon nanotubes were taken from24 and25 respectively. Atomic charges on cholesterol molecule were taken from.26 The integration time step was set to Δt = 0.5 fs for all simulation runs. The standard NAMD integrator (Brünger− Brooks−Karplus algorithm) was used.27 All molecules used in simulations (cholesterol, protein, CNT, SiCNT, water) were modeled on the full atomistic level. Cutoff distance for all nonbonding interactions was set to 13 Å. All simulations were performed at the physiological temperature T = 310 K and with periodic boundary conditions.28,29

most stable SiCNT has the ratio of Si to C atom equal to 1. In other studies,16,17 it was demonstrated that SiCNT can be applied in hydrogen storage components.

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THEORETICAL METHODS We decided to examine two issues concerning cholesterol− nanostructure interactions. First, we surrounded the nanotubes by 30 cholesterol molecules to show the impact of CNT and SiCNT on the cholesterol molecules covering it. We compared the obtained results with pure sample (without nanotube). In the second part of our studies we investigated the process of extraction of cholesterol molecules from the domain by nanotubes. The model we applied is simplified when compared to the biological systems, as it consists only of endothelial protein, cholesterols, water, and nanotube. In living organisms, there are many additional components of blood (i.e., proteins, liposomes, blood cells), that could interact with nanotubes and impact the proposed extraction process. In the present work, the term cholesterol domain is referred to a group of cholesterol molecules located on the surface of endothelium protein. This system is a simplification of an early stage cholesterol domains, which are known to be the precursors of atherosclerosis. The molecular dynamics (MD) simulations were performed using the NAMD 2.8 program18,19 with the all-atom CHARMM force field.20 The initial configurations of all systems were obtained from a series of NPT simulations. In these preliminary simulation runs pressure was controlled using Langevin barostat. The reference pressure was set to 1 atm. The equilibration in the NPT ensemble was performed over 5 × 106 time steps (Δt = 0.5 fs). Next the systems were equilibrated in the NVT (constant number of particles, constant volume, and constant temperature) ensemble for time t = 1 ns. After that the “production phase” was started for all systems. During this stage the systems were simulated in the NVT ensemble and the data were collected every 200 simulation steps. The cholesterol domain on 1LQV protein surface, obtained from previous NPT and NVT simulations, was used as a starting point to initiate our investigations. The extracellular domain protein 1LQV was chosen (see Protein Data Bank21). 1LQV protein occurs in the endothelium which provides an interface between circulating blood and the rest of the vessel wall. We placed 21 cholesterol molecules near the surface of the 1LQV protein. Next, to make the environment of the described system similar to that appearing in biological samples, water was added (TIP3P CHARMM adapted model22). It should be noted, that the applied water model does not reproduce water translational diffusion correctly,23 nevertheless the entire CHARMM force field was fitted to describe interaction of biological compounds with this particular model. Initially, nanotubes were placed at the distance of approximately 20 Å from the cholesterol domain surface and the system was equilibrated first in the NPT ensemble and then in NVT, as discussed earlier. Next, steered molecular dynamics simulation (SMD; see ref 19) was performed to bring the nanotube near the cholesterol domain and to remove it after the extraction process. When the nanotube was placed next to the cholesterol domain, NVT simulation with fixed nanotube position was performed and the data were collected. All interactions in the simulated systems were described with adapted form of CHARMM potential. The CHARMM adapted

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RESULTS AND DISCUSSION Properties of Cholesterol Molecules Surrounding Nanotubes. We prepared two ensembles composed of 30 cholesterol molecules surrounding (a) pure CNT and (b) heterogeneous SiCNT. We prepared also the pure cholesterol cluster (30 molecules), as a reference. These ensembles after equilibration were simulated in water environment for 5 ns at T = 310 K. The cholesterol molecules surrounding CNT or SiCNT formed a thin layer on the surface of the corresponding nanotube (Figure 1).

Figure 1. Snapshot of instantaneous configurations of cholesterol molecules covering (a) CNT and (b) SiCNT. The nanotubes are colored in gray, cholesterol in black. OH groups of cholesterols are additionally marked.

Figure 1, shows that cholesterol molecules tend to orient along the nanotube following their amphiphilic nature−polar OH groups prefer the neighborhood of other OH groups rather than hydrophobic hydrocarbon chains. The mean square displacement ⟨|Δr(⃗ t)|2⟩ of the mass center of cholesterols, where Δr(⃗ t) = r(⃗ t) − r(⃗ 0) and r ⃗ is the position of the center of mass of a single molecule, was calculated. Figure 2 shows ⟨|Δr(⃗ t)|2⟩ for cholesterols surrounding CNT (black line) and SiCNT (gray line). We did not observe that the movement of cholesterol molecules along the nanotube 30116

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and the diameter of SiCNT is R = 16 Å. The lengths of CNT and SiCNT are l = 55 Å and l = 71 Å, respectively. The smallest distance between nanotube surface and cholesterol molecules defined as the distance between the vertical line and first significant peak is bigger for the system with CNT (r = 2.1 Å in system with CNT compared to r ≈ 1.9 Å in system with SiCNT). In case of the system with CNT, two clearly visible peaks can be observed, which correspond to the two, parallel to nanotube axis, orientations of cholesterol molecules (see Figure 1a). The small peak observed in the second system around ∼1 Å from SiCNT surface is an effect of lower stiffness of SiCNT in comparison to CNT. While the shape of CNT is stable during the simulation, SiCNT tends to bend and to become more elliptical in cross-section than CNT. Extraction of Cholesterol Molecules by Nanotubes. The impact of CNT and SiCNT on the cholesterol domain were studied for three different orientations of the nanotube, with the respect to the protein−cholesterol cluster: perpendicular (orientation 1, Figure 4a), parallel (orientation 2, Figure 4b) and directed toward the domain (orientation 3, with one end of the nanotube oriented to the domain, Figure 4c). In each case, the nanotube was first moved toward the domain, than maintained close to the domain for 25 ns and finally retracted with the velocity v = 2.5 m/s which is typical for SMD simulations of nanostructures.32−34 The same simulation scheme was repeated for CNT. To compare the mobility of cholesterol molecules in a domain without the nanotube and after the nanotube was moved closer, we chose the orientation 2, because in this case comparable numbers of cholesterols were pulled out of the protein surface. In Figure 5 plots of ⟨|Δr(⃗ t)|2⟩ for the compared systems are shown. The calculated self - diffusion coefficient of cholesterol is equal to D = 0.0027 Å2/ps without nanotube, D = 0.0007 Å2/ps with CNT and D = 0.0005 Å2/ps with SiCNT. These coefficients were estimated with error smaller than 0.00005 Å2/ps. The data presented above were obtained from the last 5 ns of the simulations (when the nanotube was kept close to the domain). Our results show that the presence of nanotube reduces the mobility of cholesterol molecules. The SiCNT more effectively decreases the mobility of cholesterols than the CNT. It should be also emphasized, that in case of the two systems with protein, nanotube and cholesterols, the dynamics is slower than in the cases discussed in previous section, i.e. without protein, when cholesterol molecules form layers around nanotubes. When cholesterols are localized on protein surface (without nanotube), the translational diffusion coefficient is similar to values obtained for layers surrounding nanotubes, while for the systems with protein and nanotube it is an order of magnitude lower. The process of removing cholesterol by nanotubes is quite efficient (see Supporting Information in mpg format, available online for visualization of domain extraction by SiCNT, orientation 2). Our simulations show that in each simulated system some cholesterols move from the domain and form a layer around the nanotube. The efficiency of removing cholesterols from the domain depends both on the orientation and on the type of nanotube. Table 1 presents the average efficiency of the two types of nanotubes, obtained from five simulation runs for each system. Each simulation run of the extraction process was performed with different starting configuration generated from previous simulation with nanotube fixed close to the cholesterol domain surface.

Figure 2. Mean square displacement of the center of the cholesterol molecules surrounding CNT (black line) and SiCNT (gray line) and for the system without nanotube (black dotted line).

long axis dominates over the movement in the plane perpendicular to the axis. Accordingly, we decided to study the diffusion in three dimensions rather than only along the nanotube with is typical procedure for smaller molecules like noble gases or methane.30,31 It should be also noted, that we did not observe cholesterol molecules desorbing from nanotube surface. In case of pure cholesterol cluster ⟨|Δr(⃗ t)|2⟩ was calculated with respect to its mass center. Small drift of the mass center of the cluster, induced by thermostatting procedure, was excluded from calculations. The diffusion coefficient D of cholesterol, estimated from the linear part of the ⟨|Δr(⃗ t)|2⟩ plot, is about 0.0033 Å2/ps for both systems with nanotubes, whereas for pure cholesterol cluster it is equal to 0. 0177 Å2/ps, thus it is about 1 order of magnitude higher. This difference in D values shows that the presence of CNT or SiCNT significantly slows down the translational dynamics of cholesterol molecules, due to physisorption of cholesterol on the nanotube surface. Figure 3 shows the radial distribution of cholesterol molecules with respect to the nanotube axis. Black solid line represents system with CNT and gray onewith SiCNT. Vertical dashed lines correspond to the approximated radii of the respective nanotubes. The diameter of CNT is R = 13 Å

Figure 3. Radial distribution function of the center of cholesterol mass with respect to the main axis (longitudinal) of CNT (black line) and SiCNT (gray line). Dashed vertical lines represent the approximated diameters of the corresponding nanotubes. 30117

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Figure 4. Snapshot of instantaneous configurations of three different orientations of SiCNT with respect to the cholesterol domain. SiCNT was oriented perpendicularly to the domain (a), along the domain (b) or toward the domain (c). Identical arrangements were used in the case of CNT.

potential energy (averaged over the last 5 ns of the simulations) of interaction between cholesterol molecules and SiCNT is −305.3 kcal/mol whereas for CNT it is −228,4 kcal/mol. The fluctuations in interaction energy are not larger than 30 kcal/ mol. The attractive interaction between SiCNT and cholesterols is therefore about 1/4 larger than in case of CNT. The lowest efficiency (less than 30%) was achieved for orientation 3. In this orientation the contact surface between protein and cholesterol is the smallest. In this case CNT achieves slightly larger efficiency than SiCNT (one more cholesterol molecule extracted). We observed that after pulling CNT out of the domain the cholesterols were both around and inside it, while for the SiCNT all the extracted cholesterols were inside. To increase the extraction efficiency in this orientation, the bundle of nanotubes might be considered. Except orientation 3 the SiCNT reaches higher efficiency than CNT. The repeating of the extraction process (orientation 1 and 2) using clean SiCNT should remove these cholesterols which remain after the first intervention of nanotube. The ability of nanotubes (including heterogeneous SiCNT) to extract the cholesterol domain is quite appealing.

Figure 5. Comparison of the mean square displacement of the mass center of cholesterols in systems with CNT (black solid line) and SiCNT (gray solid line) and without nanotube (black dashed line).

Table 1. Efficiency of the Nanotubes in Removing Cholesterol Molecules from the Domaina orientation orientation 1 orientation 2 orientation 3 a

type of nanotube CNT SiCNT CNT SiCNT CNT SiCNT

amount of extracted cholesterols

average efficiency [%]

± ± ± ± ± ±

33 86 76 86 29 24

7 18 16 18 6 5

2 3 1 2 1 1

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CONCLUSIONS We have shown that at physiological temperature T = 310 K the discussed types of nanotubes can quite efficiently reduce the cholesterol domain. The efficiency of extraction process depends on the nanotube orientation and type of the nanotube. SiCNT is more effective than CNT. Both nanotubes significantly reduce the mobility of cholesterol molecules in the domain localized on the surface of endothelial protein 1LQV. Our results might be of interest for future medical applications, especially taking into account the quest for the development of future tools of molecular medicine. In this context, however, some corresponding experimental studies would be of great value and are therefore called for.

The obtained results were rounded to integers.

CNT reaches the highest efficiency for orientation 2, i.e., when it is arranged along the domain. In this orientation, the contact area of the CNT with cholesterols is the largest and the extraction process is the easiest. This result is consistent with the results presented in ref 6, where we examined the process of reducing domain by CNT arranged similarly to orientation 2. Significant differences in the efficiency of the two nanotubes occur in orientation 1, i.e., when they are oriented perpendicularly to the domain. The efficiency of SiCNT is over three times higher than the efficiency of CNT. This effect could be explained by the stronger interaction SiCNT− cholesterol and bigger diameter of SiCNT, causing that the contact area with the cholesterol cluster is larger. The average

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ASSOCIATED CONTENT

S Supporting Information *

Visualization of cholesterol extraction process (mpg movie). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(P.R.) E-mail: [email protected]. 30118

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Notes

Empirical Potential for Molecular Modeling and Dynamics Studies of Proteins†. J. Phys. Chem. B 1998, 102, 3586−3616. (21) Oganesyan, V.; Oganesyan, N.; Terzyan, S.; Qu, D.; Dauter, Z.; Esmon, N. L.; Esmon, C. T. The Crystal Structure of the Endothelial Protein C Receptor and a Bound Phospholipid. J. Biol. Chem. 2002, 277, 24851−24854. (22) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. Comparison of Simple Potential Functions for Simulating Liquid Water. J. Chem. Phys. 1983, 79, 926. (23) Mark, P.; Nilsson, L. Structure and Dynamics of the TIP3P, SPC, and SPC/E Water Models at 298 K. J. Phys. Chem. A 2001, 105, 9954−9960. (24) Alexiadis, A.; Kassinos, S. Molecular Simulation of Water in Carbon Nanotubes. Chem. Rev. 2008, 108, 5014−5034. (25) Moradi Garakani, F.; Kalantarinejad, R. A Molecular Dynamics Simulation of Water Transport through C and SiC Nanotubes: Application for Desalination. Int. J. Nano Dimens. 2012, 2, 151−157. (26) Henin, J.; Chipot, C. Hydrogen-Bonding Patterns of Cholesterol in Lipid Membranes. Chem. Phys. Lett. 2006, 425, 329− 335. (27) Brünger, A.; Brooks, C. L.; Karplus, M. Stochastic Boundary Conditions for Molecular Dynamics Simulations of ST2 Water. Chem. Phys. Lett. 1984, 105, 495−500. (28) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Clarendon Press; Oxford University Press: Oxford, England, and New York, 1989. (29) Rapaport, D. C. The Art of Molecular Dynamics Simulation; Cambridge University Press: Cambridge, England, 2004. (30) Bartuś, K.; Bródka, A. Methane in Carbon Nanotube: Molecular Dynamics Simulation. Mol. Phys. 2011, 109, 1691−1699. (31) Bartuś, K.; Bródka, A. Temperature Study of Structure and Dynamics of Methane in Carbon Nanotubes. J. Phys. Chem. C 2014, 118, 12010−12016. (32) Pogodin, S.; Baulin, V. A. Can a Carbon Nanotube Pierce through a Phospholipid Bilayer? ACS Nano 2010, 4, 5293−5300. (33) Gangupomu, V. K.; Capaldi, F. M. Interactions of Carbon Nanotube with Lipid Bilayer Membranes. J. Nanomater. 2011, 2011, e830436. (34) Raczynski, P.; Gorny, K.; Pabiszczak, M.; Gburski, Z. Nanoindentation of Biomembrane by Carbon Nanotubes - MD Simulation. Comput. Mater. Sci. 2013, 70, 13−18.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the computational center at ICM University of Warsaw-Poland, Grant No. G53-6, for CPU time allocation. REFERENCES

(1) Lusis, A. J. Atherosclerosis. Nature 2000, 407, 233−241. (2) Alberts, B. Molecular Biology of the Cell; Garland Science: New York, 2008. (3) Róg, T.; Pasenkiewicz-Gierula, M.; Vattulainen, I.; Karttunen, M. Ordering Effects of Cholesterol and Its Analogues. Biochim. Biophys. Acta BBABiomembr. 2009, 1788, 97−121. (4) Scott, H. L. Modeling the Lipid Component of Membranes. Curr. Opin. Struct. Biol. 2002, 12, 495−502. (5) Feller, S. E. Molecular Dynamics Simulations of Lipid Bilayers. Curr. Opin. Colloid Interface Sci. 2000, 5, 217−223. (6) Gburski, Z.; Gorny, K.; Raczynski, P. The Impact of a Carbon Nanotube on the Cholesterol Domain Localized on a Protein Surface. Solid State Commun. 2010, 150, 415−418. (7) Gburski, Z.; Gorny, K.; Raczynski, P.; Dawid, A. Impact of the Carbon Allotropes on Cholesterol Domain: MD Simulation. In Carbon NanotubesGrowth and Applications; Naraghi, M., Ed.; InTech: 2011. (8) Bianco, A. Carbon Nanotubes for the Delivery of Therapeutic Molecules. Expert Opin. Drug Delivery 2004, 1, 57−65. (9) Kostarelos, K.; Bianco, A.; Prato, M. Promises, Facts and Challenges for Carbon Nanotubes in Imaging and Therapeutics. Nat. Nanotechnol. 2009, 4, 627−633. (10) Parthasarathi, R.; Tummala, N. R.; Striolo, A. Embedded SingleWalled Carbon Nanotubes Locally Perturb DOPC Phospholipid Bilayers. J. Phys. Chem. B 2012, 116, 12769−12782. (11) Shi, L.; Shi, D.; Nollert, M. U.; Resasco, D. E.; Striolo, A. SingleWalled Carbon Nanotubes Do Not Pierce Aqueous Phospholipid Bilayers at Low Salt Concentration. J. Phys. Chem. B 2013, 117, 6749− 6758. (12) Miyamoto, Y.; Yu, B. D. Computational Designing of Graphitic Silicon Carbide and Its Tubular Forms. Appl. Phys. Lett. 2002, 80, 586−588. (13) Sun, X. H.; Li, C. P.; Wong, N. B.; Lee, C. S.; Lee, S. T.; Teo, B. K. Templating Effect of Hydrogen-Passivated Silicon Nanowires in the Production of Hydrocarbon Nanotubes and Nanoonions via Sonochemical Reactions with Common Organic Solvents under Ambient Conditions. J. Am. Chem. Soc. 2002, 124, 14856−14857. (14) Pham-Huu, C.; Keller, N.; Ehret, G.; Ledoux, M. J. The First Preparation of Silicon Carbide Nanotubes by Shape Memory Synthesis and Their Catalytic Potential. J. Catal. 2001, 200, 400−410. (15) Mavrandonakis, A.; Froudakis, G. E.; Schnell, M.; Mühlhäuser, M. From Pure Carbon to Silicon−Carbon Nanotubes: An Ab-Initio Study. Nano Lett. 2003, 3, 1481−1484. (16) Lithoxoos, G. P.; Samios, J.; Carissan, Y. Investigation of Silicon Model Nanotubes as Potential Candidate Nanomaterials for Efficient Hydrogen Storage: A Combined Ab Initio/Grand Canonical Monte Carlo Simulation Study. J. Phys. Chem. C 2008, 112, 16725−16728. (17) Mpourmpakis, G.; Froudakis, G. E.; Lithoxoos, G. P.; Samios, J. SiC Nanotubes: A Novel Material for Hydrogen Storage. Nano Lett. 2006, 6, 1581−1583. (18) Kale, L.; Skeel, R.; Bhandarkar, M.; Brunner, R.; Gursoy, A.; Krawetz, N.; Phillips, J.; Shinozaki, A.; Varadarajan, K.; Schulten, K. NAMD2: Greater Scalability for Parallel Molecular Dynamics. J. Comput. Phys. 1999, 151, 283−312. (19) Phillips, J. C.; Braun, R.; Wang, W.; Gumbart, J.; Tajkhorshid, E.; Villa, E.; Chipot, C.; Skeel, R. D.; Kalé, L.; Schulten, K. Scalable Molecular Dynamics with NAMD. J. Comput. Chem. 2005, 26, 1781− 1802. (20) MacKerell; Bashford, D.; Bellott; Dunbrack; Evanseck, J. D.; Field, M. J.; Fischer, S.; Gao, J.; Guo, H.; Ha, S.; et al. All-Atom 30119

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