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Journal of Colloid and Interface Science 329 (2009) 351–356

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Journal of Colloid and Interface Science

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Surface films of short fluorocarbon–hydrocarbon diblocks studied by moleculardynamics simulations: Spontaneous formation of elongated hemimicelles

Ángel Piñeiro ∗, Gerardo Prieto, Juan M. Ruso, Pedro V. Verdes, Félix Sarmiento

Biophysics and Interfaces Group, Department of Applied Physics, Faculty of Physics, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 July 2008Accepted 2 October 2008Available online 15 October 2008

Keywords:Surface filmsMolecular dynamics simulationsPerfluoroalkanesDiblocksTwo-dimensional self-assemblyHemimicelles

Using grazing incidence small-angle X-ray scattering (GISAXS), and atomic force microscopy (AFM) ithas been recently demonstrated that linear fluorocarbon–hydrocarbon diblocks (FnHm) self-assemble inwater/air interfaces forming elongated and circular hemimicelles. Those structures have been observedfor diblocks with at least eight fluorinated carbons. Based on the lack of a collapse pressure for F6H16,and due to the fact that no stable surface pressure values are reached under compression, it has beenconcluded that these molecules do not form stable monolayers. It has been also suggested that F6H16and shorter diblocks desorb from the water surface under compression. It is not easy to accept thata significant concentration of so hydrophobic molecules can be stable in aqueous solution even whenthe employed experimental techniques were not able to clearly detect a well defined structure on theinterface. In the present work the adsorption and arrangement of F6H16 and F6H10 at the water surfaceare studied by molecular dynamics (MD) simulations as a function of the available area per molecule.Starting from a random mixture, the spontaneous formation of elongated hemimicelles is observed forboth systems when the area per molecule is higher than ∼50 Å2. For intermediate areas two pseudo-phases, one rich in hydrocarbons and the other with higher fluorocarbon concentration, are formed. Forthe systems with less than ∼30 Å2 available per molecule the formation of multilayers is observed. Thisis the first time that the dynamics and structure of perfluoroalkane (PFA) films, and in particular ofhemimicelles on a liquid surface, are observed and characterized at atomic level.

© 2008 Elsevier Inc. All rights reserved.

1. Introduction

Interfaces are ubiquitous in nature. Biological systems use themto build containers for molecular transport, to separate or fil-ter molecules between different tissues or cells, or to protectcomplete organisms from the environment, among many otherfunctions. Most of the biological interfaces consist of many rel-atively small molecules that self-assemble in a well organizedpattern whose structure and physicochemical properties dependon its composition and on external variables such as the tem-perature or the application of electric fields [1]. Very complexmolecular processes, like the whole hormonal system, are basedon the efficient communication between opposite sides of mem-brane cells, which represent the most important type of biologicalinterfaces. Trying to mimic nature, it has been proved that it ispossible to take advantage of interface-based systems, mainly li-posomes, for controlled drug delivery, reducing the toxicity andincreasing the stability of therapeutic agents [2]. Releasing rate

* Corresponding author.E-mail address: angel.pineiro@usc.es (Á. Piñeiro).

of liposome-trapped drugs can be regulated by affecting mem-branes composition [3]. For instance, mixtures of standard phos-pholipids and fluorinated molecules proved to increase the sta-bility of the resulting vesicles and to decrease the releasing rateof the encapsulated molecules [4,5]. Fluorocarbons are very at-tractive for pharmaceutical purposes due to their biocompatibility.Thus, a large number of applications based on these moleculesare being studied or even already being commercialized [6,7]. Thefirst step towards the rationale control of membrane propertiesbased on a certain system is to get a good knowledge of monolay-ers or surface films built with the same molecules. Fluorocarbonself-assembled monolayers (F-SAMs) have been studied mainly us-ing surface X-ray scattering, surface potential, Brewster angle andatomic force microscopies, and surface pressure measurements [8–10]. F-SAMs resulted to be interesting by themselves due to theirsimultaneous hydrophobic and lipophobic character that makesthem ideal candidates for nanoscale coating materials [11]. Spe-cial attention was put on mixed fluorocarbon-hydrocarbon diblocks(named CnF2n+1CmH2m+1 or, in a more compact way, FnHm). Thesemolecules showed unexpected aggregation patterns such as the re-cently reported elongated or pit- and tip-centered circular hemimi-celles which were proposed as building blocks for nanolithography

0021-9797/$ – see front matter © 2008 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2008.10.018

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Fig. 1. Illustration of the methodology employed to build up the simulation systems. Top row: adsorption of PFA molecules producing two surface films. Middle row:construction of larger films with exactly the same number of molecules in both interfaces. Bottom row: compression of the system to obtain boxes with specific values ofthe area available per molecule. Carbon (yellow spheres) and fluorine (green spheres) atoms are shown together with the water molecules (blue semitransparent surface).(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

[12,13]. Such supramolecular structures were studied by grazing in-cidence small-angle X-ray scattering and atomic force microscopy.Surface pressures isotherms as a function of molecular areas werealso measured using a Wilhelmy plate coupled to a Langmuir filmbalance. For diblocks larger than F6H16, collapse pressures be-tween 12 and 22 mN m−1 with limiting surface areas of ∼31 Å2,were reported. Based on the lack of a collapse pressure and tothe fact that no stable surface pressure values are reached undercompression for F6H16, it was concluded that this compound doesnot form stable monolayers [13]. Monolayer stabilities were foundto be higher as longer the diblock chain is. It was thus suggestedthat F6H16 and shorter diblocks desorb from the water surface un-der compression [13]. Similar arguments were provided to explainthe profile of F8H8 and F8H10 isotherms [14]. Our skepticism to-wards this conjecture, mainly based on the high hydrophobicityof PFA molecules with almost 20 carbon atoms and on the weakevidences in favor of their absorption to the water bulk phase,encouraged us to perform the comprehensive molecular dynamicsstudy presented here. Our results indicate that the studied diblocksmay form elongated hemimicelles at the water surface when theavailable area per molecule is higher than ∼50 Å2. At lower areasthe same molecules would organize in hydrocarbon/fluorocarbonpseudo-phases. The mechanism leading to these structures as wellas the structures by themselves are observed and characterized

for the first time at atomic level. A few MD simulations on PFAor related molecules have been carried out in the past by sev-eral authors [15–20]. The closest approximations to the presentwork are the studies by Kim and Shin [16,17], based on 10-ns-long simulations for F12H18 at several temperatures using implicitsolvent. Highly ordered structures with clusters of different chainorientations relative to the interface were observed by these au-thors. Studies considering different surface concentrations of PFAmolecules have not been performed. Consequently, no structuressimilar to those reported in the present work have been previouslyobserved.

2. Materials and methods

2.1. Setup of the simulation boxes

Two random mixtures consisting of 2400 waters and 80 F6H10or F6H16 molecules were introduced in boxes of 5 × 5 × 15 nm.The larger size of the z dimension was chosen to allow the forma-tion of two liquid/air interfaces perpendicular to this axis. For eachsystem, a 10-ns-long MD simulation at 298 K and constant volumeproduced two PFA monolayers (labeled as A and B in Fig. 1, toprow) due to the fast adsorption of the fluorinated molecules to thesurface of the solution. The system was enlarged by a factor of four,

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pasting two inverted copies of itself adjacent to the original box inthe xy plane and an exact replica in the diagonal (Fig. 1, middlerow). The resulting box consists of 9600 water molecules in to-tal with two monolayers of exactly 160 F6H10 or F6H16 moleculeseach. The size of such a simulation box is 10 × 10 × 15 nm andthe average area available per PFA molecule is 62.5 Å2. An en-ergy minimization of the final system using the steepest descentmethod was carried out. Then a MD simulation at constant boxheight with semi-isotropic control pressure in the x and y dimen-sions coupled to a Berendsen barostat [21] at 1 bar and at 298 Kwas performed. As expected, a compression of the system in thexy plane was observed (Fig. 1, bottom row). From this trajectory,frames with an average of 50, 45, 40, 35, 30, 25, and 20 Å2 perPFA molecule were extracted. Each of these latter systems togetherwith the original one (with 62.5 Å2 per molecule), were simulatedat constant volume and 298 K for 50 ns in order to observe therearranging of the solutes on the water surface.

2.2. MD simulation parameters

All MD simulations were performed using the GROMACS pack-age [22–24] version 3.3.2. PFA molecules were modeled using theGROMOS96 (53a6) force field [25] with the bonded parametersthat involve fluorine taken from Borodin et al. [26]. Following theseauthors, the partial atomic charges of PFA molecules were assumedto be negligible. The extended simple point charge (SPC/E) model[27] was utilized for water molecules. Periodic boundary condi-tions with rectangular boxes were used for all the trajectories.Except for the case of the semi-isotropic compression describedabove, all simulations were performed at 298 K and at constantvolume. Water and PFA molecules were separately coupled to aBerendsen thermostat with a coupling constant of 0.1 ps [21]. Sincethe PFA molecules do not have partial atomic charges, a simplecutoff at 1.4 nm was used instead of more expensive methodstypically employed, to evaluate long range interactions. Randominitial velocities were assigned to the systems from a Maxwell–Boltzmann distribution at 298 K. The equations of motion wereintegrated using the leapfrog method [28] with a time step of 2 fs.Bond lengths and angles in water were constrained using the SET-TLE algorithm [29], while the LINCS algorithm [30] was used toconstrain bond lengths within the PFA molecules. During the MDsimulations, coordinates, velocities, and energies were stored every2 ps for further analysis.

2.3. Analysis of MD trajectories

The viewers RASMOL 2.7 [31], VMD 1.8.2 [32], and PyMOL [33]0.99 were employed to roughly inspect the arrangement of PFAmolecules on the water surface and to capture images throughoutthe trajectories. The distributions of hydrocarbon and fluorocarbonatoms as a function of the box height obtained every 10 ns wereplotted and employed to measure the width of the surface lay-ers. With this aim, the simulation boxes were divided in 2 Å widthslides perpendicular to the z axis and the number of atoms of eachtype in every slide was counted. The width of the total carbons dis-tribution at one third of its maximum height was identified withthe width of the PFA surface film. Average deuterium order param-eters for the carbon atoms of PFA molecules were also calculatedevery 10 ns for all the trajectories. The analysis was performed us-ing programs from the GROMACS package.

3. Results and discussion

The methodology employed to build the systems (Fig. 1) al-lowed us to simulate simultaneously two replicas of each film un-der the same conditions. Eight simulations with different area per

molecule each were performed using F6H10 and another eight us-ing F6H16. Area per molecule values both larger and shorter thanthe expected limiting surface area of the employed PFAs (between30 and 35 Å2) were selected. Four views of the final conformation,after 50 ns of MD simulation, of each of the sixteen trajectories areshown as Supplementary material. Distributions of carbon atoms(both fluorinated, hydrogenated, and total) corresponding to thelast conformation of every system are shown in Figs. S1 and S2,and two selected examples are presented in Figs. 2A and 2B. Dif-ferent relative distributions of the fluorinated and hydrogenatedcarbons were observed as a function of the area per molecule.A strong dependence of the surface film width (Fig. 2C) on thearea was also noted. For 40, 45, and 50 Å2 per PFA molecule thedistribution of hydrogenated carbons is shifted towards the wa-ter surface while the distribution of the fluorinated segments areshifted in the opposite direction (Fig. 2A). This reflects the simulta-neous hydrophobic and lipophobic character of fluorocarbons thattend to separate both from water and from the hydrocarbon seg-ments of the diblocks. At lower area per molecule two separatedpeaks in the fluorocarbon distribution indicating two regions withhigh concentration of the fluorinated segments can be observed,one close to the water surface and the other one in the externalside of the film (Fig. 2B). This profile appears combined with amaximum of the hydrocarbon segment density in between the re-gions with high concentration of fluorine. This suggests that below35 Å2 a second PFA layer is formed on the surface, with the fluo-rinated tails of one of the layers in direct contact with water andthose of the second layer exposed to the vapor phase. The area permolecule at which this second layer starts to be formed compareswell with the limiting surface area obtained for larger diblocks. Al-though the abovementioned peaks and shifts in the distributionsare clear, they coexist with the presence of fluorine and hydro-gen throughout the film region, the mixture between both groupsbeing almost random at 20 Å2, which corresponds to the high-est surface concentration of PFA molecules studied in this work.Results for the systems with F6H16 and F6H10 are similar. Theheight of the hydrocarbon distributions is higher in the case ofF6H16 than for F6H10 due to the difference in the length of thecorresponding segment between the two molecules. For the sim-ulations at 62.5 Å2 per molecule the height of the distributions islower than that at 50 Å2 and both the hydrogenated and the fluori-nated contributions seem shifted towards the water surface. This isclearly due to the presence of the elongated hemimicelles formedunder such conditions. The hemimicelles formation process can beobserved in the snapshots shown in Fig. 3 for one of the F6H16films. Similar structures were obtained for the opposite film in thesame simulation as well as for the two films corresponding to thesimulation with F6H10 at the same area per molecule (snapshotsare shown in Figs. S3–S6). As observed in Fig. 3 or S5 and S6, theemployed time scale, 50 ns, was suitable and necessary to observethe whole hemimicelle formation process for F6H16. Such a pro-cess seems faster for F6H10 (Figs. S3 and S4). Moreover, for F6H10at 50 Å2 small water-holes were observed in one of the surfacefilms, indicating the onset of the PFA two-dimensional aggregation.

For both PFA molecules the structure of the hemimicelle is sim-ilar, the hydrocarbon chains forming the core of the structure andthe fluorinated segments mainly orientated towards its externalside, exposed to the vapor phase. However the hemimicelle is notperfectly symmetric in the sense that a few fluorocarbons coex-ist with the hydrocarbons in the core of the structure. Close upsof hemimicelles transversal sections obtained for F6H10 and F6H16are shown in Fig. 4. Film width values obtained for the simulationsperformed at 62.5 Å2 (Fig. 2C) may be taken as an estimation ofthe hemimicelle radius. Thus, hemimicelles of F6H16 seem to be6–8 Å wider than hemimicelles of F6H10. There is a clear depen-dence of the film width with the available area per PFA, lower

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Fig. 2. (A) Distribution of total carbons (black solid line), hydrogenated carbons (solid red line) and fluorinated carbons (dashed green line) as a function of the box heightat 40 Å2 per F6H10 molecule. (B) Idem to (A) for 30 Å2 per F6H16 molecule. (C) Film widths calculated from the carbon distributions as indicated in the text for the twointerfaces of F6H10 (P and ) and of F6H16 (× and 1) as a function of the available area per molecule. (D) Carbon atoms order parameters after 50 ns of MD simulation for62.5 (dashed orange line), 50 (solid orange line), 45 (dashed green line), 40 (solid green line), 35 (dashed red line), 30 (solid red line), 25 (dashed black line) and 20 (solidblack line) Å2 per F6H16 molecule. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the area higher the width (Fig. 2C). However the correspondingplots show a minimum at 50 Å2 and a subsequent increase at62.5 Å2. Such a behavior was found for the two replicas. For bothPFA molecules this is clearly due to the self-assembly on the watersurface.

It is interesting to observe the order parameters as a functionof the available area per molecule. They were calculated every10 ns, the corresponding plots being shown in Figs. S7 and S8.Fig. 2D shows the order parameters obtained for the last con-formation of the eight trajectories involving F6H16. In all cases,fluorocarbon chains appear more ordered than hydrocarbon seg-ments. Unexpectedly the average orientation of all carbon atomsis far from the perpendicular to the plane of the interface sinceall order parameters are much closer to zero than in typical lipidbilayers [34,35]. This may be also observed in the final configu-rations shown as Supplementary material. Thus, although the dis-tributions of fluorinated and hydrogenated segments indicate thatthere is certain trend to separate from each other forming twopseudo-phases, they seem to be partially miscible and the result-ing pseudo-mixtures present a significant disorder.

4. Conclusions

The biocompatibility of fluorinated compounds, as well as thesimultaneous hydrophobic and lipophobic character of fluorocarbon–hydrocarbon diblocks justify the interest of these molecules for anumber of applications including the stabilization of liposomes fordrug transport and delivery, the generation of two-dimensionalspecific structures for nanolithography, and the nanoscale coatingof materials [4–7,11–13]. The particular structural features of thesemolecules are expected to be reflected in self-assembly patters

under favorable conditions. Interestingly, tip and pit-centered cir-cular, as well as elongated hemimicelles formed by relatively largePFA diblocks were recently found in water/air interfaces, by usinggrazing incidence small-angle X-ray scattering and atomic forcemicroscopy [13]. Based on surface pressure isotherms versus thearea per F6H16 molecule—measured using a Wilhelmy plate cou-pled to a Langmuir film balance—it has been recently suggestedthat this and shorter diblocks desorb from the water surface atlow area per molecule values [13]. Such a proposal deserves to beverified since short PFA’s are expected to be more suitable to becombined with lipids (amongst other molecules) than very largefluorocarbon–hydrocarbon diblocks. The absorption of PFA’s to theaqueous solution would indicate that they are at least partially sol-uble and this would affect their potential applications. In this work,a comprehensive study of surface films consisting of two perfluo-roalkanes, F6H10 and F6H16, as a function of the available area permolecule is presented. It is observed by molecular dynamics sim-ulations that both F6H10 and F6H16 are stable on the liquid/airinterface and that the compression of the films may induce theformation of multilayers with separated fluorinated/hydrogenatedpseudo-phases on the surface of the liquid. The formation of mul-tilayers would explain the lack of a collapse pressure observed inthe experimental π–A isotherms. Interestingly, a two-dimensionalself-aggregation of both compounds leading to elongated hemim-icelles was observed at areas larger than ∼50 Å2 per molecule.The formation process as well as the structural characterizationat atomic level of such structures is reported for the first time inthe present work. These results cannot be considered as an artifactof the employed methodology since the same protocol has beenused in our lab for molecules of different nature (ionic surfactants)

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Fig. 3. Snapshots corresponding to one of the surface films of the simulation at 62.5 Å2 per F6H16 molecule, showing hemimicelles formation. The simulated system was aquadrant of the shown images, enlarged here with its periodic images for clarity. PFA and water molecules representations are the same as in Fig. 1.

providing completely different patterns (work in progress). More-over, similar structures have been previously observed for longerPFA’s by using atomic force microscopy [13]. In addition to thedescription at atomic level of the different structures consistingof F6H10 and F6H16 perfluoroalkanes at the water/air interface,our results question the interpretation of recently reported π–Aisotherms for short PFA molecules. As stated by Kwok et al. [36],the operation of Langmuir film balances depends on a direct com-pression of the surface. This could seriously affect π–A profilesof some molecules—like PFA’s—that do not have a polar or ionicgroup which anchor to the water surface. As suggested by Kwoket al. [36] an axisymmetric drop shape analysis based tensiometercould be more suitable for this kind of systems.

Acknowledgments

We thank M.P. Krafft (Institut Charles Sadron, Strasbourg) for acritical reading of this work and for useful suggestions. This studywas supported by grants from the Spanish “Ministerio de Edu-cación y Ciencia” (Project MAT2005-02421) and the “European Re-gional Development Fund (ERDF)”. Á.P. and P.V.V. thank “Xunta de

Fig. 4. Close ups of transversal sections for hemimicelles consisting of F6H10 (left)and F6H16 (right) molecules. PFA and water molecules representations are the sameas in Fig. 1.

Galicia” for their “Isidro Parga Pondal” research positions. F.S. andJ.M.R. thank “Consellería de Educación e Ordenación Universitariade Xunta de Galicia” and “Dirección Xeral de Promoción Científicae Tecnolóxica do Sistema Universitario de Galicia” for financial sup-port. We are grateful to the Centro de Supercomputación de Galicia(CESGA) for computing time and for their excellent services.

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Supplementary material

The online version of this article contains the following addi-tional Supplementary material: One pdf file containing Figs. S1–S8.Two additional pdf files—one for the systems involving F6H10 andanother for those with F6H16—showing four views for the finalconformation (after 50 ns) of each simulation.

Please visit DOI: 10.1016/j.jcis.2008.10.018.

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