pubs.acs.org/Langmuir © 2009 American Chemical Society
Influence of Lipid Vesicle Composition and Surface Charge Density on Vesicle Adsorption Events: A Kinetic Phase Diagram Kristian Dimitrievski* and Bengt Kasemo Department of Applied Physics, Chalmers University of Technology, S-412 96 G€ oteborg, Sweden Received May 15, 2009 Lipid vesicle adsorption on a solid surface, from a bulk liquid solution, results in different final situations on the surface depending on the vesicles’ composition, properties of the solution (pH, ion types, and concentration), and surface properties. The main alternative outcomes of the adsorption event are (i) a lipid bilayer with vesicle rupture immediately upon the adsorption event or (ii) bilayer formation only at and after a critical vesicle coverage, (iii) adsorption of intact vesicles, or (iv) repulsion (no adsorption). We have simulated these different events for the case of vesicles consisting of pure neutral (zwitterionic) lipids and mixtures of neutral and positive or negative lipids, keeping the bulk conditions fixed, and have compiled the different resulting lipid structures on the surface as a function of vesicle composition and surface charge density, in a kinetic phase diagram.
Supported lipid bilayers (SLBs) and supported (lipid) vesicular layers (SVLs), formed on a solid surface from unilamellar vesicles in the bulk phase, have received rapidly increasing interest in the past decade. Pioneering work was done even earlier by Tamm and McConnell,1 demonstrating the possibility to make SLBs by this approach. Since the demonstration that the whole process from the first vesicles landing on the surface to completion of an SLB (or an SVL) could be followed in real time2 with the quartz crystal microbalance with dissipation (QCM-D) technique,3 the field has developed rapidly. It has been demonstrated that electrostatic interactions play a major (although not the only) role in the overall process of SLB and/or SVL formation, and constitute a major governing factor for the vesicle-surface interaction. Work by several groups4-10 has shown how different charges in the vesicles, keeping the surface charge state constant (usually SiO2, mica, or TiO2), give rise to a number of prototype cases, ranging from immediate rupture of the first vesicle hitting the surface (strongly positively charged vesicles) to intact vesicles at low coverage, which transforms to a bilayer at a critical higher coverage (e.g., palmitoyl-oleoyl-phosphatidylcholine (POPC) vesicles), to intact vesicle adsorption at all coverages (e.g., POPC on TiO2 and partly negatively charged vesicles on SiO2) and even complete repulsion (strongly negatively charged vesicles on SiO2). Results for a given type of lipid vesicles on different surfaces with different charge densities reinforce this picture. Furthermore, *Corresponding author. Tel.: +46 317726114; fax: +46 317723134. E-mail address:
[email protected].
(1) (a) Tamm, L. K.; McConnell, H. M. Biophys. J. 1985, 47, 105. (b) McConnell, H. M.; Watts, T. H.; Weis, R. M.; Brian, A. A. Biochim. Biophys. Acta 1986, 864, 95. (2) Keller, C. A.; Kasemo, B. Biophys. J. 1998, 75, 1397. (3) Rodahl, M.; H€oo€k, F.; Krozer, A.; Brzezinski, P.; Kasemo, B. Rev. Sci. Instrum. 1995, 66, 3924. (4) Richter, R. P.; Mukhopadhyay, A.; Brisson, A. R. Biophys. J. 2003, 85, 3035. (5) Richter, R. P.; Brisson, A. R. Biophys. J. 2005, 88, 3422. (6) Reviakine, I.; Rossetti, F. F.; Morozov, A. N.; Textor, M. J. Chem. Phys. 2005, 122, 204711. (7) Solon, J.; Streicher, P.; Richter, R.; Brochard-Wyart, F.; Bassereau, P. Proc. Natl. Acad. Sci. U.S.A. 2006, 103(33), 12382. (8) Sapuri, A. R.; Baksh, M. M.; Groves, J. T. Langmuir 2003, 19(5), 1606. (9) Wikstr€om, A.; Svedhem, S.; Sivignon, M.; Kasemo, B. J. Phys. Chem. B 2008, 112, 14069. (10) Kunze, A.; Svedhem, S.; Kasemo, B. Langmuir 2009, 25(9), 5146.
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it has been shown that ion strength and type of ion (monovalent or divalent) have a strong influence on the results (see ref 11 and references therein). Various experimental techniques in addition to QCM-D have been utilized in these studies, e.g., optical techniques such as surface plasmon resonance (SPR), fluorescence microscopy, reflectometry,5,8-10,12 and atomic force microscopy (AFM).4,6,13 Theoretical modeling is lagging behind this rapid experimental development (see, e.g., the review by Shillcock and Lipowsky14). The reason for this is the complicated nature of the interface between a bilayer and its support, at the molecular and nanoscale. Atomistic resolution, at the theoretical level, of this interface is today limited to a few lipid molecules sitting on a substrate, which are surrounded by buffer molecules (such as water and ions).15,16 Typically, such molecular dynamics simulations are limited to a few tens of nanoseconds, which is many orders of magnitude shorter than the time scale of adsorption and rupture of a single vesicle on a surface (which takes seconds to minutes). Therefore, coarse-grained models for vesicles and substrates are needed, as a complement, to probe this physical region theoretically. As mentioned above, electrostatics is a major component in the lipid-surface interaction. We have recently17 theoretically explored several of the cases mentioned above, with different vesicle charges and a given, fixed charged surface (SiO2), by using a coarse-grained model together with the Monte Carlo technique. Our goal with the present work is to extend and articulate the work in ref 17. In the latter study, only one type of surface was used for the vesicle adsorption simulations (namely a representation of a SiO2 surface). Here, the surface type is used as a governing parameter, by changing the surface charge density. The other governing parameter is (11) Seantier, B; Kasemo, B. Langmuir 2009, 25(10), 5767. (12) Reimhult, E.; Z€ach, M.; H€oo€k, F.; Kasemo, B. Langmuir 2006, 22(7), 3313. (13) Dimitrievski, K.; Z€ach, M.; Zhdanov, V. P.; Kasemo, B. Colloids Surf., B 2006, 47(2), 115. (14) Shillcock, J. C.; Lipowsky, R. J. Phys.: Condens. Matter 2006, 18(28), S1191. (15) Roark, M.; Feller, S. E. Langmuir 2008, 24(21), 12469. (16) Fortunelli, A.; Monti, S. Langmuir 2008, 24(18), 10145. (17) Dimitrievski, K.; Kasemo, B. Langmuir 2008, 24, 4077.
Published on Web 07/21/2009
DOI: 10.1021/la9025409
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Figure 1. Panel (a) shows a “kinetic phase diagram” of the fate of an adsorbing vesicle. The x-axis indicates the extent to which the surface is charged (in percent), and the y-axis indicates the bead mixture in the vesicle (in percent). Positive and negative percentages for the bead mixture means “p” beads and “n” beads mixed with “0” beads, respectively (y=0 means a 100% neutral vesicle). Red data points (red area) mean that the vesicle ruptured, blue data points (blue area) mean that the vesicle stayed intact on the surface, green data points (green area) mean that the vesicle adsorbed and then desorbed from the surface, and black data points (gray area) mean that the vesicle did not adsorb to the surface (the other three colored areas indicate mixed phases). For each data point there are 10 runs, and the color indicates the fate of the vesicle for the majority of the 10 runs. Two data points overlapping means that the fate of the vesicle was shared between two cases (e.g., a red spot inside a blue spot means rupture in five runs and an intact vesicle in five runs). The solid and dashed lines are connected to Figure 2 (see caption of Figure 2). Panel b shows snapshots of a vesicle for three cases, indicated by (1), (2), and (3). For case 1, the vesicle ruptures (at the left side), for case 2, the vesicle stays intact on the surface, and for case 3, the vesicle does not adsorb to the surface. Red, yellow, and blue beads indicate “p”-, “0”-, and “n”-type beads, respectively (blue surface beads indicate negative surface charges, while white surface beads indicate neutral surface sites).
the charge of the adsorbing vesicle. These two governing parameters determine the fate of an adsorbing vesicle: spontaneous vesicle rupture upon adsorption, intact vesicle adsorption, adsorption and then desorption from the surface, or no adsorption at all (repulsion). We propose a compact scheme to compile and present such data, in the form of a “kinetic phase diagram”. Such diagrams, widely used in, e.g., heterogeneous catalysis and in some other areas as well, 8866 DOI: 10.1021/la9025409
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describe different kinetics regimes or, in other words, “phases” depending on governing parameters. One kinetic phase is characterized by similar kinetics behavior, while the characteristics of the kinetics is quite different in another kinetic phase. The reason for choosing the term “kinetic phase diagram” and not just “phase diagram” is that, in general, the systems under consideration are far from thermodynamic equilibrium, and kinetic steady states shown in a diagram are not the lowest energy states of a system, i.e., they are not “phases” in a thermodynamic sense. What we aim to obtain is a kinetic phase diagram representation of the different possible outcomes of a vesicle-surface encounter displaying the different behaviors briefly outlined above. A complete treatment including all governing parameters is far beyond our goal here: Such a treatment would have to display data in a multidimensional space, where important components would be lipid composition of the vesicles (e.g., different mixtures of charged and neutral lipids), different surface charges and densities, different ion concentrations in the solution, different types of ions (monovalent, divalent, etc.), different temperatures, and different pH-values. Here, we restrict ourselves to keeping these parameters constant except the lipid composition of the vesicles and the surface charge density of the support, which are both varied in the simulations. A detailed description of our two-dimensional (2D) model is presented in ref 17. Here, we only give a brief summation. The vesicle is modeled as a closed string of beads, where the beads can be of three types, namely, “0”, “n”, and “p”, representing neutral (zwitterionic), negatively, and positively charged lipids, respectively (Figure 1b). The surface, onto which the vesicle is allowed to adsorb, is represented by Ns fixed surface sites, separated by a distance b. A surface site may be neutral or occupied by a negative surface charge. Electrostatic interaction is mimicked via a 1/r dependence of the energy between beads, a screening effect via an exp(-r/R) factor, and a parameter Cij that represents relative differences in the strength of electrostatic interaction between lipids with different head groups, that is, the value of Cij depends on the specific bead types “i” and “j”. The vesicle energy is given by E ¼ Eb þ Ee þ Ev þ ELJ þ Es
ð1Þ
where Eb ¼ A
N X ð1 -cos θi Þ
ð2Þ
i ¼1
is the bending energy (θi is the angle between si ri - ri-1 and si+1, where ri is the position vector of vesicle bead i); Ee ¼ B
N X
ðjsi j -aÞ2 =2
ð3Þ
i ¼1
is the energy of the elastic stretching of the chain (a is the equilibrium distance between nearest-neighbor beads); Ev ¼
N -1 X i ¼1
N X Cij -rij =R e r j ¼i þ1 ij
ð4Þ
is the vesicle bead-type contribution to the energy between vesicle beads, where Cij is a coupling parameter that depends on the specific types of beads i and j, R is a parameter corresponding to Langmuir 2009, 25(16), 8865–8869
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Table 1. Fixed and Governing Model Parametersa fixed
governing
A = 50 σ = 0.1 vesicle bead-mix N = 50 F θc = 80° B = 100 Ns = 100 a = 1.0 b = 0.5 C = 4.0 rcutoff = 2.0 κ = 0.25 yLJcutoff = 0.4 R = 0.5 kBT = 1.0 ε = 1.0 δx = δy = 0.1 a The first column contains, respectively, the bending energy prefactor, the critical bending angle that defines vesicle rupture, the elastic stretching prefactor, the equilibrium bead-bead distance in the model vesicle, the coupling constant for electrostatic-like interaction, the fraction of lipid head-group charge for zwitterionic lipids compared to positively charged lipids (as seen by the substrate), the Debye screening length, and the Lennard-Jones potential-depth parameter. The second column contains, respectively, the Lennard-Jones potential-width parameter, the number of beads in the vesicle, the number of beads defining the substrate, the fixed spacing between two substrate beads, the cutoff distance for electrostatic-like interaction, the cutoff distance for the Lennard-Jones potential, the definition of temperature, and the vesicle coordinate sampling. The third column contains the governing parameters, including the specific mixture of bead types in the vesicle, and the average surface charge density.
the length scale for which the bead-bead interaction becomes weak, and rij is the distance between the beads; ELJ ¼
N X
4ε½ðσ=yi Þ12 -ðσ=yi Þ6
ð5Þ
i ¼1
is the Lennard-Jones potential between the vesicle and the substrate, where yi is the (vertical) distance between vesicle bead i and the substrate; and Es ¼
Ns0 N X X Cij i ¼1 j ¼1
rij
e -rij =R
ð6Þ
is the energy between vesicle beads and surface sites occupied by a surface charge (N0s indicates summing over surface sites, which are occupied by surface charges), where Cij depends on the specific type of the vesicle bead in question (surface charges are of type “n” by definition), and rij is the distance between vesicle bead i and surface site j. The values for Cij (Cij=Cji) between vesicle beads are taken to be Cnn=Cpp=+C, C00=C0p=C0n=0, and Cpn=-C. Between a vesicle bead and a negative surface charge (indicated by n0 ) we take C0n =-κC, Cpn0 =-C, and Cnn0 =+C. The parameter κ (κ < 1) is introduced for “0” beads to represent a weaker interaction with surface charges compared to “p” and “n” beads (see ref 17). The vesicle is prescribed to have ruptured as soon as the bending angle for a bead reached a critical bending angle, θi g θc (eq 2). This occurs at the transition zone where some beads are attached to the surface and some are exposed to the liquid bulk phase. The fixed and the governing model parameters are summarized in Table 1. With this prescription, beads of the same charge in a vesicle tend to stay away from each other, while those of opposite sign attract each other, giving a uniform distribution of beads of different kinds when a vesicle is far from the surface. When a vesicle gets close to a surface, the bead distribution in the vesicle is changed, because negative beads are repelled from the surface and positive ones are attracted, causing a polarization effect on the vesicle. This polarization is fast compared to the velocity of approach to the surface of the whole vesicle, because lateral lipid diffusion in a lipid leaflet is very fast. Langmuir 2009, 25(16), 8865–8869
Initially, an equilibrated bulk vesicle (equilibrated with respect to shape and bead-type distribution) is put above the substrate at a distance of 1 length unit (lu), and at the center of the substrate laterally. Time starts to run in units of Monte Carlo steps (MCS), where one MCS is defined as the following steps: (i) A bead is selected at random. One attempt to move the selected bead is performed according to the Metropolis rule (see text below). (ii) Two nearest-neighbor (nn) beads are selected at random. An attempt to exchange the beads is performed according to the Metropolis rule. (iii) Steps (i) and (ii) are repeated N times. The new bead coordinates are selected randomly in the range xi ( δx and yi ( δy, where (xi, yi) is the initial position of the bead. The bead move is realized with probability P=1 if ΔE e 0, and with probability P=exp(-ΔE/kBT) if ΔE > 0 (i.e., according to the Metropolis rule), where ΔE is the energy difference between the final and initial states of the vesicle. Lateral diffusion of lipids in the outer leaflet of the bilayer membrane is simulated via step (ii), where a bead exchange event is realized with probability P=1 if ΔE e 0, and with probability P=exp(-ΔE/kBT) if ΔE > 0. In one MCS there are N bead move trials and N nn bead exchange trials, respectively. For the coordinate sampling we used δx=δy= 0.1. The surface charges are distributed randomly in the beginning of each simulation run, and the surface charges are fixed in location during a run. The outcome of a vesicle adsorption simulation is shown schematically in a “Kinetic Phase Diagram” in Figure 1a. If the vesicle adsorbs and then ruptures, the data point is red. This corresponds to the formation of a lipid bilayer platelet on the surface as has been seen, e.g., in ref 4. If the vesicle adsorbs and stays intact on the surface up to tmax, then the data point is blue (tmax=106 MCS is the maximum time defined in order to make a run stop). This corresponds to adsorption of an intact vesicle on the surface as has been seen in, e.g., refs 4 and 13. If the vesicle first adsorbs and then desorbs out into the bulk again, the color is green. This corresponds to transient (reversible) adsorption and has not, to our knowledge, been observed experimentally. It is likely to occur, however, for some combination of surface charge state and mixture of neutral, positive, and negative lipids in the vesicles. It would require a search for a situation where an adsorption- desorption equilibrium could be established with sufficient surface coverage to be measurable. If the vesicle does not adsorb at all to the surface, the color is black. This corresponds to a purely repulsive surface and has been seen in, e.g., ref 4. The different outcomes of a vesicle adsorption simulation depend on the bead mixture in the vesicle and on the density of charged surface sites. Since we have worked with a coarse-grained 2D model of a vesicle and a surface, one should not expect a perfect match between our simulation results and reality. A second reason is the interaction potentials. We have chosen a simplified set of effective potentials and have not treated ions (anions, cations) explicitly but only via the effective potentials and screening. The most interesting aspects are the trends and qualitative/semiquantitative results. The trends given by the phase diagram are likely to be generic, and we can draw some general conclusions about vesicle adsorption for different situations. For example, highly positively charged vesicles tend to not rupture spontaneously at low surface charge densities, while they do for moderately and highly charged surfaces. Highly negatively charged vesicles tend to touch/adsorb to the surface at low surface charge densities, while, for moderately and highly charged surfaces, they do not DOI: 10.1021/la9025409
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adsorb at all to (i.e., are repelled from) the surface. This scheme of compiling the simulation data (a kinetic phase diagram) is also very useful to compile corresponding experimental data. We point out again that the surface charges stay on the same location throughout a simulation run. This means that the possibility of dissociation/association of surface OH/O- groups during an adsorption run are not treated. However, if the rate of such dissociation/association events is comparable to or larger than the rate of vesicle adsorption events, one might expect slightly different outcomes for certain regions of the phase diagram. For example, a highly positively charged vesicle adsorbing on a surface with relatively low surface charge density may form more and more contact points with the surface as surface charges happen to appear underneath the vesicle that “locks” positively charged lipids to these (induced) surface charges, leading eventually to a quite deformed vesicle, which then might rupture spontaneously. That is, in the upper left part of the phase diagram in Figure 1a, the data points might be red instead of blue in reality. In fact, this detail might be clarified by QCM-D experiments. Spontaneous vesicle rupture gives a specific fingerprint in QCM-D measurements. A significant population of nonruptured vesicles on the surface also gives a specific fingerprint in such measurements. Using different pH values, for example, in the buffer solution would shift the surface charge density on, e.g., a SiO2 surface. However, generally we emphasize the general scheme of the presenting results and the kinetic phase diagram as such, for displaying simulation or experimental data and for comparing them and the trends, rather than the exact results (we did not study adsorption on positively charged surfaces because available experimental results are on negatively charged surfaces, and in order to keep the presentation as short as possible). For a given surface, the average shape (deformation) of an adsorbed vesicle depends on the composition of the vesicle. For example, the more positively charged a vesicle is, the more deformed it is on the surface. If the vesicle is positive enough, the vesicle ruptures spontaneously (Figure 1b, no. 1). On the other hand, the more negative a vesicle is, the more “rounded” it is on the surface (Figure 1b, no. 2). If the vesicle is negative enough, the vesicle does not adsorb at all (Figure 1b, no. 3). For a given vesicle composition, the deformation of an adsorbed vesicle depends on the type of surface it adsorbs to. The larger the surface charge density (of opposite charge) on the surface is, the larger the deformation of an adsorbed vesicle. Figure 2 shows the vesicle deformation for two cases: the deformation (aspect ratio) versus vesicle composition for a fixed surface (solid line) where the surface is taken to be 100% charged, and the deformation versus surface type for a fixed vesicle composition (dashed line) where the vesicle is 100% neutral. In general, the degree of deformation of an adsorbed vesicle most likely reflects the tendency of vesicle rupture on that particular substrate. If the vesicle deformation is large enough (i.e., if the bilayer-substrate interaction is attractive and strong enough), then spontaneous vesicle rupture occurs on the surface (note the small aspect ratio of a vesicle that is 40% positively charged on a 100% charged surface in Figure 2, and also the snapshot in Figure 1b, no. 1). If, on the other hand, the vesicle deformation is not too strong, then vesicles stay intact on the surface (all data points in Figure 2 except the rightmost one on the solid line; see also Figure 1b, no. 2). However, even if vesicles do not rupture spontaneously, vesicle rupture might occur as a consequence of other vesicles adsorbing close by that touch and push on the original vesicle (note that lipid vesicles are not rigid structures, but are rather soft and flexible, 8868 DOI: 10.1021/la9025409
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Figure 2. The solid line shows the aspect ratio (height-to-width ratio) of an adsorbed vesicle versus vesicle composition, on a 100% charged surface (corresponding to the solid line in Figure 1a). Aspect ratios are only calculated for cases where the vesicle stayed adsorbed (intact) on the surface the entire time up to t = tmax, and each data point is the average value of the vesicle aspect ratio. The rightmost data point on the solid line (where the vesicle bead mixture is 40% “p” and 60% “0”) is the average of the four runs (out of 10 runs) where the vesicle stayed intact on the surface up to t = tmax (in the remaining six runs, the vesicle ruptured spontaneously; see Figure 1a). For vesicles with a bead composition containing g 50% “p” beads, the vesicles always rupture spontaneously. For vesicles containing 60% and 70% “n” beads (the rest being “0” beads [the two leftmost data points on the solid line]), the vesicle stayed adsorbed on the surface up to t = tmax in 7 out of 10 runs and 6 out of 10 runs, respectively. Vesicles with g80% “n” beads do not adsorb. The dashed line shows the vesicle aspect ratio versus surface type, for neutral vesicles (corresponding to the dashed line in Figure 1a). The percentages indicated along the dashed line correspond to the different surface charge densities of the surface. The solid and dashed curves are plotted in the same figure to facilitate comparison of the aspect ratios (the lateral placement of the dashed curve is arbitrary).
and exhibit thermal shape fluctuations). [Vesicles are immobile on SiO2 and mica, which means that vesicle-vesicle “pushing” between neighboring vesicles takes place, if not constantly, then at least during an “equilibration” time during which neighboring vesicles have pushed themselves away from each other.18 Whether vesicles on TiO2 are immobile is unclear at the moment.] This interaction between adjacent vesicles probably lowers the barrier for vesicle rupture. The degree of deformation of adsorbed vesicles and the amount of neighbors is likely to influence the probability for vesicle rupture. If the deformation is large enough (but not so large that a single vesicle ruptures spontaneously), a critical amount of neighbors should be sufficient to initiate rupture. When this happens, we talk about a “critical coverage” of vesicles on the surface, and an SLB is eventually formed as more vesicles adsorb on the surface from the bulk liquid in standard experiments.2,12 The SLB formation proceeds fast as soon as the critical coverage is reached, because bilayer patches from ruptured vesicles induce rupture in nearby vesicles via the bilayer edge, which is a relatively fast process and accelerates the SLB formation.4,19 However, if the deformation of adsorbed vesicles is moderate, then it does not matter how many vesicles are adsorbed to the surface, vesicle rupture will not be initiated, and we end up with an SVL on the surface. The type of substrate used, and the type of vesicles used (together with buffer conditions), determine the outcome of an adsorption experiment. If we link vesicle deformation to the (18) Klacar, S; Dimitrievski, K.; Kasemo, B. J. Phys. Chem. B 2009, 113(17), 5681. (19) Dimitrievski, K.; Reimhult, E.; Kasemo, B.; Zhdanov, V. P. Colloids Surf., B 2004, 39(1-2), 77.
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tendency of vesicle rupture (and thereby the tendency of SLB or SVL formation), Figure 2 tells us that, for surfaces with a moderate surface charge density, neutral vesicles probably form a SVL on the surface instead of a SLB via a critical coverage (all this under “standard” buffer conditions; changing buffer conditions might result in completely different results, because buffer conditions is another “free variable” [not treated here] in a multidimensional kinetic phase diagram). Moreover, Figure 2 tells us that various negatively charged vesicles on a highly charged surface probably form an SLB via a critical-coverage pathway, which has indeed been observed on SiO2.4 Note that our model was constructed such that a surface with about 75% surface charges would represent a SiO2 surface.17 On SiO2, neutral vesicles form an SLB via a critical coverage (under normal experimental conditions). According to Figure 2, the deformation of neutral vesicles on surfaces having lower surface charge densities than 75% is significantly milder. We know that, on TiO2, an SVL is formed from neutral vesicles.6 Noting that the isoelectric points of SiO2 and TiO2 are about 2 and 4, respectively, the TiO2 surface is probably less charged than the SiO2 surface, and Figure 2 is thus consistent with these results. In ref 4, negatively charged vesicles were adsorbed on SiO2, with the observation that, for 30% negatively charged vesicles, an SLB
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was formed after a significantly delayed critical-coverage pathway, while, for 50% negatively charged vesicles, an SVL was formed. Figure 2 reflects this tendency via the aspect ratios of the different vesicles (note that the solid line should be translated vertically slightly to represent a 75% charged surface instead of a 100% charged one). Another interesting detail is that the boundary regions of rupture/intact (red/blue) and intact/desorption (blue/black) in Figure 1a do not change for surfaces ranging from about 40% up to 100% surface charges. This fact indicates that it is primarily the lipid composition of the vesicle that determines its fate upon adsorption, and the surface type plays a secondary role (for fixed other conditions, such as pH, temperature, ion concentration, etc.). In ref 17, we explained the reasons for the rupture/intact boundary for 40% positively charged vesicles on a surface containing 75% surface charges. The same reasons are applicable for surfaces containing 40% up to 100% surface charges (these reasons are discussed at length in ref 17 and are not discussed here). Acknowledgment. Financial support was obtained from the Swedish Research Council (VR) (Contract No. 16254111 and Contract No. 16254099).
DOI: 10.1021/la9025409
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