8650
Langmuir 2007, 23, 8650
Reply to the Comment on Kinetics of the Adhesion of DMPC Liposomes on a Mercury Electrode. Effect of Lamellarity, Phase Composition, Size and Curvature of Liposomes, and Presence of the Pore Forming Peptide Mastoparan X
Zˇ utic´ et al. write that we did not study the potential dependence of adhesion behavior. That is not true, and the respective information can be found in our previous publication.1 In the Langmuir paper, we reported the data for the electrode potential of -0.9 V vs Ag/AgCl because all necessary information could be derived from these measurements. Zˇ utic´ et al. report in Figures 2 and 3 adhesion signals that they measured, for which they claim to have carried out measurements similar to those reported in our paper. However, the composition of their liposomes completely differs from that of our liposomes, so that a comparison is not possible. Further, Zˇ utic´ et al. recorded the adhesion signals with a much too coarse time spacing of data (approximately 80 µs), so that they could not resolve the fine structure of the signals. Our measurements were performed with a resolution down to 1 µs. Zˇ utic´ et al. write that they come to another interpretation of the adhesion signals; however, they do not provide a quantitative model accounting for the capacity changes of highresolution signals. Zˇ utic´ et al. claim that “Equation 1 for the time evolution of the total displaced charge neglects the initial adhesion step of the liposome at the electrode surface and the flattening of the intact liposome.” This is wrong, as we explicitly describe that the first term on the right side of eq 1 represents the contact making (i.e., the attachment of an intact liposome to the mercury surface). There is a fundamental difference between a gold and a mercury electrode surface: whereas the gold surface is hydrophilic, the mercury surface is strongly lipophilic, and thus it follows that intact liposomes cannot attach to a mercury surface, stay there intact and just flatten, as they do on gold. Zˇ utic´ et al. know that oil droplets flatten at mercury electrodes until their detachment at very negative electrode potentials (see their paper2); however, that is due to their hydrophobic surface. With the hydrophilic surface of a liposome, this is not possible. At least that process is confined to a much smaller time span, and the flattening cannot play a significant role. Thus the statement of Zˇ utic´ et al. that “these assumptions are in direct contradiction with the theory and the experimental evidence on liposome deformability and the orientation of polar groups of bilayer membranes in the process of liposome adhesion on hydrophilic and hydrophobic substrates” is incorrect. The cited AFM studies on gold surfaces cannot be taken as arguments against our interpretation of processes occurring at mercury electrodes. It is known from AFM studies that monolayers are formed when liposome are attached to hydrophobic surfaces made by the modification of hydrophilic surfaces with organic molecules.3,4 Here, one should remember that a mercury surface is even more hydrophobic than these preparations. * Corresponding author. Address: Institut fu¨r Biochemie, Universita¨t Greifswald, 17487 Greifswald, Felix-Hausdorff-Strasse 4, Germany. Tel.: +49-(0)3834-86-4450. Fax: +49-(0)3834-86-4451. E-mail: fscholz@ uni-greifswald.de. (1) Hellberg, D.; Scholz, F.; Schubert, F.; Lovric´, M.; Omanovic´, D.; Herna´ndez, V. A.; Thede, R. J. Phys. Chem. B 2005, 109, 14715. (2) Ivosˇevic´, N; Zˇ utic´, V. Langmuir 1998, 14, 231.
We did not neglect the fact of “impact and contact making” of particles, but, from our data analysis, it is clear that this effect is given by Q0 in eq 1 of the comments of Zˇ utic´ et al. What Zˇ utic´ et al. depict in part A of Figure 1 is a scenario for oil droplets, but, for liposomes, that model is too simplified. Our model comprises the contact making (impact) and the details of spreading, and thus it is much more elaborate than the picture given in panels A and B of Figure 1 of Zˇ utic´ et al. Zˇ utic´ et al. write “We argue that, analogously to fluid membranes of living cells, liposomes will first establish close contact with the charged surface of the mercury electrode with their intact outer membrane, without the “turning around of one or few lecithin molecules” or pore opening in the lipid bilayer.” In Figure 1 of our Langmuir paper we explained that L′ is the liposome attaching to the mercury surface without any “turned around” molecule and without pore formation yet. The statement of Zˇ utic´ et al. that “it is difficult to imagine the pulling out of only one or a few lecithin molecules from the supramolecular assembly of a phospholipid bilayer” is surprising because it is well-known that single molecules of liposome membranes flip around with activation energies that are similar to what we have observed for the docking step.5-7 That kind of molecular dynamics of single lecithin molecules in membranes is a well-known and established feature. Finally, Zˇ utic´ et al. criticize our interpretation of the experiments with Mastoparan X because we did not explicitly show the incorporation of that peptide in the liposome membrane. Indeed, that has been shown earlier by others,8 and there is no need to repeat these experiments. Further, Zˇ utic´ et al. write that “Schwarz and Abruzova (cited as ref 42 in H&S1) also added MPX to the aqueous suspensions of liposomes, but at higher MPX concentrations”; however, this is not correct. We worked exactly with the same concentration as these authors, that is, with their lowest concentration, because higher concentrations were detrimental to the measurements. In summary, we do not believe the comments of Zˇ utic´ et al. diminish the value of our experimental findings and their interpretation. The adhesion of liposomes indeed has similarities with the process of fusion of two vesicles, since the anchoring of a liposome on mercury and the first steps of anchoring of two liposomes for fusion need the same rearrangements of lecithin molecules. Victor Agmo Herna´ ndez and Fritz Scholz*
Institut fu¨r Biochemie, UniVersita¨t Greifswald, 17487 Greifswald, Felix-Hausdorff-Strasse 4, Germany ReceiVed April 3, 2007 LA7009435 (3) Tero, R.; Watanabe, H.; Urisu, T. Phys. Chem. Chem. Phys. 2006, 8, 3885. (4) Jass, J.; Tja¨rnhage, T.; Puu, G. Biophys. J. 2000, 79, 3153. (5) Lipowsky, R. Encyclopedia of Applied Physics; Wiley-VCH: New York, 1998; Vol. 23, p 199. (6) Liu, J.; Conboy, J. C. Biophys. J. 2005, 89, 2522. (7) Marti, J.; Csajka, F. S. Europhys. Lett. 2003, 61, 409. (8) Schwarz, G.; Arbuzova, A. Biochim. Biophys. Acta 1995, 1239, 51.
10.1021/la7009435 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/24/2007
