Comment pubs.acs.org/Langmuir
Reply to “Comment on ‘Structural Properties of POPC Monolayers under Lateral Compression: Computer Simulations Analysis’”
T
he simulation parameters and method used by Huynh et al.1 to simulate a 1-palmitoyl-2-oleoyl-sn-glycero-3phosphocholine (POPC) monolayer lead to a good reproduction of the experimental data in the 20−40 mN/m range of surface pressure, which is the range of interest from an experimental point of view. The first point raised by Lamberg and Ollila in their comments2 concern the use of a particular experimental parameter, i.e., the surface tension of the pure water at the air/water interface (γ0) in Huynh et al.1 simulations. Precise experimental data in addition to general parameters such as temperature, pressure, and volume are commonly used in simulations and constitute reference data allowing simulations validation. Concerning the work of Huynh et al., the experimental reference data are, first, the value of γ0, namely the known experimental value (71.8 mN/m), and second, the experimental pressure at which the lipid film collapse occurs (∼44 mN/m), a well-recognized structural event. When using such references, we may be confident about the extraction of experimentally unknown data from simulations, which is the main objective of simulations that are the description of atomic and molecular interactions governing microscopic and macroscopic properties of experimental systems. Using the experimental γ0, Huynh et al. observed that the POPC film collapses at a pressure of 43 mN/m. This value agrees quite well with the experimental value of 44 mN/m. The experimental isotherm published by Huynh et al.1 (page 566) is the mean isotherm obtained from three measurements, and surface pressures at collapse showed very good reproducibility. Therefore, the reported experimental value may thus be considered accurate. With the force-field, parameters, and protocol used, in the 20−40 mN/m range, the experimentally known physical parameters, Π, A, T, and pressure at collapse, are rather well reproduced. This gives us the confidence that experimentally unknown data could be correctly represented. Lamberg and Ollila stated that the “apparent agreement between the simulated and measured surface pressure-area isotherms” would be due to a compensation effect: the error in the choice of the experimental value for γ0 would be compensated by errors in the force field as described by Siu et al.3 This statement is questionable. As a matter of fact, the correction of force-field effect described by Siu et al.3 was introduced to suppress a nonexisting transition to a gel-like state. Such a transition is not observed in the work of Huynh et al.1 Concerning the suggested metastable state of the simulated monolayer, we may consider the following four points which, in our opinion, do not provide a demonstration of the metastable state of the system of Huynh et al.:1 1. Lamberg and Ollila argue that, using the simulation parameters provided by Huynh et al.,1 they proved that at 20 mN/m and below, we should observe metastable states. However, in fact they used NVT ensemble instead of the NPγT ensemble they should have used to compare with the Huynh et al. work. © 2014 American Chemical Society
2. Lamberg and Ollila refer to the existence of instability close to a first-order transition, but there is no such transition in the POPC system between 20 and 40 mN/m! 3. Importantly enough, simulation of lipid systems should take into account a certain time scale related to local perturbation dissipation dynamics. This time scale can be safely taken as the characteristic time for a lipid lateral definite diffusion step. From past experiments, this time can be estimated, for one-step diffusion, on the order of 100−200 ns, and this is the minimum simulation time that Huynh et al.1 chose. Lamberg and Ollila used a much too short 50 ns simulation time. Therefore, the state that they observed most probably results from simulation instability due to their protocol, rather than a meaningful physical metastable state. As an example, Duncan and Larson4 compared different modes of simulation, and their data suggested that simulations using the NVT ensemble “do not allow for sufficient pressure relaxation”. 4. Using the NPγT ensemble and 200 ns long trajectories, our simulation never showed any evidence of phase separation that could lead one to think that our system is in a metastable state, in the whole 10−40 mN/m range with 66 POPC monolayers, as well as with a 4 times larger monolayer at 20 mN/m (see below). This is a fact. During their work, Huynh et al.1 were of course aware of the possibility of finite size effects, and undertook long time
Figure 1. POPC monolayer at the end of the 200 ns MD simulation at 300 K and 20 mN/m surface pressure. The system is composed of 264 POPC molecules and 13 828 water molecules.
Received: October 17, 2014 Published: December 22, 2014 888
DOI: 10.1021/la504104e Langmuir 2015, 31, 888−889
Comment
Langmuir
Figure 2. POPC monolayer (2 × 33 POPC) at the end of the 200 ns MD simulation at 300 K and 30 mN/m surface pressure. Left: before heating to 320 K; right: after heating and cooling to 300 K. ‡
simulations of a monolayer system composed of 264 POPC molecules and 13 828 water molecules at different surface pressures. At 20 mN/m surface pressure, these systems never showed any feature that could lead one to think that they could be in a metastable state (see Figure 1). The starting point configuration of the MD simulation of the larger system was built from the configuration of the smaller one at the end of the 200 ns MD simulation. From these data, Huynh et al.1 concluded also that finite size effects were not detectable, and thus continued simulations with the 66 POPC monolayer for computational time saving. Again, we stress that careful preparation and an equilibration period of at least 100 ns should be applied to study lipid systems. As reported,1 the system comprising 66 POPC was also heated to 320 K and cooled to 300 K (total simulation time of 0.8 μs) (see Figure 2). The same value of the calculated surface area per lipid and the same profile of the order parameters were obtained after heating the system to 320 K and cooling it back to 300 K. No instability was observed during this temperature cycling. In conclusion, simulations of lipid monolayers are very important and complementary to bilayer simulations because the monolayer physical parameters, i.e., Π, A, T, and pressure at collapse could be well experimentally characterized, and thus be precisely confronted to simulation data. Importantly, these studies are needed for improvement of force field parameters and simulation protocols. The work of Huynh et al.1 set up the basis for such an approach for POPC monolayers and underlined the needed development of an “improved” water model and force field. This is obviously the aim of our ongoing work.
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CEA, iBiTecS, Laboratoire des Protéines Membranaires, F-91191 Gif-sur-Yvette, Cedex, France § CNRS, UMR8221, F-91191 Gif-sur-Yvette, France ∥ Université Paris-Sud, UMR8221, F-91405 Orsay, France ⊥ Université d’Evry-Val-d’Essonne, Département de Physique, F-91025 Evry, Cedex, France # Université Paris-Sud, CNRS UMR8612, Physico-chimie des surfaces, F-92296 Châtenay-Malabry, France
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
REFERENCES
(1) Huynh, L.; Perrot, N.; Beswick, B.; Rosilio, V.; Curmi, P.A.; Sanson, A.; Jamin, N. Structural properties of POPC monolayers under lateral compression: Computer simulations analysis. Langmuir 2014, 30, 564−573. (2) Ollila, O. H. S.; Lamberg, A. Comment on “Structural Properties of POPC Monolayers under Lateral Compression: Computer Simulations Analysis” . Langmuir 2015, DOI: 10.1021/la5025845. (3) Siu, S. W. I.; Vácha, R.; Jungwirth, P.; Böckmann, R. a Biomolecular simulations of membranes: Physical properties from different force fields. J. Chem. Phys. 2008, 128, 125103−125112. (4) Duncan, S.L.; Larson, R.G. Comparing experimental and simulated pressure-area isotherms for DPPC. Biophys. J. 2008, 94, 2965−2986.
Lucie Huynh† Nahuel Perrot‡,§,∥ Veronica Beswick‡,§,∥,⊥ Véronique Rosilio# Patrick A. Curmi† Alain Sanson‡,§,∥ Nadège Jamin*,‡,§,∥ †
INSERM, U829, Laboratoire Structure − Activité des Biomolécules Normales et Pathologiques, Université d’Evry-Val-d’Essonne, F-91025 Evry, France 889
DOI: 10.1021/la504104e Langmuir 2015, 31, 888−889
