Model of an Asymmetric DPPC/DPPS Membrane: Effect of Asymmetry on the Lipid Properties. A Molecular Dynamics Simulation Study J. J. Lo ´ pez Cascales,* ,² T. F. Otero, ² Bradley D. Smith, ‡ Carlos Gonza ´ lez, § and M. Ma ´ rquez |,§ UniVersidad Polite ´ cnica de Cartagena, Centro de Electroquı ´ca y Materiales Inteligentes (CEMI), Aulario II, Campus de Alfonso XIII, 30203 Cartagena, Murcia, Spain, Department of Chemistry and Biochemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556, National Institute of Standards and Technology (NIST), Building 221, Room A111, Gaithersburg, Maryland 20899, Chemistry DiVision, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and I’NEST Group, New Technology Research, PMUSA, Richmond, Virginia 23234 ReceiVed: October 31, 2005; In Final Form: December 13, 2005 The study of asymmetric lipid bilayers is of a crucial importance due to the great number of biological process in which they are involved such as exocytosis, intracellular fusion processes, phospholipid-protein interactions, and signal transduction pathway. In addition, the loss of this asymmetry is a hallmark of the early stages of apoptosis. In this regard, a model of an asymmetric lipid bilayer composed of DPPC and DPPS was simulated by molecular dynamics simulation. Thus, the asymmetric membrane was modeled by 264 lipids, of which 48 corresponded to DPPS - randomly distributed in the same leaflet with 96 DPPC. In the other leaflet, 120 DPPC were placed without DPPS - . Due to the presence of a net charge of -1 for the DPPS - in physiological conditions, 48 Na + were introduced into the system to balance the charge. To ascertain whether the presence of the DPPS - in only one of the two leaflets perturbs the properties of the DPPC in the other leaflet composed only of DPPC, different properties were studied, such as the atomic density of the different components across the membrane, the electrostatic potential across the membrane, the translational diffusion of DPPC and DPPS, the deuterium order parameters, lipid hydration, and lipid-lipid charge bridges. Thus, we obtained that certain properties such as the surface area lipid molecule, lipid head orientation, order parameter, translational diffusion coefficient, or lipid hydration of DPPC in the leaflet without DPPS remain unperturbed by the presence of DPPS in the other leaflet, compared with a DPPC bilayer. On the other hand, in the leaflet containing DPPS, some of the DPPC properties were strongly affected by the presence of DPPS such as the order parameter or electrostatic potential. 1. Introduction Biological membranes are crucial cellular components with multiple roles. For example, they maintain electrochemical gradients by controlling the diffusion of ions and biomolecules, they act as a supporting matrix for embedded enzymes and receptors, and they engage in stabilizing interactions with skeletal proteins. It is becoming increasingly clear that biological membranes are a complex and heterogeneous assembly of nonpolar and amphiphilic molecules and that theoretical methods will play an important role in developing an atomic-scale picture of the structure and kinetics of membrane assembly. In this regard, the past decade has seen the first attempts to simulate the structure of highly simplified bilayer membranes. These pioneering studies have typically employed the molecular dynamics (MD) technique to study a homogeneous bilayer membrane composed of one type of phospholipid. In 1994, Egberts et al. 1 carried out preliminary studies of DPPC (DPPC: dipalmitoylphosphatidylcholine) bilayers, in which they identified various phospholipid-phospholipid and phospho- lipid-water interactions that formed the basis of the assembly. They also estimated the rate of water diffusion across the bilayer. Subsequent studies of other bilayer systems have employed analogous methodology. Cascales et al. 2,3 carried out simulations of a bilayer composed of an anionic DPPS (DPPS: dipalmi- toylphosphatidylserine) bilayer (compared with the DPPC membrane which is uncharged in physiological conditions), the surface of which is smaller, even when it is highly anionic. The simulations of DPPS bilayers provided useful information concerning the most likely phospholipid-phospholipid interac- tions and the effect of the charged bilayer on the water structure in the vicinity of the membrane surface. In recent years, growing computing power and improved simulation algorithms have led to more complicated simulations and extended the trajectory length. Thus, for example, Pandit et al. 4 studied the effect of salt concentration on the structure of a DPPC bilayer and lipid complexation and ion binding in symmetric mixed bilayers of DPPC and DPPS. 5 Mukhopadhyay et al. 6 carried out several MD simulations of a palmitoyl- oleoylphosphatidylserine (POPS) bilayer with sodium counter- ions and additional sodium chlorides. Sach et al. 7 also studied the changes in phospholipid headgroup tilt induced by the presence of monovalent salts. However, the common denomination of all the works mentioned above was the use of symmetrical bilayers that have an equal number and the same type of phospholipids in each leaflet of the membrane. However, it is well-known that the * To whom correspondence should be addressed: javier.lopez@upct.es. ² Centro de Electroquı ´ca y Materiales Inteligentes (CEMI). ‡ University of Notre Dame. § National Institute of Standards and Technology (NIST). | Los Alamos National Laboratory and I’NEST Group. 2358 J. Phys. Chem. B 2006, 110, 2358-2363 10.1021/jp0562680 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/17/2006