Enhanced Autoionization of Water at Phospholipid Interfaces

Abstract

ABSTRACT:The structure and autoionization of water at the water−phospholipid interface are investigated by ab initio molecular dynamics and ab initio Monte Carlo simulations using local density approximation (LDA) and generalized gradient approximation (GGA) for the exchange−correlation energy functional. Depending on the lipid headgroup, strongly enhanced ionization is observed, leading to the dissociation of several water molecules into H + and OH − per lipid. The results can shed light on the phenomena of the high proton conductivity along membranes that has been reported experimentally. ■ INTRODUCTIONPhospholipids are amphiphilic molecules that, in an aqueous environment, self-assemble into bilayers and form the major structural constituents of biomembranes. Although the affinity of the lipid's headgroup to interfacial water has been widely addressed in the literature, 1,2 the exact chemical nature of this coupling is not fully understood and its strength remains to be quantified. In particular, it is not clear how this coupling affects the structure and dynamics of the interfacial water layer and the proton transport. Enhanced proton conduction along phospholipid−water interfaces was first observed in the mid 1980s.3,4 Recent studies by means of scanning tunneling microscopy (STM) confirm a significant lateral conductivity. This conductivity is believed to be of functional importance because lateral proton diffusion along membrane surfaces represents the most efficient pathway for H + transport between protein pumps. 6 The molecular mechanism underlying the high lateral proton conductivity has not yet been resolved. 7−11In this paper we report ab initio molecular dynamics (AIMD) and ab initio Monte Carlo (AIMC) simulations of interfacial water covering the headgroup of zwitterionic dipalmitoyl−phosphatidylcholine (DPPC) molecule. We perform calculations within the local density approximation and generalized gradient approximation for the exchange−correlation energy functional, with Ceperley−Alder 12 and Perdew− Burke−Ernzerhof 13 parametrizations, respectively. We show that the interfacial water exhibits a strongly enhanced autoionization that is caused by the presence of strong local electric field as well as strong hydrogen bonding. ■ COMPUTATIONAL DETAILSGround State Calculations. To calculate the ground state structure of the DPPC molecule, we first build the molecule by putting atoms together with coordinations according to the bonding rules from chemistry. To relax this structure, density functional theory (DFT) is applied as implemented in the SIESTA code.14 To this end, the DPPC molecule is placed in a unit cell with dimensions much larger than the size of the molecule. The spatial extension of the pure DPPC structure is ≈4 × 29 × 8 Å. We choose a box of 20 × 40 × 20 Å dimensions. As a consequence, because we are dealing with a molecule (a cluster), no periodic boundary conditions are adopted and only the Γ point in the reciprocal space is needed for the energy integration. The mesh cutoff is set to...

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