Effect of Molecular Structure on Electrochemical Phase Behavior of Phospholipid Bilayers on Au(111).

Philip N. Jemmett,Liam R. Cox,Sarah L. Horswell,Richard J. Nichols,David C. Milan

doi:10.1021/acs.langmuir.1c01975

Abstract

Lipid bilayers form the basis of biological cell membranes, selective and responsive barriers vital to the function of the cell. The structure and function of the bilayer are controlled by interactions between the constituent molecules and so vary with the composition of the membrane. These interactions also influence how a membrane behaves in the presence of electric fields they frequently experience in nature. In this study, we characterize the electrochemical phase behavior of dipalmitoylphosphatidylcholine (DPPC), a glycerophospholipid prevalent in nature and often used in model systems and healthcare applications. DPPC bilayers were formed on Au(111) electrodes using Langmuir-Blodgett and Langmuir-Schaefer deposition and studied with electrochemical methods, atomic force microscopy (AFM) and in situ polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). The coverage of the substrate determined with AFM is in accord with that estimated from differential capacitance measurements, and the bilayer thickness is slightly higher than for bilayers of the similar but shorter-chained lipid, dimyristoylphosphatidylcholine (DMPC). DPPC bilayers exhibit similar electrochemical response to DMPC bilayers, but the organization of molecules differs, particularly at negative charge densities. Infrared spectra show that DPPC chains tilt as the charge density on the metal is increased in the negative direction, but, unlike in DMPC, the chains then return to their original tilt angle at the most negative potentials. The onset of the increase in the chain tilt angle coincides with a decrease in solvation around the ester carbonyl groups, and the conformation around the acyl chain linkage differs from that in DMPC. We interpret the differences in behavior between bilayers formed from these structurally similar lipids in terms of stronger dispersion forces between DPPC chains and conclude that relatively subtle changes in molecular structure may have a significant impact on a membrane's response to its environment.

Highlights

We find that DPPC bilayers are slightly thicker than DMPC bilayers and the apparently small difference in molecular structure leads to different packing and different electrochemical phase behavior
An atomic force microscopy (AFM) study[51] explored the effect of temperature on bilayer thickness as supported bilayers become thinner in the liquid crystalline phase; a step in bilayer thickness was observed at 20−22 °C and attributed to the gel−liquid crystalline phase transition,[51] supporting the conclusion in IR studies that bilayers were in the gel phase.[35,49,50] (The slight decrease in temperature compared with other dispersions was ascribed to decoupling of, and stress within, the bilayer.51) Our results suggest that DPPC has fewer gauche conformers than DMPC, which would be expected for a longer-chain molecule at a similar temperature in a similar phase
The effect of an applied electric field on the behavior of DPPC bilayers supported on Au(111) has been studied with a combination of electrochemical methods, AFM and in situ PM-IRRAS

Summary

Introduction

Dipalmitoylphosphatidylcholine (DPPC) is one of the most prevalent phospholipids found in nature and is a common constituent of mammalian cell membranes.[1−4] As a result, it has been widely used in many applications, including eye drops,[5−7] synthetic lung surfactants,[8,9] cosmetics, and liposome solutions for drug delivery.[10,11] It has proved a popular model system for investigating simpler models of biological cell membranes, sometimes as a matrix for the study of membrane proteins, receptors used in sensing, or drug−membrane interactions,[12−15] sometimes to build an understanding at a fundamental level of the behavior of the lipid bilayer that makes up the cell membrane.[16−18] It is increasingly being recognized that the lipid composition of the cell membrane is crucial to function,[19−22] and a range of techniques has been employed to investigate the structures of lipid ensembles, including vesicles, lipid monolayers, stacked lipid bilayers, suspended lipid layers, and supported lipid bilayers.[23,24] For DPPC, data have been collected to determine molecular volume,[25] phase behavior,[26,27] molecule arrangement in monolayers at the air|water interface,[16,28] and thickness of layers in multilayers and monolayers.[29−31] The packing arrangements, numbers of associated water molecules per lipid, and the location of such associated water molecules, in various phases of DPPC, have been characterized,[32] but the influence of static electric field on DPPC has not yet been investigated. One of the roles of a natural membrane is to separate environments within a cell or to separate the intracellular and extracellular environments; in so doing, concentration gradients of ions across the membrane are formed[33] and charge separation occurs. This charge separation, or even the charge asymmetry resulting from the asymmetric distribution of lipids in mammalian plasma membranes, results in strong electric fields of up to 107 V m−1.34 Changes in the field across the membrane can result in changes in structure, for example, the reorientation of dipoles. Some studies have highlighted the role of lipid molecule structure on ensemble structure and response to the applied field;[56,61,62] others have focused on the interaction of peptides and proteins with a lipid bilayer and how charge controls their insertion.[52,55,58−60]

Methods

Results

Conclusion