Abstract
The effect of molecular structure on ensemble structure and dynamics of phospholipid bilayers has been investigated. Bilayers of dimyristoyl phosphatidylserine (DMPS) supported on Au(111) surfaces were prepared by Langmuir-Blodgett and Langmuir-Schaeffer deposition and studied with a combination of electrochemical measurements and in situ Polarisation Modulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS). DMPS bilayers have relatively large capacitance when compared with those formed from similar molecules and this is attributed to a high solvent content within the bilayer, resulting from the need for solvation of the negatively charged lipid headgroups. Infrared spectra show that the ensemble of molecules is in a gel state, with extended and ordered hydrocarbon chains, similarly to bilayers of dimyristoyl phosphatidylethanolamine (DMPE) molecules, which are of similar shape. The infrared spectra also show that, in contrast to DMPE, the headgroups of DMPS are very strongly hydrated and have higher mobility. This higher mobility allows the re-orientation of the molecules under the influence of an applied electric field: re-orientation both of headgroups and hydrocarbon tail groups is observed. Thus the shape and charge of the molecules in an ensemble have a strong influence on both their structure and dynamics in the presence of an externally applied electric field.
Highlights
Phospholipids are a major component of biological cell membranes, self-assembling to form a fluid, selective barrier between the intracellular and extracellular fluids
dimyristoyl phosphatidylserine (DMPS) exhibits minimum capacity at the most positive potentials of 9–10 F cm−2. This capacity is higher than that reported for the similar molecules DMPC (7–8 F cm−2) [28] and dimyristoyl phosphatidylethanolamine (DMPE) (2–3 F cm−2) [54] and for that of bilayers of 9:1 DMPE:DMPS [44]
DMPS has a smaller headgroup than DMPC and might be expected to be oriented with its hydrocarbon chains tilted at an angle closer to the surface normal, which should result in a slightly thicker bilayer with lower capacities than observed for DMPC
Summary
Phospholipids are a major component of biological cell membranes, self-assembling to form a fluid, selective barrier between the intracellular and extracellular fluids. In situ infrared spectroscopic measurements have provided details of molecular orientation and packing as a function of applied field [26,27,28,29], scanning probe microscopies have provided information on molecular adsorption and arrangement [39,41,42] and in situ neutron reflectivity measurements have been used to determine quantitatively the degree of solvent ingress into the membranes as the applied field is varied [43,44] Taken together, this information can be used to build up a detailed picture of how the structure of a lipid film affects its properties and the mechanism by which the film desorbs from the surface. The presence of the charge on the molecules leads to a difference in solvation, which has an impact on the electrochemical properties of the film and on the degree to which an applied electric field can cause changes in molecular organisation
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