Web Release Date: November 30,
Study of Gramicidin A-Phospholipid Interactions in Langmuir Monolayers: Analysis of Their Mechanical, Thermodynamical, and Electrical Properties
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Department of Physics, Faculty of Electrical Engineering and Information Technology, Slovak University of Technology, 812 19 Bratislava, Slovak Republic, and the Department of Nuclear Physics and Biophysics, Faculty of Mathematics, Physics and Computer Sciences, Comenius University, 842 48 Bratislava, Slovak Republic
Received: July 19, 2006
In Final Form: October 6, 2006
Abstract:
The mechanisms of interactions between gramicidin A (gA) and dimyristoylphosphatidylcholine (DMPC) in monolayers formed at the air-water interface were studied by analyzing their mechanical, thermodynamical, and electrical properties evaluated from measurements of pressure-area isotherms and of Maxwell displacement currents (MDC). A contactless method of recording MDC enabled us to monitor changes in the charge state of the monolayer-constituting molecules and to find the relation between a phase state of the monolayer and structural transitions of gA. The peptide-lipid interactions were quantified in terms of the excess of Gibbs free energy, excess entropy, as well as the molecular dipole moments at various gA/DMPC molar ratios, at various temperatures (in the gel phase and also in the liquid-crystalline phase of DMPC molecule), and at various surface pressures. It was found that the strongest interactions between gA and DMPC took place at the gA/DMPC molar ratio at around 0.25. At this monolayer composition, the phospholipids, via their carbonyl moieties, dominantly interact with the single helical gA, which mostly stands upright on the surface and is anchored by its C-terminus to the water surface, and prevent the formation of the intertwined helical gA dimers. The optimum ratio was confirmed also by anomalous electrical behavior of electrical dipole moments derived from MDC measurements.
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