Effect of Cations on the Nanomechanical Response of Phospholipid Model Membranes. A Force Spectroscopy Study

Abstract

How do metal cations affect the stability and structure of phospholipid bilayers? Which role does ion binding play in the insertion of proteins and in the overall mechanical stability of biological membranes? To characterize such effects, several theoretical and microscopic approaches have been proposed in the past to study the mechanical properties of lipid bilayers. While providing crucial information, molecular dynamics simulations can not completely deal with the extraordinary complexity of biological membranes. Experimental techniques also have problems when it comes to test ion binding to lipid bilayers in an accurate way. Hence, a new perspective from the nanometric scale [1,2], where most of the specific molecular phenomena are sensitive, was introduced being Atomic Force Spectroscopy an essential tool to examine the lipid bilayers structure and behaviour. So, we used Force Spectroscopy to quantitatively characterize the nanomechanical resistance as a function of the electrolyte concentration and composition thanks to a reliable molecular fingerprint that reveals itself as a repetitive jump in the approaching force curve. By systematically testing two model membranes, DPPC and DPPE, immersed in an electrolyte containing a series of either one monovalent (Li+ to Cs+) or divalent cation (Mg2+ and Ca2+) we provide a wealth of information which unambiguously proves an independent contribution of each ion to the gross mechanical resistance, reporting quantitative measurements for the membrane elastic modulus and also for its plastic properties. So, this work deals with the need of assessing the effect of different cations in the structure of phospholipid membranes.[1] Effect of ion-binding and chemical phospholipid structure on the nanomechanics of lipid bilayers studied by force spectroscopy, Biophys J 89 (2005) 1812-1826[2] Nanomechanics of lipid bilayers:heads or tails?, under review (2009)