Effect of ion-binding and chemical phospholipid structure on the nanomechanics of lipid bilayers studied by Force Spectroscopy

¹ University of Barcelona

^* To whom correspondence should be addressed. E-mail: fsanz{at}ub.edu.

Submitted on April 5, 2005
Revised on April 28, 2005
Accepted on 7 June 2005

	Abstract

The nanomechanical response of supported lipid bilayers (SPB) has been studied by Force Spectroscopy with Atomic Force Microscopy (AFM). We have experimentally proved that the amount of ions present in the measuring system has a strong effect on the force needed to puncture a 1,2-Dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayer with an AFM tip, thus highlighting the role that monovalent (so far underestimated),(e.g. Na+) cations play upon membrane stability. The increase in the yield threshold force has been related to the increase in lateral interactions (higher phospholipid-phospholipid interaction, decrease in area/lipid) promoted by ions bound into the membrane. The same tendency has also been observed for other phosphatidylcholine (PC) bilayers, namely, 2-Dilauroyl-sn-glycero-3-Phosphocholine (DLPC), 1,2-Dipalmitoyl-sn-glycero-3-Phosphocholine (DPPC), 1,2-Dioleoyl-sn-3-Phosphocholine (DOPC) and also for phosphatidylethanolamine (PE) bilayers such as 1-Palmitoyl-2-Oleoyl-sn-3-Phosphocholine (POPE). Finally, this effect has been also tested on a natural lipid bilayer (Escherichia coli lipid extract), showing the same overall tendency. The kinetics of the process has also been studied, together with the role of water upon membrane stability and its effect on membrane nanomechanics. Finally, the effect of the chemical structure of the phospholipid molecule on the nanomechanical response of the membrane has also been discussed.

Key Words: Atomic Force Microscopy, ionic strength, membranes, nanoindentation, phosphatidylcholine

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