Large Scale Membrane Movement Induced by a Cation Switch

A biomembrane sample system where millimolar changes of cations induce reversible large scale (≥ 200 Å) changes in the membrane-to-surface distance is described. The system composes of a free-floating bilayer (FFB), formed adjacent to a self-assembled monolayer (SAM). To examine the membrane movements, differently charged FFBs in the presence and absence of Ca²⁺ and Na⁺, respectively, were examined using neutron reflectivity (NR) and quartz crystal microbalance (QCM) measurements, alongside molecular dynamics (MD) simulations. In NR the variation of Ca²⁺ and Na⁺ concentration enabled precision manipulation of the FFB-to-surface distance. Simulations suggest that Ca²⁺ ions bridge between SAM and bilayer whereas the more diffuse binding of Na⁺, especially to bilayers, is unable to fully overcome the repulsion between anionic FFB and anionic SAM. Reproduced NR results with QCM demonstrate the potential of this easily producible sample system to become a standard analysis tool for e.g. investigating membrane binding effects, endocytosis and cell signalling.