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Differential Stiffness and Lipid Mobility in the Leaflets of Purple Membranes

Kislon Voïtchovsky ^*, Sonia Antoranz Contera ^*, Miya Kamihira , Anthony Watts  and J. F. Ryan ^*

^* Interdisciplinary Research Collaboration in Bionanotechnology, Department of Physics, Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, United Kingdom; and Biomembrane Structure Unit, Department of Biochemistry, University of Oxford, Oxford OX1 3QU, United Kingdom

Correspondence: Address reprint requests to Kislon Voïtchovsky, E-mail: k.voitchovsky1{at}physics.ox.ac.uk.

Purple membranes (PM) are two-dimensional crystals formed by bacteriorhodopsin and a variety of lipids. The lipid composition and density in the cytoplasmic (CP) leaflet differ from those of the extracellular (EC) leaflet. A new way of differentiating the two sides of such asymmetric membranes using the phase signal in alternate contact atomic force microscopy is presented. This method does not require molecular resolution and is applied to study the stiffness and intertrimer lipid mobility in both leaflets of the PM independently over a broad range of pH and salt concentrations. PM stiffens with increasing salt concentration according to two different regimes. At low salt concentration, the membrane Young's normal modulus grows quickly but differentially for the EC and CP leaflets. At higher salt concentration, both leaflets behave similarly and their stiffness converges toward the native environment value. Changes in pH do not affect PM stiffness; however, the crystal assembly is less pronounced at pH 10. Lipid mobility is high in the CP leaflet, especially at low salt concentration, but negligible in the EC leaflet regardless of pH or salt concentration. An independent lipid mobility study by solid-state NMR confirms and quantifies the atomic force microscopy qualitative observations.

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