Dynamic Imaging of Membrane Hydration

Abstract

Lipid membranes are essential for life: They provide a dynamic compartmentalized environment, a localized space for proteins (e.g. ion channels) to perform their functions, and a selective environment to decide what enters the cell. Although recognized as an essential building block, water is usually treated as a background for biology; mainly because it cannot be measured on the molecular interfacial level of realistic free floating membrane systems. However, water is a crucial mediator of chemical change and determines the structure of the membrane. Water and ions determine the electrostatic environment of the membrane, whether interfacial reactions take place and provide a path for the insertion and functioning of ion channels/pores. The study of lipid membranes is generally pursued by following either a top-down approach, introducing labels to living cell membranes or a bottom-up approach with well-controlled but simplified membrane monolayer or supported membrane models. In the first approach molecular level hydration information is lost, while in the second approach the connection with real bilayer membranes is limited. Recent work in in our laboratory offers an alternative path that ultimately envisions bringing together both top-down and bottom-up approaches. By using intermediate nano-, micro- and macroscale free-floating membrane systems in combination with novel nonlinear optical spectroscopy and imaging methods, we advance the understanding of realistic membranes on a more fundamental level. In this presentation I will introduce those methods and provide an overview of recent advances in understanding membrane molecular structure, hydration, and electrostatics. We demonstrate the failure of electrostatic mean field models, the emerging roles of variable length scale, curvature, confinement and transient ordered water domains by experiments on lipid droplets, liposomes, macroscopic free floating membranes and living neurons.