Dr. Alexander Fellows, Fritz-Haber-Institut Berlin

Structural anisotropy is arguably the most important factor governing the functionality of lipid membranes, with the orientational alignment restricting the transport of molecules. Beyond anisotropy, however, heterogeneity is crucial to their physiological behaviour. For example, localised ‘lipid rafts’ are implicated in several signalling functionalities, trans-membrane proteins form switchable ion channels, and even the two membrane leaflets present pronounced compositional asymmetry that provides specific inter-cellular communicative capabilities and allosteric interactions. Understanding the wider behaviour of the membrane thus requires a molecular-to-mesoscopic picture of the structure, but obtaining this is a formidable challenge.

Here, we introduce sum-frequency generation (SFG) microscopy as a method to resolve the heterogeneous structure in lipid membranes. By combining a resonant IR excitation with an off-resonant visible laser, we obtain wide-field images of the resulting SFG signals, which, due to their selection rules, are highly sensitive to the specific packing structure and orientational ordering within the membrane. Through polarisation- and azimuth-dependent imaging, we can map the different components of the 3D molecular structure and gain unprecedented access to the orientational distribution.

The capabilities of SFG microscopy are demonstrated using model membrane systems consisting of phase-separated lipid monolayers, wherein we extract images of the molecular composition, tail-group tilt angles, and the in-plane chiral packing structure. Going forward, we aim to take the capabilities of this microscope to more complex systems and even live cell imaging to better understand the local lipid environment in the membrane and how this connects to wider membrane functionality.