The importance of biological membranes for regulating transport of molecules in and out of cells or organelles is well-recognized. To understand how membranes achieve this we need to look at how molecules that constitute the membranes are organized. This has been a notoriously difficult task. Vibrational sum-frequency scattering (VSFS) is a non-linear optical spectroscopic technique that promises to overcome this problem. The underlying process of VSFS is sum-frequency generation, a second-order optical nonlinearity, which makes VSFS sensitive to breaking of inversion symmetry. We demonstrate how this can be elegantly used to probe transmembrane asymmetry in liposomes.
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