Sum-frequency vibrational spectroscopy, a tutorial: Applications for the study of lipid membrane structure and dynamics.

This article focuses on the use of sum-frequency vibrational spectroscopy (SFVS) combined with planar supported lipid bilayers (PSLBs) to study lipid membrane structures and dynamics, particularly phospholipid orientation and transmembrane translocation ("lipid flip-flop"). It details the construction of asymmetric PSLBs via the Langmuir–Blodgett/Langmuir–Schaefer (LB/LS) method and explains the SFVS theory underlying measurements at the silica–water interface under total internal reflection. The article presents how SFVS’s sensitivity to membrane asymmetry enables determination of lipid chain and headgroup tilt angles and allows kinetic measurements of flip-flop rates and activation thermodynamics for various phospholipids. Experimental results demonstrate that flip-flop occurs rapidly even in gel-phase membranes, with rates and activation energies dependent on lipid chain length, supporting the biological relevance of native flip-flop processes.