Structural characterization of phosphatidylcholine lipid monolayers across the liquid-expanded / liquid-condensed phase transition

Lipid layers are the foundational element of biological membranes and can exhibit heterogeneous structural ordering that impact membrane function. However, important thermodynamic aspects of transitions between fluid and ordered lipid phases are still not fully understood. Using state-of-the-art grazing-incidence X-ray diffraction, we are able to re-assess the structural transition between the fluid, liquid-expanded (LE) phase and the chain-crystalline, liquid condensed (LC) phase of a Langmuir monolayer of a saturated double-chain phosphatidylcholine (1,2-dipentadecanoyl-sn-glycero-3-phosphocholine, diC15PC) at the air-water interface, induced by lateral compression. While the sharp diffraction peaks characteristic for the LC phase are seen at phase coexistence but not in the LE phase, the broad peak indicative of LE-like short-range correlations persist throughout the entire transition and beyond. The monolayer's structural parameters are found to depend on the transition progress. These observations indicate that lateral compression at typical speeds is not quasi-static and thereby shed some light on the non-horizontality of transition plateau in pressure-area isotherms.