A Molecular View of Cholesterol-Induced Condensation in a Lipid Monolayer

We investigate the intermolecular interactions in a mixed phospholipid/cholesterol monolayer using the surface-specific technique of vibrational sum frequency generation. This technique allows us to monitor the conformational and orientational order in the phospholipid as the surface pressure and cholesterol content are varied. We find a disproportionally large increase in phospholipid molecular order upon addition of relatively small amounts of cholesterol, a witness to the condensation effect at the molecular level.