Cholesterol Rich Domains Identified in Unilamellar Supported Biomimetic Membranes via Nano-Viscosity Measurements

Abstract

Understanding the distribution of cholesterol in phospholipid membranes is of key importance in membrane biophysics, primarily since cholesterol enriched regions, rafts, are known to play a special role in protein function. In this work, quartz crystal microbalance with dissipation (QCM)-based viscosity measurements were used to study cholesterol-induced domain formation in partially suspended single bilayer membranes. 1,2-Dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and its mixtures with different amounts of cholesterol were studied. QCM temperature ramping experiments identified domains of different phase transition temperatures in the mixed membranes. The phase transition of DMPC shifted from 23.4 °C toward lower temperatures with increasing cholesterol content. A second, continuous but much broader, transition peak has been observed for the DMPC: cholesterol mixtures suggest that a separate cholesterol rich domain coexists with the DMPC rich domain. Importantly, the sharp DMC phase transition peak gradually diminished and eventually disappeared over 15% cholesterol content, suggesting that the cholesterol rich domain has a definite stoichiometry and once this cholesterol concentration is reached the DMPC-rich domain disappears. DSC control experiments do not show the second domain, suggesting that the phase separation only happens in nontensioned (flat) membranes.