In Situ and Real-Time SFG Measurements Revealing Organization and Transport of Cholesterol Analogue 6-Ketocholestanol in a Cell Membrane.
Cholesterol organization and transport within a cell membrane are essential for human health and many cellular functions yet remain elusive so far. Using cholesterol analogue 6-ketocholestanol (6-KC) as a model, we have successfully exploited sum frequency generation vibrational spectroscopy (SFG-VS) to track the organization and transport of cholesterol in a membrane by combining achiral-sensitive ssp (ppp) and chiral-sensitive psp polarization measurements. It is found that 6-KC molecules are aligned at the outer leaflet of the DMPC lipid bilayer with a tilt angle of about 10°. 6-KC organizes itself by forming an α-β structure at low 6-KC concentration and most likely a β-β structure at high 6-KC concentration. Among all proposed models, our results favor the so-called umbrella model with formation of a 6-KC cluster. Moreover, we have found that the long anticipated flip-flop motion of 6-KC in the membrane takes time to occur, at least much longer than previously thought. All of these interesting findings indicate that it is critical to explore in situ, real-time, and label-free methodologies to obtain a precise molecular description of cholesterol's behavior in membranes. This study represents the first application of SFG to reveal the cholesterol-lipid interaction mechanism at the molecular level.