Direct imaging of rippled structures of lipid–cholesterol membranes using cryo-SEM and AFM

Abstract

Cholesterol is a major component of many eukaryotic cell membranes and hence its effect on the structure of lipid membranes is of considerable interest. One major role of cholesterol is to stabilize the biologically relevant fluid phase over the crystalline gel phase. The suppression of the gel phase occurs above a threshold concentration of cholesterol. There have been reports on the formation of modulated phases at lower cholesterol concentrations, characterized by one-dimensional periodic rippling of the membranes. However, some of these structures have not been investigated in much detail. We have probed the structure of the gel phase of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) membranes containing cholesterol using two direct-imaging techniques, cryogenic-scanning electron microscopy (cryo-SEM) and atomic force microscopy (AFM). In addition to confirming the results of earlier studies, we found evidence for the formation of two novel modulated structures in DPPC–cholesterol membranes. We also report the first direct observation of the cholesterol-induced modulated phase of DPPC membranes using AFM, which had so far been observed only using X-ray scattering.