AFM observations of (DMPC/cholesterol) mixed monolayer on aqueous solution of Vibrio cholerae hemolysin

The aggregation structure of monolayer, consisting of phospholipid and cholesterol, on a pure water and an aqueous solution of Vibrio cholerae hemolysin (VCH) was investigated on the basis of surface pressure–area (π–A) isotherm measurements and atomic force microscopic observations. The behavior of π–A isotherm by addition of VCH revealed changes in the monolayer state due to interaction with VCH. Small dots with a diameter of about 10 nm, assigned to VCH subunits, were observed in the matrix region of the mixed monolayer but not in the domain. This result implies that VCH penetrates into the mixed monolayer, suggesting that a stronger aggregation between cholesterols prevents to its penetration into cholesterol-rich phase (domain region).