Abstract
A synthetic charged lipid, DC-Chol (3β-[N-(dimethylaminoethane)-carbamoyl]-cholesterol), was used to mix with the DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) to form monolayers at the air-liquid interface. The mixed DPPC/DC-Chol monolayer at the air-water interface was studied by measuring the surface pressure versus molecular area isotherms and BAM. The pressure-area isotherm shows that typical plateau of DPPC disappears by adding DC-Chol more than 30%. It was also found that the DPPC/DC-Chol system has only one minimum excess area point at X DC-Chol = 0.2. The absence of the minimum excess area point at higher DC-Chol mixing ratio should be due to the charge interaction of DC-Chol. It was observed by BAM that the domains formed in the plateau region of the isotherm become smaller by adding small amounts of DC-Chol. These domains as well as the plateau region disappear at X DC-Chol ≧ 0.3. The characteristics of the mixed monolayer seems to be dominated by the charged DC-Chol at X DC-Chol ≧ 0.3. The results of X-ray reflectivity measurements of the mixed DPPC-d62/DC-Chol LB monolayer supported on silicon wafer show that the DC-Chol molecules are positioned in the DPPC-d62 hydrophobic layer. © 2006 Elsevier B.V. All rights reserved.