Effect of Functional Groups on Incorporation Behavior of Amino Acid-Type Surfactant into Phospholipid Vesicle Membrane

The incorporation behaviors of some N-dodecanoyl amino acid-type surfactants into L-alpha-dipalmitoylphosphatidylcholine (DPPC) vesicles in aqueous solution were investigated. From the leakage measurements of a vesicle-entrapped fluorescence probe, it was found that these surfactants did not affect the DPPC vesicle so much at a very low concentration less than a one-tenth of the critical micelle concentration (CMC), but caused a significant release of the probe form the vesicles even at just below the CIVIC. The leakage induced by amino acid-type surfactants; was promoted by increasing hydrophobicity of the amino acid. However the polarization of DPH embedded in the DPPC membrane was almost unchanged by incorporation of the surfactants; at the concentration below the CMC except for the N-dodecanoylphenylalanine system. The binding enthalpy and the binding constant of the surfactant to DPPC vesicles were estimated by isothermal titration calorimetry. While the binding enthalpy was independent of the kind of surfactant, the binding constant increases with increasing hydrophobicity of the amino acid species in the surfactants. The molar ratio of bound surfactant to DPPC molecule in the saturated state was much greater than that for sodium dodecyl sulfate system. Further the ratio was not influenced by the structure of the amino acid side chain.