Probing the steric barrier of nonionic surfactant vesicles with melittin

The role of the surface polymer brush of nonionic surfactant vesicles (NSV) in inhibiting interactions with small membrane-perturbing molecules was investigated using the bee venom peptide melittin as a probe. The interaction between melittin and NSV was compared with that of distearoylphosphatidylcholine (DSPC) vesicles and sterically stabilised liposomes (SSL) containing 5 mol% pegylated distearoylphosphatidylethanolamine (DSPE.E44). The degree of melittin interaction with the various vesicles was determined by measuring peptide binding and folding, using intrinsic tryptophan fluorescence and circular dichroism respectively, in addition to monitoring the release of encapsulated carboxyfluorescein dye. NSV composed of 1,2-di-O-octadecyl-rac-glyceryl-3-(omega-dodecaethylene glycol) (2C18E12) showed a strong affinity for melittin, whilst exhibiting approximately 50% less bound peptide than SSL. 2C18E12:Chol vesicles showed reduced melittin interaction, in a manner consistent with Chol incorporation into DSPC vesicles. These results are discussed with respect to the effect of Chol on the in-plane order of 2C18E12 bilayers and consequent attenuation of hydrophobic interactions with the peptide. NSV formed from equimolar mixtures of polyoxyethylene-n-stearoyl ethers C18E2 and C18E20 showed a greater interaction with melittin than 2C18E12. However, replacing C18E20 with C18E10 was sufficient to achieve an attenuation of melittin interaction similar to that observed in 2C18E12:Chol vesicles. This indicates that the presence of surface polymer brush alone may confer resistance to melittin, provided hydrophobic interactions between the peptide and the vesicles can be minimised, through improved in-plane bilayer order.