Incorporation of pepsin within zwitterionic, anionic, and cationic lipid monolayers: A comparative study

The incorporation of water-soluble surface-active enzyme pepsin (PEP) within insoluble zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and anionic stearic acid (SA) monolayer is studied. Furthermore, the results are compared with cationic octadecylamine (ODA). Adsorption of PEP is found to be higher in ODA as compared to that of DPPC and SA. PEP adsorption kinetics in lipids monolayer follows two-step process: diffusion and unfolding. The unfolding of PEP is lower in the case of DPPC than in SA and ODA. pi-A isotherm together with high-resolution field emission scanning electron microscope (FE-SEM) images indicate that at higher pressure, PEP molecules tend to squeeze out from the lipids monolayer. PEP forms larger irregular intermolecular aggregates by increment of beta-component on SA and ODA monolayer. However, in zwitterionic (DPPC) matrix the a-helix increases with smaller intermolecular aggregates. The overall results specify that zwitterionic (DPPC) monolayer is better choice to obtain protein lipid mixed film than anionic (SA) and cationic (ODA) monolayer.