Comparative analysis of the electrostatics of the binding of cationic proteins to vesicles: asymmetric location of anionic phospholipids.

The role of electrostatics is studied in the adsorption of cationic proteins to zwitterionic phosphatidylcholine (PC) and anionic PC/phosphatidylglycerol (PG) mixed small unilamellar vesicles (SUVs). For model proteins the interaction is monitored vs. PG content at low ionic strength. The adsorption of lysozyme and myoglobin (isoelectric point, pI 7-11) is investigated in SUVs, along with changes of the fluorescence emission spectra of the cationic proteins, via their adsorption on SUVs. In the Gouy-Chapman formalism, the activity coefficient goes with the square of charge number. Deviations from the ideal model could indicate the asymmetric location of the anionic phospholipid in the bilayer inner leaflet, in mixed zwitterionic/anionic SUVs for both lysozyme- and myoglobin-PC/PG systems, in agreement with experiments and molecular dynamics simulations. Fitted effective SUV charge stays constant. Effective-formal difference increases 0.417 e.u. Effective protein charge increases as PC/PG<PC being greater for myoglobin. The molar free energies of the protein in aqueous and lipid phases increase as PC<PC/PG. Both free-energy changes are greater for myoglobin. Effective interfacial charge stays constant for anionic PC/PG SUVs being greater for myoglobin.