Combined mass and structural kinetic analysis of multistate antimicrobial peptide-membrane interactions.

Kinetic analysis of peptide-membrane interactions generally involves a curve fitting process with no information about what the different curves may physically correspond to. Given the multistep process of peptide-membrane interactions, a computational method that utilizes physical parameters that relate to both peptide binding and membrane structure would provide new insight into this complex process. In this study, kinetic models accounting for two-state and three-state mechanisms were fitted to our previously reported simultaneous real-time measurements of mass and birefringence during the binding and dissociation of the peptide HPA3 (Hirst, D.; Lee, T.-H.; Swann, M.; Unabia, S.; Park, Y.; Hahm, K.-S.; Aguilar, M. Eur. Biophys. J. 2011, 40, 503-514); significantly, the mass and birefringence are constrained by the same set of kinetic constants, allowing the unification of peptide binding patterns with membrane structure changes. For the saturated phospholipid dimyristoyl-phosphatidylcholine (DMPC) the two-state model was sufficient to account for the observed changes in mass and birefringence, whereas for the unsaturated phospholipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) the two-state model was found to be inadequate and a three-state model gave a significantly better fit. The third state of interaction for POPC was found to disrupt the bilayer much more than the previous two states. We propose a hypothesis for the mechanism of membrane permeabilization based on the results featuring a loosely bound first state, a tightly bound second state, and a highly membrane-disrupting third state. The results demonstrate the importance of the difference in membrane fluidity between the gel phase DMPC and the liquid crystal phase POPC for peptide-membrane interactions and establish the combination of DPI and kinetic modeling as a powerful tool for revealing features of peptide-membrane interaction mechanisms, including intermediate states between initial binding and full membrane disruption.