Charge-Driven Interaction of Antimicrobial Peptide NK-2 with Phospholipid Membranes

NK-2, derived from a cationic core region of NK-lysin, displays antimicrobial activity toward negatively charged bacterial membranes. We have studied the interaction of NK-2 with various phospholipid membranes, using a variety of experimental techniques, such as, isothermal titration calorimetry (ITC), zeta potential, and dynamic light scattering. As bacteria mimicking membranes, we have chosen large unilamellar vesicles (LUVs) composed of negatively charged phospholipid and neutral phospholipids. ITC and zeta potential results show the stronger binding affinity of NK-2 to negatively charged membranes than to neutral membranes. Saturation of the isotherm, obtained from ITC, at a given lipid to NK-2 ratio, was found to be consistent with the charge compensation, determined from zeta potential. A surface partition model with electrostatic contribution was used to estimate the intrinsic binding constant and other thermodynamical parameters of binding kinetics of NK-2. The size distribution of negatively charged LUV in the presence of NK-2 was found to increase drastically, indicating the presence of large aggregates. Such a large aggregate has not been observed in neutral membranes, which supports the ITC and zeta potential results.