Structural Dynamics of a Lytic Peptide Interacting with a Supported Lipid Bilayer

The interaction of a melittin mutant with a 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC)-supported lipid bilayer was studied with the use of time-resolved evanescent wave-induced fluorescence spectroscopy (TREWIFS) and evanescent wave-induced time-resolved fluorescence anisotropy measurements (EW-TRAMs). The mutant peptide was labeled at position K14 with AlexaFluor 430 and retained the lytic activity characteristic of native melittin. The fluorescence decay kinetics of the conjugate was found to be biexponential with a short-lived component, τ1, due to photoinduced electron transfer between AlexaFluor 430 and proximal side chains within or between the peptides. The longer-lived component, τ2, was sensitive to the polarity of the microenvironment at or near the K14 position of the peptide. Upon interaction with a DPPC-supported bilayer, the proportional contribution of τ1 increased, indicating a conformational change of the peptide. The values of τ1 and τ2 indicate that the AlexaFluor 430 probe experienced an environment with an equivalent polarity no less than that of methanol. EW-TRAMs data from the melittin mutant revealed hindered rotational motions of the AlexaFluor 430 probe both in the plane and perpendicular to the plane of the supported lipid bilayer. The data indicate a highly ordered and polar environment near the center of the melittin helix consistent with the formation of a toroidal pore.