Binding of cationic model peptides (KX)K to anionic lipid bilayers: Lipid headgroup size influences secondary structure of bound peptides.

Differential Scanning Calorimetry (DSC) and Fourier transformed Infrared (FT-IR) spectroscopy were used to test the influence of acyl chain length, acyl chain saturation, and chemical structure of anionic phospholipids on the interaction with cationic model peptides (KX)K, with amino acid X=A, Abu, and L. The lipids used were phosphatidylserine (PS), phosphatidylglycerol (PG), phosphatidic acid (PA), and cardiolipin (CL). DSC was used to monitor the phase transition of lipid vesicles before and after peptide binding. The electrostatic attraction is the main driving force for binding. The hydrophobicity of the amino acid X influences the binding strength as well as the secondary structure of the bound peptide. Binding of peptides leads to an upshift of the lipid phase transition. Lipids with smaller headgroups show a larger upshift of the main phase transition temperature. Data from FT-IR spectroscopy show in addition that the stability of the bound β-sheets of (KX)K depends on the hydrophobicity of the uncharged amino acid X and on the size of the lipid headgroup. For lipids with large anionic headgroups, such as PS, the antiparallel β-sheet of (KAbu)K bound to gel phase bilayers is converted to an unordered structure upon heating through the lipid phase transition. Reducing the size of the headgroup, as in PG, increases the stability of the bound peptide β-sheets. For the smallest headgroups, present in PA and CL, stably bound β-sheets are observed even above the lipid phase transition.