Binding of cationic peptides (KX)4K to DPPG bilayers. Increasing the hydrophobicity of the uncharged amino acid X drives formation of membrane bound β-sheets: A DSC and FT-IR study

The binding of cationic peptides of the sequence (KX)(4)K to lipid vesicles of negatively charged dipalmitoylphosphatidylglycerol (DPPG) was investigated by differential scanning calorimetry (DSC) and temperature dependent Fourier-transformed infrared (FT-IR) spectroscopy. The hydrophobicity of the uncharged amino acid X was changed from G (glycine) over A (alanine), Abu (alpha-aminobutyric acid), V (valine) to L (leucine). The binding of the peptides caused an increase of the phase transition temperature (T-m) of DPPG by up to 20 degrees C. The shift depended on the charge ratio and on the hydrophobicity of the amino acid X. Unexpectedly, the upward shift of T-m increased with increasing hydrophobicity of X. FT-IR spectroscopy showed a shift of the CH2 stretching vibrations of DPPG to lower frequency, particularly for bilayers in the liquid-crystalline phase, indicating an ordering of the hydrocarbon chains when the peptides were bound. Changes in the lipid C=0 vibrational band indicated a dehydration of the lipid headgroup region after peptide binding. (KG)(4)K was bound in an unordered structure at all temperatures. All other peptides formed intermolecular antiparallel beta-sheets, when bound to gel phase DPPG. However, for (KA)(4)K and (KAbu)(4)K, the beta-sheets converted into an unordered structure above T-m. In contrast, the beta-sheet structures of (KV)(4)K and (KL)(4)K remained stable even at 80 degrees C when bound to the liquid-crystalline phase of DPPG. Strong aggregation of DPPG vesicles occurred after peptide binding. For the aggregates, we suggest a structure, where aggregated single beta-sheets are sandwiched between opposing DPPG bilayers with a dehydrated interfacial region. (C) 2016 Elsevier B.V. All rights reserved.