Homooligopeptides composed of hydrophobic amino acid residues interact in a specific manner by taking alpha-helix or beta-structure toward lipid bilayers.

In order to investigate the role of each amino acid residue in determining the secondary structure of the transmembrane segment of membrane proteins in a lipid bilayer, we made a conformational analysis by CD for lipid-soluble homooligopeptides, benzyloxycarbonyl-(Z-)Aaan-OEt (n = 5 - 7), composed of Ala, Leu, Val, and Phe, in three media of trifluoroethanol, sodium dodecyl sulfate micelle, and phospholipid liposomes. The lipid-peptide interaction was also studied through the observation of bilayer phase transition by differential scanning calorimetry (DSC). The CD studies showed that peptides except for Phe oligomers are present as a mainly random structure in trifluoroethanol, as a mixture of alpha-helix, beta-sheet, beta-turn, and/or random in micelles above the critical micellization concentration and preferably as an extended structure of alpha-helical or beta-structure in dipalmitoyl-D,L-alpha-phosphatidylcholine (DPPC) liposomes of gel state. That the beta-structural content of Val oligomers in lipid bilayers is much higher than that in micelles and the oligopeptides of Leu (n = 7) and Ala (n = 6) can take an alpha-helical structure with one to two turns in lipid bilayers despite their short chain lengths indicates that lipid bilayers can stabilize the extended structures of both alpha-helical and beta-structures of the peptides. The DSC study for bilayer phase transition of DPPC/peptide mixtures showed that the Leu oligomer virtually affects neither the temperature nor the enthalpy of the transition, while Val and Ala oligomers slightly reduce the transition enthalpy without altering the transition temperature.(ABSTRACT TRUNCATED AT 250 WORDS)