Orientation and aggregation of hydrophobic helical peptides in phospholipid bilayer membrane

Hydrophobic sequential peptides with various chain lengths, Boc-(Ala-Aib)n-OMe (n = 2, 4, 6, 8) and Boc-Ser(CH2Ant)-(Ala-Aib)n-OMe (n = 2, 4, 6, 8, 10, Ant represents O-anthrylmethyl; abbreviated as A2-A10), were synthesized and their orientation and aggregation in a lipid bilayer membrane were investigated. Circular dichroism (CD) measurements revealed that the peptides took a partially helical structure, and that the helix content increased with increasing chain length and upon distribution to phospholipid vesicles. When long-chain peptides, A8 and A10, were incorporated into lipid bilayer membranes, the membrane fluidity was reduced, while 5/6-carboxyfluorescein (CF) leakage through the bilayer membranes was enhanced. Fluorescence quenching of the anthracene group with 12-doxylstearic acid suggested that these peptides took a perpendicular orientation in the membrane. Detection of excimer emission and large fluorescence depolarization of the peptides indicated an aggregation in the membrane. In addition, Boc-(Ala-Aib)n-OMe (n = 4, 8) showed a channel-like activity in a bilayer lipid membrane (BLM). The channel-forming ability of the hexadecapeptide was higher than that of the octapeptide. Taken together, the long-chain hydrophobic helical peptides tend to aggregate in lipid bilayer membranes with a transmembrane orientation.