Studies of the Minimum Hydrophobicity of α-Helical Peptides Required To Maintain a Stable Transmembrane Association with Phospholipid Bilayer Membranes

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Abstract
The effects of the hydrophobicity and the distribution of hydrophobic residues on the surfaces of some designed α-helical transmembrane peptides (acetyl-K2-Lm-An-K2-amide, where m + n = 24) on their solution behavior and interactions with phospholipids were examined. We find that although these peptides exhibit strong α-helix forming propensities in water, membrane-mimetic media, and lipid model membranes, the stability of the helices decreases as the Leu content decreases. Also, their binding to reversed phase high-performance liquid chromatography columns is largely determined by their hydrophobicity and generally decreases with decreases in the Leu/Ala ratio. However, the retention of these peptides by such columns is also affected by the distribution of hydrophobic residues on their helical surfaces, being further enhanced when peptide helical hydrophobic moments are increased by clustering hydrophobic residues on one side of the helix. This clustering of hydrophobic residues also increases peptide propensity for self-aggregation in aqueous media and enhances partitioning of the peptide into lipid bilayer membranes. We also find that the peptides LA3LA2 [acetyl-K2-(LAAALAA)3LAA-K2-amide] and particularly LA6 [acetyl-K2-(LAAAAAA)3LAA-K2-amide] associate less strongly with and perturb the thermotropic phase behavior of phosphatidylcholine bilayers much less than peptides with higher L/A ratios. These results are consistent with free energies calculated for the partitioning of these peptides between water and phospholipid bilayers, which suggest that LA3LA2 has an equal tendency to partition into water and into the hydrophobic core of phospholipid model membranes, whereas LA6 should strongly prefer the aqueous phase. We conclude that for α-helical peptides of this type, Leu/Ala ratios of greater than 7/17 are required for stable transmembrane associations with phospholipid bilayers.

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