The Membrane Affinities of the Aliphatic Amino Acid Side Chains in an α-Helical Context Are Independent of Membrane Immersion Depth

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Abstract
Understanding, predicting, and designing the binding of peptides and proteins to bilayers require quantifying the intrinsic propensities of individual amino acid residues to bind membranes as a function of structural context and bilayer depth. A host−guest study was performed using the peptide host named helix5 in order to determine the membrane affinities of the aliphatic side chains both in an α-helical context and as a function of bilayer depth. Use of the α-helical host with a constrained geometry allowed the placement of guest sites at three different depths in bilayers and minimized secondary structural changes due to guest substitutions. Circular dichroism and electron paramagnetic resonance (EPR) were used to characterize the aqueous and bilayer-bound structures of the peptide variants. EPR was also used to measure the bilayer−water partition constants of the peptide variants, and the ΔΔGtr values (relative to Gly) of the aliphatic amino acid side chains were subsequently calculated. Surprisingly, the ΔΔGtr values did not significantly vary as a function of the guest site depth in bilayers. In addition, the ΔΔGtr values determined in an α-helical context are reduced to approximately two-thirds of ΔΔGtr values determined in other studies for the bilayer−water and octanol−water partitioning of amino acid side chains in extended and unstructured hosts. Both the relative reduction in ΔΔGtr values in the context of an α-helical host and the invariance of ΔΔGtr values with respect to bilayer depth are consistent with the membrane affinities of the aliphatic residues being largely determined by the classical hydrophobic effect.