Interfacial Choline-Aromatic Cation-Pi Interactions Can Contribute as Much to Peripheral Protein Affinity for Membranes as Aromatics Inserted Below the Phosphates

Membrane binding interfaces of peripheral proteins are restricted to a small part of their exposed surface so the ability to engage in strong selective interactions with membrane lipids at various depths in the interface, both below and above the phosphates, is an advantage. Driven by their hydrophobicity aromatic amino acids preferentially partition into membrane interfaces often below the phosphates. Yet enthalpically favorable interactions with the lipid headgroups, above the phosphate plane, are likely to further stabilize high interfacial positions. Using Free Energy Perturbation we calculate the energetic cost of alanine substitution for 11 interfacial aromatic amino acids from 3 peripheral proteins. We show that involvement in cation-π interactions with the headgroups (i) increases the ΔΔG_transfer as compared to insertion at the same depth without cation-π stabilization and (ii) can contribute at least as much as deeper insertion below the phosphates, highlighting the multiple roles of aromatics in peripheral membrane protein affinity.