Experimental evidence for hydrophobic matching and membrane-mediated interactions in lipid bilayers containing gramicidin

Hydrophobic matching, in which transmembrane proteins cause the surrounding lipid bilayer to adjust its hydrocarbon thickness to match the length of the hydrophobic surface of the protein, is a commonly accepted idea n membrane biophysics. To test this idea, gramicidin (gD) was embedded in 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) and 1,2-myristoyl-sn- glycero-3-phosphocholine (DMPC) bilayers at the peptide/lipid molar ratio of 1:10. Circular dichroism (CD) was measured to ensure that the gramicidin was in the β^6.3 helix form. The bilayer thickness (the phosphate-to- phosphate distance, or PtP) was measured by x-ray lamellar diffraction. In the L(α) phase near full hydration; PtP is 30.8 Å for pure DLPC, 32.1 Å for the DLPC/gD mixture, 35.3 Å for pure DMPC, and 32.7 Å for the DMPC/gD mixture. Gramicidin apparently stretches DLPC and thins DMPC toward a common thickness as expected by hydrophobic matching. Concurrently, gramicidin- gramicidin correlations were measured by x-ray in-plane scattering. In the fluid phase, the gramicidin-gramicidin nearest-neighbor separation is 26.8 ≡ in DLPC, but shortens to 23.3 Å in DMPC. These experiments confirm the conjecture that when proteins are embedded in a membrane, hydrophobic matching creates a strain field in the lipid bilayer that in turn gives rise to a membrane-mediated attractive potential between proteins.