Hydrophobic matching and membrane mediated interactions in lipid bilayers
Harroun, Thad Alan
Huang, Huey W.
Doctor of Philosophy thesis
Hydrophobic matching, in which transmembrane proteins cause the surrounding lipid bilayer to adjust its thickness to match the hydrophobic surface of the protein, is a commonly accepted idea in biophysics, but one that until now has not been experimentally tested. One important consequence is that protein interactions will be mediated by the energy cost of deforming the membrane from its protein free state. With X-ray scattering techniques we tested these ideas with the peptide gramicidin embedded in DLPC and DMPC bilayers. Gramicidin pushes the different membranes to a common thickness as expected from hydrophobic matching. Concurrently, gramicidin-gramicidin nearest neighbor distance decreases with increasing mismatch, which confirms that the strain in the lipid bilayer gives rise to an attractive potential between the proteins. We have taken a continuum theory approach to the analysis of the experimental results. This approach treats the energetics of membrane-protein interactions as a function of the material properties of the membrane such as bending rigidity and compressibility. Using numerical methods and a novel simulation technique, we have successfully demonstrated the theoretical relationship between membrane thickness change and protein correlation. By quantitatively reproducing our experimental results, we have shown that the theory of membrane deformation is sufficient to explain the phenomena of hydrophobic matching. We also include a study on the peptide melittin as an example of the type of protein-lipid system we want to understand better. We answer the question of the orientation of the peptide when making membrane pores.
Physical chemistry; Biophysics