Browsing by Author "Harroun, Thad Alan"
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Item Hydrophobic matching and membrane mediated interactions in lipid bilayers(2000) Harroun, Thad Alan; Huang, Huey W.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.Item Membrane-mediated peptide-peptide interaction: Gramicidin in-plane distribution by x-ray scattering(1997) Harroun, Thad AlanWhile there has been much speculation on the role the lipid matrix plays in controlling and mediating the inter-molecular dynamics of proteins, there has been little direct evidence for how the lipid bilayer can mediate the supra-molecular organization of these proteins. With the technique of X-ray in-plane scattering, we can directly measure the correlation function of gramicidin channels in the plane of the bilayer. We have studied the simple case of gramicidin incorporated into model Dilauroyl and Dimyristoyl-Phosphocholine (DL-,DMPC) membranes, and have seen how changing the membrane thickness changes peptide-peptide correlation. In the fluid phase, DLPC and DMPC present an elastic medium that can perhaps match the hydrophobic length of the peptide. The resulting membrane deformation energy is expected to cause inter-peptide attraction. Indeed, the scattering data shows that the gramicidin-gramicidin correlation distance is smaller in DMPC than in DLPC. Thus we have obtained direct evidence of membrane mediated peptide-peptide interaction. As the temperature drops and DMPC approaches its main transition temperature, gramicidin exhibits close packing. This may indicate that the channels are being excluded from gel lipid regions and are packing closer together.