Thursday, October 16, 2008, Condensed Matter Biological Physics Seminar, Jian-Jun Pan, Dept. of Physics, Carnegie Mellon Univ., "The Superstructure of an Antimicrobial Peptide, Alamethicin, in Lipid Membranes", 4:30 PM, Wean 7316*, CMU (*Note the special location)

Abstract

Alamethicin (Alm) is one of the most extensively studied antimicrobial peptides that affect the plasma membrane, and several models that involve a channel structure formed by monomer aggregation have been proposed. The Alm channel structure has been observed experimentally by x-ray and neutron in-plane scattering, and channel size has been estimated. The barrel stave model suggests that there are 6-8 Alm monomers per channel.

In this work we investigate the effect of membrane hydration and hydrophobic mismatch on the Alm channel superstructure in an oriented multilayer sample by x-ray scattering. Wide angle x-ray (WAXS) scattering near 1.4 inverse Angstroms indicates that the lipid chain region is not much perturbed by the incorporation of up to 10 mole percent Alm. Low angle x-ray scattering (LAXS) indicates that when the sample is very dry, which promotes interactions between neighboring bilayers, a body centered tetragonal crystal packing of Alm channels is formed. As the hydration level increases closer to biological conditions, the separation between bilayers increases, the interbilayer interactions weaken, and the crystalline order disappears while considerable diffuse scattering remains. The effect of hydrophobic mismatch is examined for two mono-unsaturated lipids, diC18:1PC and diC22:1PC, that differ in bilayer thickness by 7.3 Angstroms. There is also additional in-plane scattering at a medium q of 0.7 inverse Angstroms that our analysis suggests may not be from the Alm channel structure.