Membranes prepared by the adsorption of phospholipid vesicles on solid supports are much-used model systems in biomedical research. However, there is accumulating evidence that such membranes may not always be equivalent to the free-standing cellular membranes that they are modeling. In the present study, sonicated DOPC/DOPS (80/20 mol %) vesicles were adsorbed on hydrophilic silica surfaces, a system that has been demonstrated to produce confluent bilayers. In addition, pure DOPC and DLPC membranes were studied. It is demonstrated that ethanol-induced membrane interdigitation, as demonstrated for free-standing bilayers, does not occur in these supported membranes.
The present study has two main objectives. The first is to characterize antimicrobial peptide (AMP) cryptdin-4 (Crp-4) interactions with biological membranes and to compare those interactions with those of magainin 2. The second is to combine the complementary experimental approaches of laser scanning microscopy (LSM), ellipsometry, and Z-scan fluorescence correlation spectroscopy (FCS) to acquire comprehensive information on mechanisms of AMP interactions with supported phospholipid bilayers (SPBs)-a popular model of biological membranes. LSM shows appearance of inhomogeneities in spatial distribution of lipids in the bilayer after treatment with Crp-4. Ellipsometric measurements show that binding of Crp-4 does not significantly change the lipid structure of the bilayer (increase in adsorbed mass without a change in thickness of adsorbed layer). Furthermore, Crp-4 slows the lateral diffusion of lipids within the membrane as shown by Z-scan FCS. All changes of the bilayer induced by Crp-4 can be partially reversed by flushing the sample with excess of buffer. Bilayer interactions of magainin 2 are significantly different, causing large loss of lipids and extensive damage to the bilayer. It seems likely that differences in peptide mode of action, readily distinguished using these combined experimental methods, are related to the distinctive beta-sheet and alpha-helical structures of the respective peptides.
