Hydrophobic mismatch in gramicidin A'/lecithin systems

Gramicidin A' (GA') has been added to three lipid systems of varying hydrophobic thicknesses: dimyristoyllecithin (DML), dipalmitoyllecithin (DPL), and distearoyllecithin (DSL). The similarity in length between the hydrophobic portion of GA' and the hydrocarbon chains of the lipid bilayers has been studied by using 31P and 2H NMR. Hydrophobic mismatch has been found to be most severe in the DML bilayer system and minimal in the case of DSL. In addition, the effects of hydrophobic mismatch on the cooperative properties of the bilayer have been obtained from 2H NMR relaxation measurements. The results indicate that incorporation of the peptide into the bilayer disrupts the cooperative director fluctuations characteristic of pure multilamellar lipid dispersions. Finally, the GA'/lecithin ratio at which the well-known transformation from bilayer to reverse hexagonal (HII) phase occurs (Van Echteld et al., 1982; Chupin et al., 1987) is shown to depend on the acyl chain length of the phospholipid. A rationale is proposed for this chain length dependence.