The orientation effect of gramicidin A on bicelles and Eu3+-doped bicelles as studied by solid-state NMR and FT-IR spectroscopy

We have explored the effect of gramicidin A (gA) on bicelle (Bic) orientation in the absence and presence of Eu3+ by 31P and 2H NMR at different DMPC/gA ratios. FT-IR spectroscopy was used to assess the lipid chain ordering and verify the transmembrane peptide conformation. Our results show a time-dependent flipping of the bilayer normal alignment at high temperatures and high proportion of gA. The results are explained by both the diamagnetic susceptibility anisotropy of the β6.3 helical peptides and viscosity of the lipid mixture. The concentration effect of gramicidin on Bic/Eu3+ is compared to that on Eu3+-doped DMPC liposomes. The Bic/Eu3+ system is no longer oriented in the presence of gA and adopts a vesicular morphology while the peptide incorporation induces the formation of ellipsoidal DMPC/Eu3+ assemblies aligned with their normal parallel to the magnetic field. The difference is explained in terms of lipid chain disorder and size of the bilayers.