Modulation of concentration fluctuations in phase-separated lipid membranes by polypeptide insertion.

The lateral membrane organization and phase behavior of the binary lipid mixture DMPC (1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine) - DSPC (1,2-distearoyl-sn-glycero-3-phosphatidylcholine) without and with incorporated gramicidin D (GD) as a model biomembrane polypeptide was studied by small-angle neutron scattering, Fourier-transform infrared spectroscopy, and by two-photon excitation fluorescence microscopy on giant unilamellar vesicles. The small-angle neutron scattering method allows the detection of concentration fluctuations in the range from 1 to 200 nm. Fluorescence microscopy was used for direct visualization of the lateral lipid organization and domain shapes on a micrometer length scale including information of the lipid phase state. In the fluid-gel coexistence region of the pure binary lipid system, large-scale concentration fluctuations appear. Infrared spectral parameters were used to determine the peptide conformation adopted in the different lipid phases. The data show that the structure of the temperature-dependent lipid phases is significantly altered by the insertion of 2 to 5 mol% GD. At temperatures corresponding to the gel-fluid phase coexistence region the concentration fluctuations drastically decrease, and we observe domains in the giant unilamellar vesicles, which mainly disappear by the incorporation of 2 to 5 mol% GD. Further, the lipid matrix has the ability to modulate the conformation of the inserted polypeptide. The balance between double-helical and helical dimer structures of GD depends on the phospholipid chain length and phase state. A large hydrophobic mismatch, such as in gel phase one-component DSPC bilayers, leads to an increase in population of double-helical structures. Using an effective molecular sorting mechanism, a large hydrophobic mismatch can be avoided in the DMPC-DSPC lipid mixture, which leads to significant changes in the heterogeneous lipid structure and in polypeptide conformation.

NMR study of translational and rotational dynamics in monoolein-water mesophases: obstruction and hydration effects

Using a variety of NMR methods (magnetic field gradient echo decays, 2H one- and two-dimensional spectra, spin-lattice relaxation rates), both translational and rotational dynamics of the constituents of monoolein MO/H(2)O mesophases have been studied. The experiments lead to the following conclusions. The translational dynamics of the lipid molecules is essentially dominated by obstruction effects due to the topologies of the diverse mesophases. On the other hand, water dynamics-in the regime of small water concentrations-is strongly influenced by hydration of the lipid head groups. Hydration is seen in diffusion data, in spectra and in spin-lattice relaxation of the water molecules. This work represents an involved extension of our recently published work [Chem. Phys. Lipids 106, 115 (2000)].