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.

Relating structure and translational dynamics in aqueous dispersions of monoolein.

The temperature dependence of the molecular diffusion in monoolein/water systems is investigated at several levels of hydration. Using the proton/deuteron selectivity, field gradient NMR allows the simultaneous determination of the diffusion constants of both, lipid and water molecules in the various lamellar and non-lamellar phases. Due to the mesoscopic structure of the monoolein/water phases, the diffusion coefficients are interpreted as 'reduced' or 'effective' diffusion coefficients, and are related to the microscopic molecular displacements by a so-called 'obstruction factor'. Changes in the microscopic structure at the phase transition from the bicontinuous cubic phases to the inverse hexagonal phase are reflected in the obstruction factor of the monoolein diffusion coefficients. The reduction of the water diffusion coefficients is too high to be explained by an obstruction factor only, implying a mechanism of molecular motion, which strongly differs from that of bulk water. Experiments on samples prepared with isotopic labeled water (2H(2)O and H(2)(17)O) indicate a chemical exchange of protons between the water molecules and the lipid headgroups on a millisecond timescale.