Phase separation of giant vesicles composed of neutral saturated lipid, negatively charged unsaturated lipid, and cholesterol, is observed at different calcium concentrations. Confocal microscopy provides the information where the phase separation becomes distinct as the calcium concentration is increased. The negatively charged lipid domains tend to bud toward the interior of the vesicle. This budding is assumed to be due to an increase in the osmotic pressure, in cooperation with the spontaneous curvature change in the outer leaflet of the bilayer caused by the adsorption of calcium ions and charge screening effect. We interpret the effect of small cations on the phase separation based on the theoretical model with the Poisson-Boltzmann equation.

The hydration state of biomolecules is believed to affect their self-assembly. The hydration state of phospholipid bilayers is studied precisely by terahertz spectroscopy, by which water perturbed by a lipid membrane is detected sensitively from the observation of the relaxation dynamics of water molecules in the subpicosecond time scale. Combined with x-ray observation of the lamellar structure of the lipid, a long-range hydration effect on up to 4-5 layers of water is confirmed. Most water molecules in the lamellae fall into the hydration water, and condensation of them is also indicated.

It has been unclear whether the role of water in the self-assembly of soft materials and biomolecules is influential or water is just a background medium. Here we investigate the correlation between hydration state of lipid membrane and structural phase transition of the membrane between lamellar and inverted-hexagonal phases, as an intermediate process of membrane fusion, by using the complementary techniques of X-ray scattering and terahertz (THz) spectroscopy. By comparing two lipid species, our results indicate that the structural changes of the lipid membrane depend on the behavior of the surrounding water, especially in the second hydration layer, in addition to the molecular shape of lipids. The water behaves differently at each membrane surface owing to the different hydrophilicities of the lipid head groups.

No abstract

There is a long, ongoing debate on how small molecules (osmolytes) affect the stability of proteins. The present study found that change in collective rotational dynamics of water in osmolyte solutions likely has a dominant effect on protein denaturation. According to THz spectroscopy analysis, osmolytes that stabilize proteins are accompanied by bound hydration water with slow dynamics, while the collective rotational dynamics of water is accelerated in the case of denaturant osmolytes. Among 15 osmolytes studied here, there is a good correlation between the change in mobility in terms of water rotational dynamics and the denaturation temperature of ribonuclease A. The changes in water dynamics due to osmolytes can be regarded as a pseudo-temperature-change, which agrees well with the change in protein denaturation temperature. These results indicate that the molecular dynamics of water around the protein is a key factor for protein denaturation.