Water isotope effect on the lipidic cubic phase: Heavy water-Induced interfacial area reduction of monoolein-Water system.

Heavy water (D2 O) affects various functions of cells and living things. In order to gain fundamental insight into the molecular mechanism on biological effects of heavy water, D2 O-effects on fully hydrated monoolein (MO) systems were investigated from the structural viewpoints. At room temperature, the MO fully hydrated by pure light water (H2 O) forms a bicontinuous cubic (Pn3m) phase, and then, the Pn3m cubic phase transforms into an inverted hexagonal (HII ) phase at about 90°C. Temperature-scan X-ray diffraction measurements showed that substitution of D2 O for H2 O lowers the Pn3m-to-HII phase transition temperature and reduces the lattice constants of both phases. The structural analysis of the Pn3m phase using the diffraction intensity data indicated that D2 O reduces the surface occupied area of MO at the interface by 12% in comparison with H2 O. This change is probably due to the difference of the strength of hydrogen bond.