Structural Effects, Mobility, and Redox Behavior of Vitamin K1 Hosted in the Monoolein/Water Liquid Crystalline Phases

The solubilization of vitamin K1 (VK1), a highly hydrophobic molecule, into a 1-monoolein/water (MO/W) system, is investigated by NMR self-diffusion, small angle X-ray scattering (SAXS), optical microscopy, and electrochemical methods. The various MO/W phases, namely L2, Lα, CG, and CD, can accommodate different amounts of VK1. In particular, the Lα and the cubic CG phases can solubilize up to 8 and 5 wt % VK1, respectively, without modifing the microstructure substantially. By contrast, the cubic CD phase can accommodate only about 1 wt % VK1. Larger addition of VK1 produces a transition from the lamellar and cubic phases to a reverse hexagonal phase HII, which in the MO/W binary system occurs only for the cubic phases and at temperatures above 80 °C. In practice the solubilization of VK1 induces almost the same phase transitions as would a temperature increase in the binary system. The SAXS and NMR self-diffusion data strongly suggest that the VK1 molecules are well intermingled with the MO hydrophobic chains. Consequently the swelling of the CG and Lα phases does not seem to be affected by the amount of VK1 present. However, if a sufficient number of VK1 molecules has penetrated into the lipid bilayer, the local change of the bilayer curvature is so large that a transition from cubic or Lα phase to a reverse hexagonal phase will occur. Electrochemical measurements indicate that, when solubilized in the cubic phase, the naphthoquinone group of VK1 reaches the bilayer/aqueous interface during the redox cycle, as the formal redox potential of the group is pH-dependent. The potential use of MO/W cubic phases with electrochemically active bilayer components in bioanalytical systems is discussed.