The lipidic cubic phase is a complex, bicontinuous sponge-like structure in which water channels are interwoven periodically. The structure is capable to include protein molecules either in the water channels for globular proteins or in the bilayers for membrane proteins. We have been studying a globular protein lysozyme included in the water channels of lipid monoolein cubic phases. The size of water channels of monoolein cubic phases is about 10 nm at most whereas the size of lysozyme molecules is about 4 nm. These facts suggest that the lysozyme molecules in the cubic phase are highly confined in the water channels. We measured the diffusion coefficient of lysozyme using FRAP in order to elucidate how this confinement affects lysozyme dynamics.

Figures 1(a)-(c) above show a typical FRAP measurement. Only lysozyme molecules are dyed so that we can trace their movement. From the time scale of the fluorescence recovery we could estimate the diffusion coefficient as 4-10 micron^2/s, which is significantly smaller than the one in bulk solutions, 140 micron^2/s. The reason of this slow diffusion is twofold: Firstly, lysozyme molecules are confined in cavities (or `cells') which is a wide part of the water channels, and they have to find narrow paths connecting these cavities in order to diffuse. Secondly, passing through the narrow paths is accompanied by the deformation of bilayers since lysozyme molecules are slightly larger than the path. We are now trying to construct more quantitative model to describe the effect of the confinement on lysozyme diffusion.