The Bi-modality Diffusion of Water Molecules in Liposome/Water Dispersion Systems Analyzed by Pulsed Field Gradient Spin Echo NMR Method

Abstract

The translational diffusion coefficient of water molecules in DPPC liposome/water dispersion systems was obtained by Pulsed Field Gradient (PFG) NMR method. Two types of diffusions of water molecules existing in dispersions were distinguished each other as the fast and the slow components by bi-exponential analysis of the diffusion process. The fast component exhibits likely to the pure water dynamics. On the other hand, the slow component shows two or three orders of magnitude smaller than the fast component. By the temperature and diffusion time dependences, the slow component was clearly characterized as hydrated water molecules interacting with the phospholipid membrane and moving with liposome particles. Above the main transition temperature of 1,2-dipalmitoyl-rac-glycero-3-phosphocholine (DPPC), the slow component showed a drastic increase in the diffusion coefficient and the tendency did not follow the liposome particle dynamics obtained by Stokes-Einstein relation. These results indicate that the fluidity and dynamic behavior of phospholipid membranes were abruptly changed according to the phase transition and dynamics of water molecules were treated as the probe. Both components show a typical "restricted diffusion" which means that the diffusion coefficient was dependent on the diffusion time, and this attribution provides a spatial information of such dispersion system.