Activation Energies for Water Diffusion in ex-vivo White Matter

Bibek Dhital¹, Christian Labadie^1,2, Harald E. Mller¹, Robert Turner¹

We made MR measurements of diffusion up to very high b-factor in excised human corpus callosum, over a wide temperature range. Above a freezing phase transition at -20 C, data showed a robust bi-exponential dependence on b-factor. Below this temperature only half of the slow component remained, suggesting two water pools within this component. An Arrhenius plot revealed significantly different activation energies for the fast and slow components. The lower of these corresponds well to breaking a hydrogen bond in a locally ordered region. This suggests that unfrozen water consists of hydration layers close to the membranes.