Investigation of water in normal and dehydrated rabbit lenses by H-1 NMR and calorimetric measurements

Water molecules in the lenses of animal eyes exist in two different states: water which is bound to proteins forming their hydration hull (nonfreezable) and free bulk water (freezable). Between these two types of water exchange processes take place on a time scale between approximately 10 and 100 ms, as has been shown by previous NMR relaxation studies of the lens. In this contribution we present results of experiments where we varied the relative amount of these two types of water by dehydration of the lens. The dehydration process removes primarily the free water from the lens. We monitored the changes of the relative fractions of water protons by measurements of the proton spin-lattice relaxation times and from the calorimetric curves of the eye lens. Both curves exhibit a strong singularity at the freezing point of the bulk water for the normal lens: which becomes smoother for increasing dehydration. To explain this effect on the spin-lattice relaxation time, a cross-relaxation process between bound water protons and protons in the frozen state must be assumed. From these experiments we were able to separate the individual relaxation times T-1A and T-1B of the two types of water, which are averaged out for the normal lens by the exchange process. We obtained also the activation parameters for free and bound water in the lens. We point out that the dynamic processes responsible for the water relaxation above the freezing point at 500 and 60 MHz are different. The relaxation data of the higher frequencies can be explained by assuming a fast motion (rotational diffusion of water molecules) with a single correlation time whereas for the lower frequencies additional type of molecular motions have to be taken into account.