Ganglioside hydration study by 2H-NMR: dependence on temperature and water/lipid ratio

Dynamic properties of 2H2O in samples of ganglioside aggregates hydrated at water/lipid ratios ranging from 25:1 to 8000:1 mole/mole were studied by using deuterium nuclear magnetic resonance (2H-NMR). We present a physical model for the interpretation of the measured spin-spin relaxation times (T2). For all the concentrations studied the model provides evidence for the existence of at least two kinds of water environments: one in which the rotational correlation time is in the range of 10(-9) to 10(-8) s, and a second in which it lies between 10(-11) to 10(-10) s. A detailed study on the temperature dependence was performed for two of the concentrations, one corresponding to the hexagonal phase (100:1 mole/mole) and the other involving a micellar phase (200:1 mole/mole). In the 100:1 2H2O/ganglioside molar ratio sample, most of the water is tightly bound to long cylindrical structures. For the 200: 1 sample, there are on average approximately 30 water molecules tightly bound to the polar head group of each ganglioside molecule. The relative number and dynamics of molecules in this environment are essentially insensitive to temperature variations in the range 220-300K The rest of water molecules are also influenced by the aggregate, having a different mobility from that observed in the free liquid state.