The effects of metal cations bound at phospholipid headgroups on hydration water molecules were investigated by quasi-elastic neutron scattering experiments. The hydration water could be categorized into three types—tightly bound water, loosely bound water, and free water—as observed in the case of a mixture of phospholipid and water without salt. When CaCl2 was added to the mixture, the numbers of these three types of water molecules remained unchanged. In contrast, when MgCl2 or FeCl2 was added to the mixture, the number of tightly bound water molecules increased, while the number of loosely bound water molecules remained unaffected. These results are consistent with the x-ray diffraction results, which suggested that Mg2+ and Fe2+ ions bind near the phosphate groups and enhance the extent of hydration, while Ca2+ ions bind near the glycerol groups with no significant effect on hydration water. The results demonstrate that the number of loosely bound water molecules is determined by the nature of the phospholipid membrane.

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