²D NMR Study of the Dynamics of Bound Water Molecules in Dipalmitoyl-Phosphatidylcholine–D₂O System at a Low Water Content

Abstract

We found two doublet signals A and B in ²D-NMR of dipalmitoyl-phosphatidylcholine–D₂O system at a low water content below the temperature of the onset of the main phase transition, i.e. in the β′-crystalline (L_β′) phase. The splitting of each doublet becomes minimum at the onset of the transition. The signal A decreases in intensity with a slight increase of its splitting as the temperature increases further, accompanying the marked growth of the signal B in its intensity and splitting. These features of two doublets in the L_β′ phase and at higher temperatures have never been noticed. The signals A and B were ascribed to the tightly bound water and the loosely bound water, respectively. These assignments were confirmed by the theoretical calculations of the splitting of the doublet A for all possible number of the tightly bound water molecules.