The dependence of phospholipid head-group mobility on hydration as studied by deuterium-NMR spin-lattice relaxation time measurements

The dependence of the head-group mobility of DOPC-d9(dioleylphosphatidylcholine, deuterated at its choline methyl groups) on hydration has been studied in multibilayer dispersions by deuterium nuclear magnetic resonance (2H-NMR) spin-lattice (T1) relaxation time measurements. Both the mean amplitude and the activation energy of motion of the deuterated head-group segment did not change with water content of the multibilayers. The rate of effective rotation, as determined from T1measurements, increased almost linearly with the addition of water up to a limiting hydration number of 22 ± 2 water molecules per lipid molecule. Beyond this limiting hydration level, the motional correlation time remained constant with increasing water content. The effects of bilayer hydration on lipid head-group dynamics correlate with structural results from diffraction studies. The observations are interpreted in terms of swelling of the hydrophilic bilayer surface region as water becomes intercalated directed by hydration forces. © 1990.