DYNAMIC PROPERTIES OF WATER AT PHOSPHATIDYLCHOLINE LIPID-BILAYER SURFACES AS SEEN BY DEUTERIUM AND PULSED-FIELD GRADIENT PROTON NMR

The dynamic properties of water in phosphatidylcholine lipid/water dis persions have been studied, applying a combination of H-2-NMR techniqu es (quadrupole splitting and spin-lattice relaxation time) and self-di ffusion measurements using pulsed field gradient (PFG) H-1-NMR. The hy dration properties of POPC lmitoyl-2-oleoyl-sn-glycero-3-phosphatidylc holine) were compared with those of DOPC (1,2-dioleoyl-sn-glycero-3-ph osphatidylcholine) and EYL (egg yolk phosphatidylcholine (lecithin)). A model is presented that assumes an exponentially decaying influence of the bilayer surface on water dynamics as well as on water orientati on with increasing hydration. This assumption is based on an exponenti ally decaying hydration potential which results from direct lipid-wate r and water-water interactions. The model describes successfully the e xperimental data for a large water concentration range, especially at low hydration, where other models failed. With the exception of a smal l fraction of water which is significantly influenced by the surface i n slowing down the mobility, the interbilayer water has isotropic, fre e water characteristics in terms of correlation times and molecular or der. Hydration properties of POPC are comparable with those of EYL but differ from DOPC. At very low water content the correlation times of headgroup segmental reorientation and water are similar, indicating a strong coupling of this water to the lipid lattice. The hydration prop erties of the three lipids studied are explained in terms of slightly different headgroup conformations due to different lateral packing of the molecules by their fatty acid chain composition.