An NMR line shape and relaxation analysis of heavy water powder spectra of the Lα, Lβ′ and Pβ′ phases in the DPPC/water system

Abstract

The ²H₂O NMR powder line shapes and relaxation times, T₁ and T₂, of the liquid crystal L_α, the intermediate P_β′ and the gel L_β′ phases of dipalmitoylphosphatidylcholine (DPPC)/2H₂O-system are analysed. The water structure and dynamics of the lipid/water interfaces of DPPC in the hydration regime, where all water molecules are associated to the interface, are described in terms of orientational order parameters and correlation times. The line shape of the ripple phase (P_β′) is analysed assuming model parameters of the gel or liquid crystalline phase. The narrow line shape of the ripple phase is partly due to an extra average of the quadrupole interaction because of lateral diffusion along the curved surface, reducing the splitting with a factor 0.5–0.2 depending on the nature of the curved ripple surface. However, more importantly, an extra reduction of the quadrupole splitting may be due to the same reorganization of water, among bound sites with different signs of the order parameter, which also explains the increase in the quadrupole splitting with temperature observed in the liquid crystalline phase. The linewidths in ¹⁴N MAS NMR spectra clearly indicate slow dynamics of the polar headgroup in the ripple phase. The results indicate that the headgroup hydrations of the ripple and liquid crystalline phases are similar, while the acyl chains are still in their gel state in the ripple phase. The increased headgroup area introduces a stress, as confirmed by the slow headgroup dynamics, which causes the bilayer to curve in the ripple phase.