, Volume 24, Issue 6, pp 423-431

Thermotropism and hydration properties of POPE and POPE-cholesterol systems as revealed by solid state2H and31P-NMR

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Abstract

The partial phase diagram and the hydration properties of the 1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE)-water system, in the absence and presence of 30 mol% cholesterol, have been investigated by solid state phosphorus NMR of the lipid and deuterium NMR of heavy water. The POPE-D2O phase diagram resembles other phosphatidylethanolamine (PE)-water systems: below water-to-lipid molar ratios (Ri) of 3 the lamellar gel (Lβ or Lc)-to-hexagonal type II (HII) phase sequence is observed on increasing the temperature. For Ri≥3 the thermotropic sequence (Lβ or Lc)-Lα-HII is detected. On increasing hydration from Ri=3, the HII phase is detected from 40°C to 85°C whereas the gel phase is observed from 40°C to 30°C. The limiting hydrations of the gel, Lα and HII phases are Ri ≈ 3, 17 and 20, respectively. The number of bound water molecules per lipid is ca. 8 in both the La and HII phases. The presence of cholesterol stabilizes the hexagonal phase 20°C below temperatures at which it is observed in its absence and reduces the limiting hydration of the fluid and hexagonal phases to Ri ≈ 9 and 14, respectively. The structure and/or dynamics of the water bound to the interface are markedly modified on going from the Lα to the HII phase.