NMR and X-Ray Investigation of the Phase Behavior of Phosphatidylethanolamines

Abstract

The second moments of the proton magnetic resonance lines of DPPE and DMPE, below the main phase transition, vary within a similar range as those of the corresponding phosphatidylcholines. In contrast to the sharp “subtransition” between L[sgrave]’ and Lβ’ phase, observed in the latter class of lipids, both DMPE and DPPE exhibit a broad transition (∼15 K width) to a less mobile state at lower temperatures ∼ -20[ddot]C. The second moment of the hydrocarbon chain resonance line in this state is between the value observed in the L[sgrave]’ phase of DPPC and the higher value of the rigid lattice. In addition to these phenomena a time-dependent transition from the gel phase to a rigid crystalline state is observed in DMPE, with rigid lattice values for the second moments. This behaviour is presumably analogous to that found for DLPE.

The X-ray diffraction patterns indicate that a hexagonal lattice exists for the hydrocarbon chains throughout the entire observed temperature range below the main transition, with increasing density of chain packing towards lower temperatures. In the low temperature state, the corresponding reflection is appreciably broadened, due to the reduced long-range order. In addition, the lamellar spacing is drastically reduced (∼10%) resulting from the loss of interlamellar water. The hydrocarbon chains are shown to remain untilted.