Solid-state carbon-13 nuclear magnetic resonance of the lecithin gel to liquid-crystalline phase transition.

The temperature dependence of the 13C NMR spectra of dipalmitoylphosphatidylcholine (DPPC) which has been 13C labeled at the carbonyl position of the sn-2 chain, 2-[1-13C]DPPC, is reported. In the L beta' phase an axially symmetric spectrum of 112-ppm breadth is observed, and this transforms to an isotropic-like line ((delta sigma) approximately 7 ppm) in the L alpha phase. In the intermediate P beta phase a temperature-dependent superposition of these spectra is observed, which suggests that this phase exhibits microscopic structural and dynamical properties of both the L beta' and L alpha phases. An analysis of the spectral line shapes leads to the conclusion that the appearance of the isotropic-like line in the P beta' phase is primarily due to a conformational change at the sn-2 carbonyl which is complete at the main transition. Increased rates of axial diffusion in the P beta' phase may contribute to the narrowing.