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

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Abstract
The temperature dependence of the 13C NMR spectra of dipalmitoylphosphatidylcholine (DPPC) 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 (.ltbbrac..DELTA..sigma..rtbbrac. .apprx. 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 primarly 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.

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