Phospholipids chiral at phosphorus. Dramatic effects of phosphorous chirality on the deuterium NMR properties of the choline head group of phospholipids in the liquid crystalline phase

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Abstract
To probe the motional and conformation properties of the choline head group of 1,2-dipalmitoyl-sn-glycero-3-thiophosphocholine (DDPsC), the Rp, Sp, and Rp + Sp isomers of [.alpha.-D2]DDPsC, [.beta.-D2]DPPsC, and [.delta.-D9]DDPsC in the subgel, gel, and liquid crystalline phases were investigated with deuterium NMR, and the results were compared with those of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) labeled at the same positions. In the subgel phase (5.degree.C) all isomers of [.alpha.-D2]DDPsC and [.beta.-D2]DPPsC displayed amorphous line shapes characteristic of a restricted and disordered motional environment, whereas [.delta.-D9]DDPsC showed narrower and symmetric line shapes indicating substantial motions. For all three labeled positions the apparent line width of the Rp isomer is larger than those of Sp and Rp + Sp isomers, and the amorphous line shape of the Rp isomer also persists at 25 and 35.degree.C, which confirm the previous observation that the Rp isomer is unusually stable in the subgel phase and suggest that the Rp isomer is more rigid than the other isomers in the choline head group. In the gel phase (25 and 35.degree.C) narrower and symmetric line shapes were observed for Sp and Rp + Sp isomers, and the apparent line widths were comparable to those of DPPC. In the liquid crystalline phase there are dramatic differences between the spectra of DPPC and different isomers of DDPsC. For [.alpha.-D2]DPPsC, two quadrupolar splittings with very different .DELTA..nu.Q values (6.5-7.1 and 1-2 kHz) were observed for both Rp and Sp isomers, which suggest significant differences in the average orientation of the two C.alpha.-D bonds. For [.beta.-D2]DPPsC, four sets of quadrupolar splittings were observed for the Rp isomer and two sets for the Sp isomer, with very different .DELTA..nu.Q values. These were interpreted to suggest that the .beta.-CD2 segment of the Rp isomer exists in two slowly exchanging conformational states, and the two C.beta.-D bonds have different average orientation in both conformational states. The Sp isomer, on the other hand, exists in only one conformational state with the two C.beta.-D bonds in different average orientations. The shorter T1 values (longer .tau.c) for DDPsC relative to DPPC also suggest that the motions of the C.alpha.-C.beta. segment are slower in DPPsC than in DPPC. These results indicate that the motional and conformational properties of the C.alpha.-C.beta. segment of DPPsC is very sensitive to the configuration at phosphorus. Structurally, this provides strong support for noncovalent interactions between the quaternary ammonium group of choline and the phosphate group of a neighboring molecule in the bilayers of phosphatidylcholine and suggests that such interactions are important to the motion of the choline chain.