The conformation of the polar group of lysophosphatidylcholine in H2O; conformational changes induced by polyvalent cations

The conformation of the polar group of egg lysophosphatidylcholine and 1-myristoyl-sn-glycero-3-phosphorylcholine present as micelles in aqueous solution has been studied using NMR methods. In the absence of polyvalent cations the preferred conformation derived from spin-spin coupling constants is similar, but not identical, to that of phosphatidylethanolamine in the crystal structure (cf. Hitchcock, P.B., Mason, R., Thomas, K.M. and Shipley, G.G. (1974) Proc. Natl. Acad. Sci. U.S. 71, 3036–3040). The presence of lanthanides induces a conformational change involving primarily the phosphorylcholine group, e.g. torsion angle $\text{α}{}_5$ changes from an all gauche to an approximate trans disposition. The gauche → trans transitions observed with torsion angles α₃ and α₅ produce a more extended orientation of the polar group (relative to the hydrocarbon chain axis). In the presence of lanthanides the conformation of lysophosphatidylcholine is very similar to that of the diacyl phosphatidylcholines observed in fully hydrated bilayers (cf. Hauser, H., Phillips, M.C., Levine, B.A. and Williams, R.J.P. (1976) Nature 261, 390–394) with the P-N vector at an angle of about 45° to the bilayer.