Spin-spin coupling constants derived from high-resolution 1H NMR spectra of pure 1-myristoyl-sn-glycero-3-phosphocholine (MLPC) micelles and 60:1 mol/mol been analyzed in order to determine the effects of apoprotein on phosphatidylcholine (PC) polar group conformation. The shift ratios of the polar group proton resonances after addition of the paramagnetic shift reagent Fe(CN)6(3-) to the above MLPC systems, egg PC small unilamellar vesicles, and human HDL3 have been used to compare the PC polar group conformations in all systems. The location of the largely alpha-helical apo A-I molecules in the complex with MLPC was deduced from its effects on the chemical shifts and spin-lattice relaxation times (T1) of the well-resolved 1H resonances from the various parts of the lipid molecules. The data are consistent with the apo A-I molecules lying in the surface fo the MLPC micelle with their amphipathic, alpha-helical segments intercalated among the glycerophosphocholine groups of the lipid molecules so that aromatic amino acid side chains are interspersed among the lipid hydrocarbon chains. This leads to a spacing out of the glycerol backbones and immediately adjacent methylene groups of the MLPC molecules, thereby causing an enhancement of the motions affecting T1. The presence of apo A-I at the lipid-water interface apparently does no perturb the PC polar group conformation, indicating that this conformation is determined by intramolecular effects. The preferred conformation of the phosphocholine group (Hauser, H., Pascher, I., Pearson, R. H., & Sundell, S. (1981) Biochim. Biophys, Acta 650, 21-51] is characterized by an almost exclusively gauche conformation of the choline group and predominantly antiperiplanar conformations about the C-C-O-P and P-O-C-C bonds. The PC molecules in MLPC micelles, MLPC-apo A-I complexes, egg PC vesicles, and HDL3 all have this polar group conformation.