To further understand the packing of amphipathic alpha-helices of apolipoproteins in serum lipoproteins, we have investigated the interactions with dimyristoyl phosphatidylcholine (DMPC) of a 13C-labeled, 18-residue peptide (18A) which can form an amphipathic alpha-helix. This peptide whose amino acid sequence is DWLKAFYDKVAEKLKEAF has the positive-negative residue clustering typical of the apolipoprotein class of amphipathic helix. 13CH3-alanine was introduced as the 11th residue of 18A so that the 13CH3 group protrudes on the apolar side of the amphipathic helix. [13C]NMR spectra of [13C-Ala11]18A in discoidal complexes with DMPC show three resonances from the Ala-13CH3 group; one originates from 18A in aqueous solution, while those at chemical shifts (delta) of 15.2 and 16.4 ppm are assigned to 18A in the "edge" and "faces," respectively, of the discoidal complex. The proportion of 18A in the faces of the discoidal complex increases as the size of the disk is increased by raising the lipid/peptide ratio. 18A covers the edge of the disk so that the 13CH3-Ala side chain from these molecules is in contact with DMPC acyl chains. [13C-Ala11]18A bound to the surface of an egg PC small unilamellar vesicle gives a single resonance from 18A at delta 16.3 ppm consistent with there being no edge location. Cooling 18A-DMPC disks to 15 degrees C crystallizes the DMPC bilayer and restricts the motion of the 13CH3-Ala group of the 18A molecules. The molecular motions of the side chains of the amphipathic helix are sensitive to their location in the disk and to PC molecular packing.