Plasmenylcholine and phosphatidylcholine membrane bilayers possess distinct conformational motifs

The conformation of plasmenylcholine near the hydrophobic-hydrophilic interface in membrane bilayers was deduced by determination of critical internuclear distances utilizing truncated driven nuclear Overhauser enhancement. These experiments demonstrated that the beta-vinyl ether proton in plasmenylcholine was in close spatial proximity and nearly equidistant (approximately 3 A) to both the alpha- and beta-methylene protons of the sn-2 aliphatic chain. In contrast, the distances between the alpha-vinyl ether proton and the alpha- and beta-methylene protons of the sn-2 aliphatic chain were greater than or equal to 5 A. Furthermore, the distance between the N-CH3 protons in the polar head group and the methylene protons of the glycerol backbone in plasmenylcholine vesicles is larger than that present in phosphatidylcholine vesicles. Although the proximal portion of the sn-2 acyl chain in phosphatidylcholine is bent, conformational analysis utilizing these distance constraints demonstrated that the carbon atoms which comprise the proximal portion of the sn-2 aliphatic chain in plasmenylcholine are nearly coplanar, in register, and parallel to the sn-1 aliphatic chain. Taken together, these observations indicate that modest covalent alterations in the proximal portion of the sn-1 aliphatic chain in choline glycerophospholipids result in substantial changes in the molecular conformation and packing of hydrated phospholipid bilayers.