Solubilization and localization of triolein in phosphatidylcholine bilayers: a 13C NMR study.

Abstract

Cosonicated mixtures of egg phosphatidylcholine and small amounts (< 5% wt/wt) of triolein were studied by 13C NMR spectroscopy. The 50.3-MHz 13C NMR spectrum of vesicle preparations containing 90% isotopically substituted [1-13C]triolein showed 2 carbonyl resonances at chemical shift values that indicate H-bonding of H2O molecules with the carbonyl groups. The extent of hydration, estimated from the chemical shift values (173.07 and 172.39 ppm) is .apprxeq. 50%. The data suggest that the triolein is located in the bilayer with the 3 carbonyl groups at the aqueous interface. The acyl chains are extended toward the bilayer interior, with a conformation of the glyceryl region such that the primary (.alpha.) carbonyls are closer to the aqueous medium than is the secondary (.beta.) carbonyl. Thus, triolein is present in the bilayer in an orientation appropriate for enzymatic hydrolysis, with the 2nd substrate (H2O) in close proximity to the hydrolytic site, and with a conformation that could explain, in part, enzymatic specificity for hydrolysis at the .alpha. position. Spectra of vesicles containing .gtoreq. 3% triolein showed 2 additional carbonyl peaks characteristic of pure (neat) triolein. This allowed a determination of the maximum solubility (.apprxeq. 2.8%) of surface-oriented triolein in the bilayer phase. Beyond this limit all excess triolein partitions into a separate oil phase.