Interactions of lyso 1-palmitoylphosphatidylcholine with phospholipids: a 13C and 31P NMR study.

Abstract

13C and 31P NMR spectroscopy were used to monitor interactions of lyso 1-palmitoylphosphatidylcholine (LPPC) in the interfacial region of egg phosphatidylcholine (PC) bilayers and determine the effect of LPPC on the phospholipid bilayer structure. 13C NMR spectroscopy of small amounts (0.5-10 mol%) of 13C carbonyl-enriched LPPC cosonicated with egg PC to form small unilamellar vesicles (SUVs) revealed separate carbonyl signals for LPPC in the inner and outer leaflets of the vesicles. The ratio of LPPC in the outer leaflet to that in the inner leaflet was > or = 3/1. Exchange of LPPC between bilayer leaflets ("flip-flop") was too slow to be measured (t1/2 > 12 h). Albumin added to the external buffer of LPPC/PC vesicles was shown by 13C NMR to extract LPPC only from the outer leaflet. LPPC was a poor detergent in egg PC multilayers and SUVs. Stable SUVs were prepared by cosonicating egg PC with up to 30 mol% LPPC, and preformed SUVs incorporated up to 40 mol % of LPPC (added as an aqueous solution) without undergoing any morphological changes as evidenced by 31P NMR spectroscopy. The presence of oleic or palmitic acid did not have observable effects on properties of LPPC in SUVs, such as the localization of the LPPC carbonyl in the interface, and the transbilayer distribution and movement of LPPC. The apparent pKa of the fatty acid (FA) carboxyl at the membrane interface (7.7) measured by 13C NMR was not affected by LPPC, but the FA carboxyl carbon resonance showed linewidth changes near the apparent pKa that were dependent on the FA/LPPC ratio. These data suggest weak interactions in the interfacial region between FA and LPPC when both lipids are present at low levels in PC vesicles.