Influence of the calcium-induced gel phase on the behavior of small molecules in phosphatidylserine and phosphatidylserine-phosphatidylcholine multilamellar vesicles.

The behavior of fluorescent and spin-label probes is examined in several fluid and gel phospholipid phases, with particular focus on the Ca2+-induced gel phase in phosphatidylserine (PS). These probes have behavior characteristic of the type of probe and of the type of lipid environment. Anthroyloxy- and doxyl-labeled PS [12-AS-PS and (7,6)PS, respectively] exhibit greatly restricted and/or slow probe motion in Ca(PS)2, even compared to thermotropic gel-phase lipid at the same temperature. In contrast, anthroyloxy- and doxyl-labeled phosphatidylcholine (PC), as well as fluorescent-labeled and spin-labeled fatty acid derivatives, show no apparent change in probe motion in Ca(PS)2 compared to fluid lamellar lipid. Doxyl-labeled phosphatidic acid, phosphatidylethanolamine, and phosphatidylglycerol show restricted motion in Ca(PS)2 relative to fluid-phase lipid, but the electron paramagnetic resonance (EPR) spectra could not be interpreted in terms of simple models for probe ordering. The fluorescent probes diphenylhexatriene (DPH) and trans-parinaric acid methyl ester (tPNA-Me) show motional behavior in Ca(PS)2 that is intermediate between that observed in fluid and in thermotropic gel-phase lipid. When Ca(PS)2 and fluid PS/PC phases coexist, probe molecules distribute between the two phases. Experiments using fluorescence quenching by spin-labeled PC in PS/PC in excess Ca2+ yield the distribution of several fluorophore probes between fluid liquid-crystal and Ca(PS)2 gel phases, expressed as a concentration ratio, RLC/G. The value of RLC/G = 100 in favor of the fluid phase is obtained for 12-AS-PC, 18 for 12-AS-Me, 12 for DPH, 3 for tPnA-Me, and 1 for 12-AS-PS.(ABSTRACT TRUNCATED AT 250 WORDS)