Phospholipid acyl chain rotational dynamics are independent of headgroup structure in unilamellar vesicles containing binary mixtures of dioleoyl-phosphatidylcholine and dioleoyl-phosphatidylethanolamine

We have examined relationships between phospholipid headgroup structure and acyl chain dynamics, and their respective roles in modulating the physical properties of biological membranes. Fluorescence lifetime and anisotropy m easurements were used to assess structural changes involving the lipid acyl chains in homogeneous populations of small and large unilamellar vesicles containing binary mixtures of dioleoyl-phosphatidylcholine (PC) and dioleoy l-phosphatidylethanolamine (PE) in the liquid-crystalline (L-alpha) phase. These measurements involve three different fluorescent lipid analogs contai ning diphenylhexatriene (DPH) linked to either a trimethylamine moiety (i.e ., TMA-DPH) or the sn-l position of monostearoyl-phospholipids containing P C or PE headgroups (i.e., DPH-PC and DPH-PE). The average lifetimes, rotati onal correlation times, and order parameters associated with DPH-PC and DPH -PE are virtually identical, and are not affected by alterations in the PE content of the membrane. These results suggest that the average cross-secti onal areas of the phospholipid acyl chains of DOPE and DOPC relative to the membrane normal are similar in these unilamellar vesicles. Since PC headgr oups are larger than those of PE, differences in the relative orientation o f the phosphocholine and phosphoethanolamine moieties relative to the membr ane surface probably function to maintain optimal van der Waals contact int eractions between acyl chains. On the other hand, the average lifetime asso ciated with TMA-DPH, whose chromophoric group is near the membrane surface, increases with increasing PE content. The position of TMA-DPH relative to the membrane surface does not change, since the rotational dynamics of TMA- DPH are independent of the PE concentration. Therefore, alterations in the average lifetime of TMA-DPH results from polarity differences near the memb rane surface at the level of the glycerol backbone. These results are discu ssed in terms of how differences in the average conformation of the glycero l backbones or phospholipid headgroups of PE and PC have the potential to r egulate membrane function. (C) 1998 Elsevier Science B.V. All rights reserv ed.