Effects of divalent cations and pH on phosphatidylserine model membranes: a 31P NMR study.

Received December 5,1979 SUMMARY The influence of divalent cations, and pH on the behaviour of phosphatidylserine, derived from egg phosphatidylcholine, has been examined employing 31P-NMR techniques. The addition of Ca2+ results in the observation of a "rigid lattice" 31P-NMR spectra and more than an order of magnitude increase in the spin-lattice relaxation time Tl. specific headgroup immobilization by Ca 2-k This corresponds to a strong and similar to that observed for anhydrous phosphatidylserine. At pH 7.4 the hydrated sodium salt of (egg) phosphatidylserine adopts the bilayer phase, whereas when the pH is decreased through 3.5 a bilayer to hexagonal (HII) polymorphic phase transition is observed at 5O"C, which is unaffected by equimolar cholesterol. The same transition is shown to occur at 37°C for phosphatidylserine isolated from human erythrocytes. Phosphatidylserine, the most abundant acidic phospholipid found in mammalian membranes, is also one of the most intriguing and has been implicated to play a functional role in processes as diverse as membrane fusion (1) and activation of membrane bound enzymes (2). Characteristics of phosphatidylserine which are of basic interest concern the ability of pH, as well as monovalent and divalent cations to modulate such features as the hydrocarbon transition temperature (3) and, most importantly, the strong and specific interaction it experiences with Ca '+ (4-6). This latter interaction can have dramatic manifestations, including Ca 2+ . -induced precipitation of pure phosphatidylserine to form (crystalline) cochleate lipid structures (6) as well as Ca 2+ induced lateral phase separation of phosphatidylserine in mixed lipid systems (4). In this work we have investigated the behaviour of aqueous dispersions of phosphatidylserine with two objectives in mind. First, given the specificity of the Ca 2+ phosphatidylserine interaction, it is of interest to ascertain the influence of Ca 2f on the headgroup motion and conformation. Secondly, in