Sacré M M, El Mashak E M, Tocanne J F
Chem Phys Lipids. 1977 Dec;20(4):305-18. doi: 10.1016/0009-3084(77)90071-8.
1,2-Didodecanolyl-sn-glycero-3-phosphoryl-1'-(3'-O-L-alanyl)-sn-glycerol (Ala-PG) has been synthetized. Its ionic properties have been studied at the air-water interface through film compressions and surface potential measurements as a function of subphase pH and ionic content (NaCl, Na2MoO4, CaCl2). The existence of the polar head in a loop conformation allowing for interactions between phosphate and amino groups is suggested. Ionic properties of Ala--PG clearly depended on subphase ionic strength but no specific interactions between either cations or anions in the subphase and phosphate or amino groups in the film could be detected. Results are interpreted in terms of ion-pair interactions at the interface between these two groups and anions and cations from the subphase. Occurrence of charge separation between these two groups, induced by increasing subphase ionic strength, is postulated. Since the molecular packing appeared independent of the subphase ionic content over a large domain of pH (3--7) and surface pressure (pi greater than 5 dyne/cm) and since the lipid can be considered as zwitterionic or slightly positive below pH 5--6, it is suggested that in the parent bacteria, grown under acidic conditions, Ala--PG could play a role in maintaining the membrane integrity and in preventing the passive diffusion of protons.
