Phase behaviour in monolayers and in water dispersions of mixtures of dimannosyl diacylglycerol with phosphatidylglycerol.

Mixtures of dimannosyl diacylglycerol, extracted from the membrane of Micrococcus luteus, with synthetic dipalmitoyl phosphatidylglycerol or with samples of phosphatidylglycerol and phosphatidylinositol, extracted from the same bacterium, have been studied. Through a monolayer (pi, delta V) study and from fluorescence polarization data relative to diphenylhexatriene embedded in vesicles of the mixed lipids, it is shown that the glycolipid interacts with the phospholipids. These interactions are independent of the structure and physical state of the phospholipid acyl chains, of the lipid molecular packing and of the nature of the cations (monovalent, bivalent) present in the aqueous phase. No phase separation was detected, either in monolayers or in water dispersions. Furthermore, the data presented demonstrated a marked influence of the glycolipid on the phase behaviour of phosphatidylglycerol, both in the presence of monovalent (Na+, K+) and bivalent (Ca2+, Mg2+) cations. This point is of particular interest with regard to the highly rigid phase this phospholipid is known to assume in the presence of bivalent cations. It is then suggested that the glycolipid could act as a regulator of the membrane fluidity by preventing a too high rigidity of the lipid phase when bivalent cations are present at the membrane surface.