The effects of hydration and divalent cations on lamellar-nonlamellar phase transitions in membranes and total lipid extracts from Acholeplasma laidlawii A-EF22 - a 2H NMR study

Acholeplasma laidlawii strain A-EF22 was grown in a medium supplemented with 75 μm α-deuterated palmitic acid (16:0-d2) and 75 μm α-deuterated oleic acid (18:1c-d2), or with 150 μm 18:1c-d2. The fatty acids were incorporated into the membrane lipids and 2H NMR spectra were recorded from intact membranes, total lipid extracts, and the combined glucolipid and neutral lipid fractions of a total lipid extract. The lipids in intact membranes form a bilayer structure up to at least 70 C. The same result was obtained with membranes digested with pronase, which removes a large fraction of the membrane proteins. A reversed hexagonal liquid crystalline (HII) phase was formed below 70 C by the total lipid extracts hydrated with 20 and 30% (w/w) water; in the presence of 40% (w/w) water only one of the extracts formed an HII phase below 70 C. The HII phase was formed at higher temperatures with an increasing water content. However, only a lamellar liquid crystalline (Lα) phase was formed up to 70 C by the total lipid extracts when the water concentrations were 50% (w/w) or higher. The temperature (TLH) for the Lα to HII phase transition in the combined glucolipid and neutral lipid fractions was only 2-3 C lower than for the total lipids, and the phospholipids thus have a very modest influence on the TLH value. Physiologically relevant concentrations of Ca2+ and Mg2+ ions did not affect the phase equilibria of total lipid extracts significantly. It is concluded from comparison with published data that the membrane lipids of the cell wall-less bacterium A. laidlawii have a smaller tendency to form reversed nonlamellar phases than the membrane lipids of three bacterial species surrounded by a cell wall. (orig.)