Lipid phase transitions in membrane vesicles from Thermoplasma acidophila

l’hermoplasma acidophila has been classified as a Mycoplasma. The organism grows optimally at pH 2 and 59°C [ 11. Although it has no cell wall, the cell is extremely resistant to osmotic shock and mechanical rupture [2-61. Highly purified membranes are prepared by lysing the cells at alkaline pH [6]. The lipid content of the membrane is low (19%, w/w). More than 50% of the lipid by weight is composed of two long-chain alkyl ethers [2,6], a small amount are fatty acid esters [6]. The proteins in the membranes are hydrophobic and are apparently destabilized at high pH values due to ionization of carboxyl groups [3,61. In the present work, we report about investigations of the physical properties of the lipid phase in membrane vesicles isolated from T. acidophila. For that purpose fatty acid spin labels were incorporated in the membranes lipids. The electron paramagnetic resonance (EPR) spectra of such spin labels provide information about the lipid phase of the membrane [7,81.