Physical properties of membrane lipids isolated from a thermophilic eubacterium (Thermus sp.)

Membranes from a thermophilic eubacterium, Thermus sp. strain SPS 11, isolated from thermal springs of São Pedro do Sul spa (Portugal), are characterized for having two main polar lipids, a glycolipid (GL) with four monosaccharide residues, which at 73 degrees C accounts for 95% of the carbohydrate in the total lipid extracts, and a glycophospholipid (PL) which at 73 degrees C accounts for about 90% of the lipid phosphorous. A complex mixture of carotenoids (CA) makes up 11% by weight of the total membrane lipids. The branched fatty acyl chains (iso C15 and iso C17) comprise about 90% of the alifatic moieties of the polar lipids of this bacterium. Moreover, when the growth temperature increases from 50 to 73 degrees C there is an increase of the iso C17/ iso C15 ratio and of the GL/PL ratio. We have studied the biophysical properties of bilayers (as multilamellar liposomes) prepared from GL, PL and the mixtures of PL, GL and CA in proportions found in the membranes of bacteria growing at their optimal growth temperature, using polarization of DPH fluorescence, low and wide-angle X-ray diffraction and differential scanning calorimetry. The three techniques agree in showing the presence of a broad phase transition from a gel (L beta) phase to a liquid-crystal (L alpha) phase between 8 and 30 degrees C, for all the lipid dispersions studied except for the GL. Although all the dispersions studied form a bilayer structure at all the temperatures studied, only the mixture of the three components (PL, GL + CA) avoids the phase separation present in the mixtures of PL + CA at temperatures lower than 30 degrees C and PL + GL at temperatures lower than 55 degrees C. Our results are compared with those of Pinheiro et al. (1978) obtained with the 31p-NMR technique and applied to the study of the same samples.