The thermotropic behaviour of red blood cell membrane phospholipids

Phospholipids are the major constituents of biological membranes and the lipid structural organization is important to membrane function. In this study we have examined, by differential scanning calorimetry (DSC), the thermal behaviour of phospholipids extracted from bovine red blood cell membrane and related the observed characteristics to the phospholipid classes in the mixture. Phosphol\uedp\uedd classes were separated by thin layer chromatography. DSC was performed on aqueous buffer dispersions (pH 7.4) approx. 3 mM. For all phospholipid samples at least one, or sometimes two transitions were observed. The calorimetric scans show the presence of a broad transition between 0 and 30 \ub0C (midpoint of the transition at 24.1 - 24.5 \ub0C) whereas the second phase transition occurs at a transition temperature of 32.4 - 38.1 \ub0C. The analyzed membranes contained the same amounts of phosphatidylcholine (PC; 6 - 9 % w/w of total phospholipids) and phosphatidylethanolamine (PE; 24 \u2013 32 %) but different percentual contents of sphingomyelin (SM; 16 \u2013 31 %), which contributes to rigidification, and phosphatidylserine + phosphatidylinositol (PS + PI; 30 \u2013 44 %) which may be considered as fluidizers, similar to lecithin. In all the analyzed samples practically identical temperatures at the first phase transition were obtained. The second phase transition, when occurring, was at very different temperatures. No direct correlation has been found between this second phase transition and the SM/PC ratio also if variations of this ratio in artificial bilayers and biological membranes have a pronounced effect on the system properties. The second phase transition appeared in samples characterized by an high value of the (PS + PI)/PC ratio (> 5.4) and the transition temperature increased with the value of this ratio. Further investigations seem to be necessary to consider the importance of the fatty acid composition of the studied mixtures on the thermotropic behaviour