Calorimetric evidence for phase transitions in spin-label lipid bilayers

Dispersions of pure, spin-label phosphatidylcholines in aqueous buffer have been investigated with the Privalov high-sensitivity differential scanning calorimeter. The lipids studied are mixed-chain ones in which C-2 of glycerol bears a spin-label derivative of stearic acid and the fatty acid group at C-1 is palmitate. A well-defined phase transition is observed at 30.3-30.7 degrees C for the phosphatidylcholine labeled near the polar end of the stearate chain (label at C-5). A sharp transition (32-34 degrees C) is also observed for the lipid spin-labeled near the terminal methyl of stearate (label at C-16), but the thermodynamic parameters for this lipid depend strongly on the history of the sample. Calorimetric evidence for hysteresis in the phase transition of the C-16-labeled lipid is presented. In contrast to the above spin-label lipids, the lipid labeled at C-12 does not show a sharp transition in the region 5-35 degrees C. In general, therefore, the thermal behavior of the spin-label phosphatidylcholines resembles that of phosphatidylcholines bearing double bonds or branched methyl groups at similar locations on acyl chains. During synthesis of mixed-chain lipids, migration of acyl chains occurs. Methyl esterification procedures which are compatible with the acid-labile spin-label group are described. Gas chromatographic analysis of methyl esters shows that chain migration during synthesis gives 15-20% of the spin-label fatty acid at the glycerol C-1 position.