Influence of metal ions on the phase properties of phosphatidic acid in combination with natural and synthetic phosphatidylcholines: an X-ray diffraction study using synchrotron radiation.

Synchrotron radiation was used as an X-ray source of extreme brilliance to investigate by X-ray diffraction the phase behavior of fully hydrated unoriented samples of egg phosphatidic acid (egg PA) in combination with egg phosphatidylcholine (egg PC), dimyristoylphosphatidylcholine (DMPC), or dipalmitoylphosphatidylcholine (DPPC). The diffraction method was sensitive to greater than or equal to 2 mol % gel and greater than or equal to 30 mol % liquid-crystal phase lipid. Accordingly, the liquidus boundary was determined more exactly than the solidus boundary in these phase diagrams. A more precise location of the solidus line was obtained by monitoring the electron paramagnetic resonance signal from TEMPO partitioned between fluid lipid and aqueous phases. Phase diagrams for (i) egg PA/egg PC, (ii) egg PA/DMPC, and (iii) egg PA/DPPC with and without Cd2+ were determined in the temperature range 0-55 degrees C. In system i, the addition of Cd2+ raises the midpoint of the egg PA transition temperature range from 14 to 26 degrees C and results in a wide region of gel-liquid-crystal phase coexistence. With as little as 5 mol % egg PA in egg PC, Cd2+ induces phase separation up to about 25 degrees C. The phase behavior of egg PA/spin-labeled PC in the presence of Cd2+ was very similar to that of egg PA/egg PC in the 10-35 degrees C temperature range. In systems ii and iii Cd2+ extends the temperature range over which the gel and liquid-crystal phases coexist.(ABSTRACT TRUNCATED AT 250 WORDS)