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

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Abstract
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 .gtoreq. 2 mol % gel and .gtoreq. 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 EPR signal from TEMPO [2,2,6,6-tetramethylpiperidine-1-oxyl] partitioned between fluid lipid and aqueous phases. Phase diagrams for egg PA/egg PC, egg PA/DMPC and egg PA/DPPC with and without Cd2+ were determined in the temperature range 0.degree.-55.degree. C. In the 1st system, the addition of Cd2+ raises the midpoint of the egg PA transition temperature range from 14.degree. to 26.degree. 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 .apprx. 25.degree. 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.degree.-35.degree. C temperature range. In the 2nd and 3rd systems, Cd2+ extends the temperature range over which the gel and liquid-crystal phases coexist. The phase properties of egg PA at 23.degree. C were unaffected by pH in the range 6.2-8.2 and by high concentrations of Li+, Na+, K+, Cs+ F-, Cl-, Br- and I-. The divalent cations Mg2+, Ca2+, Sr2+, Ba2+, Cd2+ and UO22+ induced gel phase formation in the egg PA/egg PC system. In the case of Ca2+ and Ba2+ sharp wide-angle lines in addition to the (4.2 .ANG.)-1 gel phase line were observed, suggesting a highly organized packing of the acyl chains.