31P NMR of tissue phospholipids: Competition for Mg2+, Ca2+, Na+ and K+ cations

Phosphatidylcholine (PC), phosphatidylethanolamine (PE), ethanolamine plasmalogen (EPLAS), sphingomyelin (SPH), phosphatidylinositol (PI), phosphatidylserine (PS), cardiolipin (CL), phosphatidylglycerol (PG) and phosphatidic acid (PA) were dispersed together in Cs(ethylenedinitrilo)tetraacetic acid-scrubbed chloroform/methanol solution, and high resolution³¹P nuclear magnetic resonance spectra were recorded. In separate titration experiments, Mg²⁺ and Ca²⁺ were added to the dispersed phospholipid mixture to determine the relative interaction potentials of each of the phospholipids for each of the added cations. The association of cations with individual phospholipids was indicated by³¹P chemical-shift changes, signal broadening, signal quenching or a combination of these. The titrations revealed that CL had the highest, and PA the next highest, interaction potential for Mg²⁺ cations. In contrast, PS and PA had the highest, and CL the next highest, interaction potential for Ca²⁺. Considering only interactions with Ca²⁺ ions, the phospholipids can be divided into three distinct groups: PS and PA (high interaction potential); CL, PI and PG (intermediate interaction potential); and EPLAS, PE, SPH and PC (essentially no interaction potential). The two phospholipids with the least interaction potential for either of the alkaline-earth cations were PC and SPH. Na⁺ and K⁺ ion interactions with PA, CL, PI and PG were unique and resulted in positive chemical-shift changes relative to the chemical shifts in the presence of Cs⁺ ions. Relative to both Cs⁺ and K⁺ ions, chemical shifts in the presence of Na⁺ ions were deshielded δ>0.1 ppm in the order PA>CL>PI>PG.