Speciation Study on O-Phosphorylethanolamine and O-Phosphorylcholine: Acid-Base Behavior and Mg Interaction

In the present study, the acid-base behavior of compounds constituting the headgroups of biomembranes, O-phosphorylethanolamine (), and O-phosphorylcholine () was investigated by potentiometric titrations in NaCl aqueous solutions at different temperatures (15 ≤ /°C ≤ 37) and ionic strength (0.15 ≤ /mol L ≤ 1) values. The complexation properties and the speciation of these ligands with Mg were defined under different temperatures (15 ≤ /°C ≤ 37) and = 0.15 mol L. The results evidenced the formation of three species for , namely, MLH, MLH, and ML and two species for , namely, MLH and ML. H-NMR titrations were performed on solutions containing ligand and metal-ligand solutions at = 25°C and = 0.15 mol L. The estimated values of ligand protonation and complex formation constants and the speciation model are in accordance with the potentiometric data. The enthalpy changes were also determined at = 25°C and = 0.15 mol L by the dependence of formation constants on the temperature, confirming the electrostatic nature of the interactions. Matrix-assisted laser desorption mass spectrometry (MALDI-MS) was applied for the characterization of Mg-L systems (L = or ). MS/MS spectra of free ligands and of Mg-L species were obtained. The observed fragmentation patterns of both Mg-L systems allowed elucidating the interaction mechanism that occurs the phosphate group generating a four-membered cycle.