Uranium(VI) complexes with phospholipid model compounds – A laser spectroscopic study

We present the complex formation of the uranyl ion (UO22+) in the aqueous system with phosphocholine, O-phosphoethanolamine and O  -phosphoserine. These phosphonates (R–O–PO32-) represent the hydrophilic head groups of phospholipids. The complexation was investigated by time-resolved laser-induced fluorescence spectroscopy (TRLFS) at pH = 2–6. An increase of the fluorescence intensity, connected with a strong red-shift of about 8 nm compared to the free uranyl ion, indicates a complex formation between UO22+ and the phosphonates already at pH = 2. Even at pH = 6 these complexes prevail over the uranyl hydroxide and carbonate species, which are generated naturally at this pH. At pH = 4 and higher a 1:2 complex between uranyl and O-phosphoserine was found. Complexes with a metal-to-ligand ratio of 1:1 were observed for all other ligands. Fluorescence lifetimes, emission maxima and complex stability constants at T = 22 ± 1 °C are reported. The TRLFS spectra of uranyl complexes with two phosphatidic acids (1,2-dimyristoyl-sn-glycero-3-phosphate and 1,2-dipalmitoyl-sn-glycero-3-phosphate), which represent the apolaric site of phospholipids, show in each case two different species.