Solution interactions between the uranyl cation [UO22+] and histidine, N-acetyl-histidine, tyrosine, and N-acetyl-tyrosine

Complexes of the uranyl cation [UO22+] with histidine (His), N-acetyl-histidine (NAH), tyrosine (Tyr), and N-acetyl-tyrosine (NAT) were studied by UV–visible and NMR spectroscopy, and by potentiometric titration. Protonation constants for each ligand are reported, as are cumulative formation constants for uranyl–amino acid complexes. Coupling constant data (JCH) for uranyl–histidine complexes indicate that inner-sphere solution interactions between histidine and uranyl cation are solely at the carboxylate site. At 25 °C the major uranyl–histidine complex has a cumulative formation constant of log β110 = 8.53, and a proposed formula of [UO2HisH2(OH)2]+; the stepwise formation constant, log KUL, is estimated to be ⩽5.6 (≈8.53 − (−6.1) − (−6.1) − 15.15). Outer-sphere interactions, H-bonding or electrostatic interactions, are proposed as contributing a significant portion of the stability to the ternary uranyl–hydroxo-amino acid complexes. The temperature dependent protonation constants of histidine and formation constants between uranyl cation and histidine are reported from 10 to 35 °C; at 25 °C, ΔG = −43.3 kJ/mol.