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 (J(CH)) 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 beta(110)=8.53, and a proposed formula of [UO(2)HisH(2)(OH)(2)](+); the stepwise formation constant, log K-UL, is estimated to be <= 5.6 (approximate to 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 degrees C; at 25 degrees C, Delta G=-43.3 kJ/mol. (c) 2008 Elsevier Inc. All rights reserved.