Complex formation of anserine with uranyl in aqueous solution: Thermodynamic studies and structural analysis

Abstract

Knowledge of uranyl (UO22+) aqueous speciation and chemical interactions with natural organic matter is the basis to mode the sorption and transport behavior of uranyl. Anserine is a natural dipeptide containing β-alanine and 1-methylhistidine, which can be found in the skeletal muscle of birds and certain species of mammals. The complexation of anserine with uranyl cation was studied thermodynamically and structurally at 25 °C, I = 0.5 mol·L−1 NaCl solution. Protonation of anserine was investigated by potentiometry, microcalorimetry and NMR spectroscopy. Experimental results confirmed that there are three successive proton reaction steps for a totally deproton anserine. The order of successive protonation sites are the terminal amine nitrogen atom of alanine, amine nitrogen atoms in imidazole ring, and finally carboxylate oxygen. This trend is in agreement with exothermic enthalpies of protonation reaction. Potentiometry was used to measure stability constant of uranyl complex in solution. Thermodynamic studies identified four uranyl/anserine complexes with different degrees of deprotonation, UO2(HL)2+, UO2L+, UO2L2(aq) and UO2(OH)L2−. Coordination modes of the three complexes [UO2(HL)2+, UO2L+ and UO2L2(aq)] were investigated by NMR spectroscopy. In these complexes, anserine holds a tridentate mode. Cyclic voltammetry was performed to understand the influence of anserine exert over redox response to uranium in solution. The results confirmed that oxidation state of uranyl cation is stabilized significantly and harder to be reduced by complex with anserine. Anserine could effectively suppress the hydrolysis of uranyl in aqueous solutions.