Solution Chemistry of Rare Earth Complexes with Amino Acids

3 Solution Chemistry of Rare Earth Complexes with Amino Acids 

While studies on the crystal structures of RE(III)-amino acid complexes can give us clear pictures on the ways in which RE(III) ions and the amino acids bond to each other, their solution chemistry, which deals with the reactions in solution, the chemical species formed, their stability, as well as their distribution over certain pH ranges, can help us understand better the in vivo behaviors of RE(III) ions and their complexes with amino acids. Work on the solution chemistry of RE-amino acid complexes has been carried out since the early 1960s [9]. It has been found that the amino acids studied behave very similarly to one another, just as we have learned from their structural chemistry. Mononuclear species with 1 1 and 1 2 (RE L) stoichiometry have been reported for all of the amino acids. In some studies, the presence of mononuclear species with stoichiometry 1 3, dinuclear species with stoichiometry 2 4 and 2 6, in addition to the hydrolyzed species, such as [RE(OH)L]+, [RE(OH)] +, and RE(OH)3(s), have been confirmed [126, 135, 136]. 

Early studies on RE(III)-neutral amino acid and RE(III)-basic amino acid systems (14 neutral amino acids Gly, Ala, Val, He, Leu, Pro, Trp, Tyr, Met, Ser, Thr, Cys, Asn, and Gin and three basic amino acids His, Lys, and Arg ) indicate that they form only 1 1 and 1 2 (RE L) mononuclear species [REL] + and [REL21+, withlogPi around 4-6 and log P2 around 6-8. Compared with the corresponding RE(III) complexes with acetic acid (log Pi = 1.53 log P2 = 2.44 pffa = 4.56) (Table 3.1), a substantial increase in the complex stability is observed for all of the RE-amino acid complexes, indicating chelation between the carboxylato-oxygen, amino-nitrogen, and the RE(III) cations. Meanwhile, thermodynamic parameters and NMR studies showed that the -OH groups in Ser and Tyr are involved in the coordination of their 

Only a few studies have been done for the RE-basic amino acid system. A comprehensive study of RE(III) complexes with His revealed that [RE(HisH)] +, [RE(His)2] , and [RE(His)] + are the major species formed in the solution, and hydroxo, such as RE(OH) + or 

The two acidic amino acids Glu and Asp have an extra carboxyl group. They can be expressed as H3GIU+ or H3ASP+ when the a-carboxylates are also protonated. Most of the studies show that they form both [RE(L)] + and [RE(L)2] species in the solution with log Pi and log P2 around 5 and 9, respectively, which are close to the data for the neutral and basic amino acids [9], This is an indication of the similar chelation modes among the RE-amino acid complexes, and the P- or y-carboxy oxygen in Asp or Glu may not be involved in the chelation of the a-carboxy oxygen and the amino nitrogen with the RE(III). 

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