The thermodynamic parameters for the complexation reactions of serine, threonine, homoserine, isoserine, 4-amino-3-hydroxybutyric acid, and /3-aminobutyric acid with copper(II) and nickel(II) in aqueous solution at 25°and µ = 0.16 are reported. Values for the formation constants were obtained from potentiometric titrations and enthalpy values determined calorimetrically. The -amino acids studied, as well as /3-aminobutyric acid, were postulated to form the typical amino-carboxyl chelates with copper(II) and nickel(II). The formation of a sixmembered chelate ring was found to be less favorable thermodynamically than the formation of a five-membered chelate ring for both copper(II) and nickel(II) complexes. The thermodynamic data seem to indicate the formation of a chelate ring involving the amino and hydroxy groups of the isoserine with the copper(II). The isoserine ligand was postulated to be functioning as either a bidentate or tridentate ligand with copper(II).It has been shown that for a number of aliphatic amino acids the formation of copper(II) complexes proceeds in a stepwise manner to form the mono and bis copper(II) species.2 These complexes are formed by the elimination of a proton from the zwitterion form of 1 and 2 moles of amino acid, respectively. Chelation occurs between the carboxyl and amino groups of the amino acid and the copper(II) ion. The formation of nickel(II) complexes with aliphatic amino acids proceeds in the same manner, except that a third mole of amino acid can be chelated to the nickel(II) ion to form the tris nickel(II) species. In the case of amino acids interacting with copper(II) there seems to be some dis-agreement3-5 as to whether or not the amino acid zwitterion can function as a monodentate ligand. Recent studies of the interaction of copper(II) with gly-(1) Based on the dissertation of J. E. Letter submitted to the Graduate
The rate constants for complexation of nickel(II) by imidazole, histidine, and histidine methyl ester have been measured at 23.7°in the pH range 5.6-6.9 and ionic strength 0.1 M (KN03). For imidazole the protona ted (k = 400 M~x sec-1) and neutral (k = 3.2 X 103 M~x sec-1) forms of the ligand are found to react. Histidine is observed to react only through the neutral form (k = 2.2 X 103 M~l sec-1), in which one proton is removed from the imidazole group. Reaction is observed for histidine methyl ester both as the monocation (k = 600 M~x sec-1) with the imidazole proton removed and as the neutral molecule (k = 2.6 X 103 M~x sec-1) in which both the amino and imidazole protons are removed. The results are consistent with a mechanism in which the imidazole part of the histidine derivatives complexes first with the rate constants showing the expected variation with ligand charge. A second slower reaction is observed in the two histidine systems. This is attributed to the formation of some dinickel species by complexing at the pyrrole nitrogen of the coordinated imidazole part of the histidine ligand.
The kinetics of complexation of nickel(II) by (V-methylimidazole, 3-methylhistidine, and 1-methylhistidine have been studied at 23.7°in 0.10 M KN03. For IV-methylimidazole the protona ted cation (k = 2.3 X 102 M"1 sec"1) and neutral molecule (k = 4.5 X 103 M"1 sec"1) are observed to react with nickel(II). Only reaction of the neutral form of 3methylhistidine (k = 2.1 X 103 Vtf"1 sec"1) was detected. This system was different from the previously studied histidine in that it did not show a second slow reaction. This observation is consistent with the previous proposal that the second reaction involves coordination of a second nickel(II) at the pyrrole nitrogen of the coordinated imidazole part of histidine. A two-step reaction was observed for 1-methylhistidine, the first step being attributed to complexing at the imidazole part of the molecule and the second step involving coordination of a second nickel(II) at the glycine part of 1-methylhistidine. Thus the latter ligand acts as two complexing units and does not show any tendency to chelate in a histidine-like manner.
The thermodynamic parameters for the complexation reactions of tyrosine, m-tyrosine, o-tyrosine, and ßphenylserine with copper(II) in aqueous solution at 25°and µ = 0.16 M are reported. The thermodynamic data indicate the formation of amino-carboxyl chelates for these amino acids with copper(II) in the pH range 3.5 to 6.5. In addition, the deprotonation reactions of the bis copper(II) complexes of tyrosine, m-tyrosine, and otyrosine were studied to determine the sites of deprotonation. The proton dissociations for the copper(II) complexes were postulated to occur from the phenolic oxygens of the bound amino acids. No bonding of the phenolic oxygen to the copper(II) was postulated for the tyrosine and m-tyrosine complexes. For the bis copper(II) complex of o-tyrosine the thermodynamic data are consistent with the formation of chelate rings involving the negatively charged phenolic oxygens and the amino nitrogens with copper(II).
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