Stability constants amino acid complexes

Although complexes of rare earths with various substituted multi-dentate amino acids have been extensively investigated, only a limited number of rare earths have been used in studying the complexes with simpler amino acids. There is still considerable scope for investigation among the simpler members of the amino acid series. The stability constants of the rare earth amino acid complexes are compared in Table 34 and are also discussed below. 

Kalchenko, O. I., Perret, F., Morel-Desrosiers, N., and Coleman, A. W. (2001) A comparative study of the determination of the stability constants of inclusion complexes of p-sulfonatocalix [4]arene with amino acids by RP-HPLC and H-l NMR, Perkin Trans. 2 258-263. 

Table 34. A comparison of the stability constants of the amino acid complexes of rare earths... 

EXPERIMENT 7.4 STABILITY CONSTANT DETERMINATION OF CD2+ AMINO ACID COMPLEXES... 

The advantage of the LMs is integrating extraction and back-extraction of the desired analyte(s) into one step. Using protonation and deprotonation reactions, selected hydrophobic carriers with carboxyl groups have been shown effective in the separation of amino acids, if the carboxyl functionality was ionized [123]. Optimum values of the stability constants of the complexes between particular amino acids and carrier(s) can be found to increase extraction efficiencies. However, the kinetics of mass transfer often has a more pronounced impact on the efficiency of extraction [118]. 

Amino-acid Complexes. A small, but reproducible, stereoselective effect has been observed in both the free energy and enthalpy changes associated with formation of [Ni(L,L-methioninate)2]. The meso-complex [Ni(o-Met)(L-Met)] is more stable in AH than the optically active NiL2 by 1.0 (0.1) kJ mol" L The stereoselectivity is attributed to terdentate co-ordination and so supports weak co-ordination of the thioether group. Formation constants of nickel(ii) and copper(ii) with N -benzyl-L-histidine and N N -dibenzyl-L-histidine and of the ternary complexes with d- and L-histidine have been measured. Stabilization of ternary complexes is small but significant stereoselectivity is found with ternary nickel complexes, when the meso configuration is preferred in each case with copper, stereoselectivity is small or absent. The i.r. spectra of tra s-[Ni(Gly)2(H20)2] and its 0-, N-, 1- C-, and 2- C-labelled... 

Kalchenko, 0.1. JDa Silva, E. Coleman, A.W. Determination of the stability constants of inclusion complexes of p-H-37-(2-carboxy-methyloxy)-calix-f6]-arone and p-sul-phonato-37-(2-carboxy-methyloxy)-calix-[6]-arene with 15 amino acids by KP-HPLC. J. Incl. Phenom. Macrocycl. Chem. 2002,43. 305-310. 

By means of calorimetric measurements reflecting the dependence of the stability constants of metal complexes on the composition of the solvent mixture, Gergely and Kiss have shown that the stability increase resulting from a decrease in the relative permittivity shows up primarily in an increase in the — dff values, whereas the dS values decrease or remain unchanged compared with those measured in aqueous medium. A comparison of the AH values for aliphatic and aromatic amino acid complexes of copper(II) has revealed that the change in the medium acts to a lesser extent on the AH values of the aromatic amino acid complexes. This is probably due to the presence of the hydrophobic phenyl group in the aromatic ligands, as a consequence of which the extent of hydration even in aqueous medium is lower than in the case of alanine. 

With different amino acids, varying heights were observed for the limiting currents of the copper complexes of individual amino acids. This behaviour was ascribed ) to the different diffusion coefficients of the particular amino acid complexes. However the differences are so great that this explanation seems unlikely, and different stability constants of the copper complexes with individual amino acids seem to be involved. 

The stoichiometry and stability constants of cadmium(II)-amino acid complexes are collected in Tables 1 and 2 together with data of the corresponding zinc(II) complexes for comparison. 

NMR signals of the amino acid ligand that are induced by the ring current of the diamine ligand" ". From the temperature dependence of the stability constants of a number of ternary palladium complexes involving dipeptides and aromatic amines, the arene - arene interaction enthalpies and entropies have been determined" ". It turned out that the interaction is generally enthalpy-driven and counteracted by entropy. Yamauchi et al. hold a charge transfer interaction responsible for this effect. 

In aqueous solutions at pH 7, there is little evidence of complex formation between [MesSnflV)] and Gly. Potentiometric determination of the formation constants for L-Cys, DL-Ala, and L-His with the same cation indicates that L-Cys binds more strongly than other two amino acids (pKi ca. 10,6, or 5, respectively). Equilibrium and spectroscopic studies on L-Cys and its derivatives (S-methyl-cystein (S-Me-Cys), N-Ac-Cys) and the [Et2Sn(IV)] system showed that these ligands coordinate the metal ion via carboxylic O and the thiolic 5 donor atoms in acidic media. In the case of S-Me-Cys, the formation of a protonated complex MLH was also detected, due to the stabilizing effect of additional thioether coordination. ... 

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