Chelating association constant

Optical activity in metal complexes may also arise either if one of the ligands bound to the metal in the first co-ordination sphere is itself optically active or if the complex as a whole lacks a centre of inversion and a plane of symmetry. Thus all octahedral cts-complexes of the tris-or bis-chelate type have two isomeric forms related by a mirror plane, the d- and /-forms. These species have circular dichroism spectra of identical intensities but opposite in sign. The bands in the circular dichroism spectrum are, of course, modified if ligand exchange occurs but they are also exceedingly sensitive to the environment beyond the first co-ordination sphere. This effect has been used to obtain association constants for ion-pair formation. There also exists the possibility that, if such compounds display anti-tumour activity, only one of the mirror isomers will be effective. 

If the binding between the paramagnetic chelate, ML, and the protein, Prot, involves a single class of equivalent binding sites, the equilibrium is characterized by the association constant, ATa ... 

Rate constants for the formation of complexes from the aquometal ion and various chelating ligands are often predicted from the expected outer-sphere association constant (K ) and... 

This phenomenon has been known for many years to inorganic chemists as the chelate effect. The magnitude may be illustrated by one of their examples, the replacement by ammonia and polyamines of some or all of the six water molecules that are coordinated to the Ni2+ ion. It is seen in Table 11.7 that there are enormous increases in the association constants of the ligands as the number of amino groups increases. 

Remarkably, enhanced binding by the chelate effect is occasionally seen not in free energies of complexation the association constants of e.g. amino acids with sulfonatocalix[4]arenes are quite close the ones observed with phenol-4-sulfonic acid as single host unit Only AH shows an advantage of the macrocycle with AH = 30 compared to 1 kJ/mol, compensated by a much smaller entropy disadvantage with the smaller monomeric host, which changes from e.g. TAS = -12 kJ/mol to favorable +12 kJ/mol.[34]... 

The equilibrium association constant, fCM +0 cicL, °ften represented as the overall metal ion stability constant /3 , associated with the binding of a metal ion M"+ of valence w+ to an immobilized monovalent chelating compound CheL can be represented as... 

Other chelating ligands have been identified with more optimal association constants for protein binding as well as lower metal ion leakage per se. The combination of hard and soft Lewis acid-base interactions100,114 between the acceptor metal ion and donor N- or O-groups of the protein or... 

Soon afterward, Kronman, who has made a long study of a-lactalbumin reactions, considered that there was an experimental artifact in the use of chelating metal ion buffers (e.g., EGTA and EDTA) in the determination of association constants for metal ions with proteins by fluorescence titration. Kronman and Bratcher (1983) concluded that their observations explained the discrepancy between As,i for Ca(ll) and bovine a-lactalbumin reported by Kronman et al. (1981) (2.7 X 10 M ), Permyakov et al. (1981) (6.3 X 10 M" ), and Murakami et al. (1982) (4 x 10 Af- ). 

Achiral lanthanide chelates can also be added to CSAs such as arylperfluoroalkyl-carbinols, the ethyl ester of 3,5-dinitrobenzoyl-L-leucine (25) , the 3,5-dinitrobenzoyl derivative of 1-phenylethylamine, Af-(l-(l-naphthyl)ethyl)trifluoroacetamide (26) and a series of l-(l-naphthyl)ethyl urea derivatives of amino acids (27) to enhance the enantiomeric discrimination. With sulfoxide or lactone substrates , the europium ion preferentially associates with the substrate in the bulk solution. Provided the enantiomers have different association constants with the CSA, the isomer that shows the weaker association with the CSA shows the larger lanthanide-induced shifts. Low concentrations of lanthanide relative to the substrate and CSA lead to enhancements of enantiomeric discrimination in the NMR spectrum. If the concentration of lanthanide is too high, binding of the substrate to the lanthanide strips the substrate from the chiral solvating agent and diminishes the chiral discrimination in the NMR spectrum. 

Lactoferrin has been isolated and identified from a wide variety of animal species. However, most of the studies on structure and iron-binding properties have involved either human or bovine proteins (2). Lactoferrin closely resembles transferrin in molecular weight of 75,000 to 90,000 and consists of a single polypeptide chain that binds two ferric ions. The pi of transferrin is 5.9 while that of lactoferrin is approximately 9.0 (8) and has an even higher association constant for iron-binding. Lactoferrin has the property of retaining its iron even in the presence of a relatively low-affir-nity iron chelator such as citrate below pH 4.0. Transferrin, on the other hand, looses its iron when the pH is lowered from 6 to 5 (7). There is extensive information in the literature concerning the physical properties of lactoferrin which will not be covered in this paper. 

If complexes and chelates involve intimate chemical interactions, the extent of association should reflect the chemical nature of the ions involved. Equilibrium constants should be different, and possibly even very grossly different, for equilibria which superficially seem very similar and alike, for instance, association of one species with ions of similar size and charge. The situation is reminiscent of that found for the dissociation constants of weak acids and bases where the magnitude of the equihbrium constants depends on the chemical nature of the species involved. It is in stark contrast to that expected for the formation of ion pairs, where the magnitude of the association constant is expected to be independent of the chemical nature of the ions involved. 

The interaction of aspartic acid and other ligands with complexes of Tb " with edta and related ligands has also been studied and association constants determined. The complex formation between Tb " or Eu " and (r)-( — )-l,2-propanediaminetetraacetic add or (r,r)-trons-1,2-cyclohexanediaminetetraacetic acid has been similarly investigated. The pH dependence of the circularly polarized and total luminescence shows a drastic configurational change of the chelate system at pH 10.5-11, corresponding, it is believed, to formation of hydroxide complexes. Tlie technique of magnetic-field-induced circularly polarized emission has been introduced for lanthanide ions the mechanisms of lanthanide transition intensities are also discussed in the paper. 

This case has additional convenience in the possibility of using colored anion (commonly picrate), which is useful for monitoring distribution but is inert with respect to complexation. This technique allowed the determination of a large body of association constants, thus providing a basis for further structure-stability correlations, and Cram s picrate method became a de facto standard. (One experienced in metal ion extraction may note that this approach evidently parallels the well-known distribution method for determining chelate stability constants [34. )... 

The bidentate diborane 126 selectively complexes fluoride anion (see Scheme 13) with an association constant >5x 10 M in THE, and can serve as a colorimetric anion sensor <2004CC1284>. Four-coordinate boron N-heterocycle complexes (L-N,N )BPh2 exhibit luminescence and electroluminescence in a broad range of visible wavelengths depending on the substituents on the chelate heterocycle <2005MI143>. 

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