Coordination numbers carboxylate groups

A hydroxyl group is situated ortho to a carboxyl group which as a bidentate ligand is terminally metallized on the fiber when aftertreated with dichromate. An example is Alizarine Yellow GG [584-42-9] (50) (Cl Mordant Yellow 1 Cl 14025). Cr(III) has a coordination number of six, and therefore normally two dye molecules of the sahcyhc type are chelated to the metal ion. 

The slow acetylation of the hydroxyl group is difficult to explain. One is inclined to suggest that this group is coordinated to the metal ion and consequently rendered inactive. Such a possibility requires either a coordination number of 7 for chromium (III) or displacement of carboxylate from the coordination sphere by hydroxyl. In the latter instance an uncoordinated functional group would still be present to react with ketene (COO or COOH) and anhydride should be detected in the crude reaction product. This is not the case. 

An example of a high coordination number for cadmium is found in aquabis(4-aminobenzoato)cadmium(II),793b which involves seven-coordinate cadmium, with two chelating carboxylate groups, a water molecule and axial positions filled by amino groups from ligands bound to the neighbouring metal centres.794... 

A number of interesting studies of lactam hydrolysis have been published. The metal(II)-catalyzed hydrolysis of some penicillin and cephalosporin derivatives displays saturation kinetics.410"412 A 1 1 complex is formed between the metal ion and penicillin which undergoes base hydrolysis up to 10s times faster than the free ligand. The catalytic activity follows the order Cu,>ZnII>NiII = Co11. Coordination of penicillin to copper(II) is believed to occur via the /3-lactam nitrogen and the carboxylate group (121)4l0-4n but other sites have been proposed.413... 

Although many of these general features are correct, these mechanistic conclusions are based upon the assumption that the properties of the enzyme in solution are conserved on crystallization. It appears from Raman studies on arsanilazotyrosine-248 carboxypeptidase that the enzyme exists in solution in a number of different forms.509 It is not certain that the form which crystallizes out is the kinetically active species. Indeed there is evidence that the kinetics of carboxypeptidase in solution differ from those of the enzyme crystals, with the crystalline enzyme being 1000-fold less active with some substrates.510 The interaction of Gly-L-tyr with Mn carboxypeptidase A, studied by 1H NMR techniques, does not involve coordination of the carboxyl group of the substrate, and may well represent a different conformation from the one studied by crystallographic techniques.511 Several conformational forms of the Cd11 carboxypeptidase A have been found in solution, while the enzyme exists in a different form in the crystal state.312... 

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