Coordination equilibria of carbohydrate-type ligands

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Carbohydrates are for the coordination chemist biologically active polyfunctional ligands. Under physiological conditions they are present in solutions containing metal ions. This alone justifies the study of their coordination equilibria. The effect of the presence of metal ions on the biological activity of carbohydrates, indicating metal complexation, has been shown in several cases, e.g. in calciumherapin systems. Metal complexes of carbohydrates are of vital importance in human and veterinary therapy ( e.g. iron(III) complexes of sugar-type ligands, etc. Investigation of the coordination equilibria of carbohydrates is made difficult, however, by the usually low stability of their complexes resulting in the appearance of competing equilibria ( e.g. hydrolysis), by the overlap of pH-dependent and pH-independent processes, by the lack of suitable electrodes for the study of the latter, by the formation of polynuclear species due to the bridge forming ability of this type of ligand. Conformational and configurational features also influence strongly the complex formation equilibria. These are the main reasons why little research has been performed so far in this field [1, 2]. The lecture covers our recent equilibrium studies on 1. iron(III) complexation of sugar-type ligands (lactose, galactose, lactobionic acid, gluconic acid, dextrane, etc.); 2. formation of mixed ligand complexes of iron(III) in the above systems; 3. copper(II) complexation of sugars and amino-sugars; 4. calcium(II) and zinc(II) complexation of heparin in the presence of alkali metal ions; 5. the protonation of carbohydrate-type ligands leading to the determination of the composition and stability constants of the complexes in each system. The experimental procedure is based on spectrophotometric, potentiometric and electrophoretic equilibrium measurements. For the separation of pH-dependent and pH-independent processes self prepared and commercial ion selective electrodes are used. The primary experimental data are processed by computer evaluation. The equilibrium studies are complemented by structural methods ( e.g Mössbauer spectroscopy). The results reflect the complexity of each system, the simultaneous presence of several species in them, the sensitivity of the complex formation equilibria on internal (structure of ligand) and external (matrix) factors. The equilibrium constants have been used for the calculation of the concentration distribution of species of different compositions.