Trace element complexation reactions, effects

It is scientifically easier to get to grips with the thermodynamics of the metal ion complexing that actually occurs in vivo rather than with the kinetics. This arises because what may well be thermodynamically feasible may not in reality be achieved because, due to slow kinetics, the time required for complex formation is longer than the effective residence time of the complexing molecule in the region of the metal. Conversely, the manifold presence of enzymes in vivo can well speed up a reaction involving a trace element by many orders of magnitude. Thus,... 

Seawater contains dissolved organic matter that may complex trace metals. The possible effects in the analysis depend on the method of analysis and the element itself In methods using a complexation step (AAS, ADPCSV) the naturally present ligand may inhibit the complexation reaction or part of it, thus causing an underestimation of the total amount of the element. The method used should therefore be tested for the complexation efficiency of the complexants used, for each element. Several proven methods have been used in the certification. Natural ligands also interfere in... 

The kinetics and mechanisms of chemical reactions in soils have been broadly studied, and comprehensive mathematical models for the particular soil conditions have been presented (Bolt 1979, Huang 2000, Sauve 2001, Schmitt and Sticher 1991, Sparks 1999, Tan 1998). The diversity of ionic species of trace elements and their various affinities to complex inorganic and organic ligands make possible the dissolution of each element over a relatively wide range of pH and Eh. In most soil conditions the effect of pH on the solubility of trace cations is more significant than that of redox potential (Chuang et al. 1996). However, redox potentials of soils also have a crucial impact on the behavior of trace elements (Bartlett 1999). 

Chelating agents, like all chemical compounds, exhibit toxic effects. These can arise from the fact that they increase the excretion of essential trace elements or from more subtle interactions. Thus all compounds which contain sulfhydryl compounds are capable of causing allergic reactions, such as the skin rash reported for DMSA (Grandjean et al. 1991), or the numerous problems which may arise from the continued administration of d-penicillamine such as nephrotic syndrome and anuria (Dubois et al. 1990). The administration of EDTA by itself may result in tetany due to the rapid drop in serum calcium which results, this being the reason for its customary administration as the calcium complex. 

The role of Cu as an essential trace element has focused attention on possible roles for copper chelation of biologically active ligands, with subsequent interference of normal transport and distribution, as well as the role of the metal in redox reactions due to the accessible oxidation states of (I) and (II). Similarly, the physiological response of copper levels in disease conditions [50] and the overall role of trace metals in health and disease [51, 52] are relevant and of considerable importance. The increase in serum copper content in infections, arthritic diseases, and certain neoplasms is well documented and, in fact, the subsequent decrease in level upon treatment has been used successfully as an indicator of cancer remission [50]. Copper complexes may be effective in therapy due in part to their ability to mimic this physiological response of elevated copper [53] and, clearly, the interplay of introduced copper with pre-existent bound copper and effects on copper—protein mediated processes will affect the ultimate biological fate of the complex. Likewise, while the excess accumulation of free Cu, and indeed Fe and Zn, caused by malfunction or absence of normal metabolic pathways is extremely damaging to the body, the controlled release of such metals may be beneficially cytotoxic. The widespread pharmacological effects of copper complexes have been briefly reviewed [54]. 

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