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OVERVIEW
Metalloenzyme controls a diverse array of biological and physiological functions important for life. Designing the synthetic model is best way to investigate metalloenzyme active-site chemistry. Functional models can provide an opportunity to examine a biological reactivity at a small-molecule level of detail through systematic and comparative studies. Biological code of reactivity can be decoded by examining the biological reactivity, creating similar chemical architectures, and determining functional reaction conditions for model systems. Reproducing complex biological behavior within a simple synthetic molecule is a challenging research field with both intellectual and aesthetic goals. Biomimetic studies need not necessarily duplicate chemical or physical characteristics of the enzyme. Rather, by careful design and systematic variations, one might help identify those factors (e.g. donor atom type, coordination geometry, metal redox potential) contributing to the observed enzyme structure, spectroscopy and mechanism. Chemical models can provide reference compounds for structural and coordination aspects, or provide insights into the plausibility of biochemically proposed ‘active’ intermediates and their reaction competency. This type of information and insight is often not available from direct enzyme studies. The reaction mechanism of naturally occurring metalloenzymes are notable for many reasons. Metalloenzyme-catalysed oxidations often exhibit exquisite substrate specificity as well as regioselectivity and/or stereoselectivity, and operate under mild conditions through inherently ‘green’ processes. Moreover, metalloenzymes are sometimes able to alter the function of inert substrates in such a ways that synthetic chemists find difficult to replicate (for example, changing methane to methanol). Although one goal of modeling is reproduction of reactivity, extension of this reactivity beyond the scope of the inspiring system is perhaps an even more important objective. Biology provides many elegant examples of selective and environmentally benign oxidants capable of performing interesting organic transformations. Model complex have an added advantage over metalloenzyme systems, insofar as they might expand the scope of possible substrates, increase the scale of production and tune selectivity and/or specificity . The study of model compounds continues to significantly contribute to our understanding of the role of transition metals at the active sites of enzymes. Thus, the understanding of the structure–function relationship of their active sites will allow the design of effective and environmental friendly oxidation catalysts. Bio-inorganic models may also lead to compounds that mimic enzyme function and provide new reagents or catalysts for practical application .
Biochemical inspirations
Vanadium and copper ions are the metal ions of choice for many biological oxidations because of their abundance in the geosphere, inherent electronic properties and accessible redox potentials.
Vandium haloperoxidase
Vanadium has been reported to be an essential bio-element for certain organisms, including tunicates, bacteria and some fungi. Vanadium with 136 ppm (0.0136 %) of the earth’s crustal rocks is the nineteenth element in the order of abundance. It is also present at very low concentrations (<10−8 M) in the cells of plants and animals.
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In this context modeling of the active site of vanadium containing protein vanadium haloperoxidase and copper containing protein galactose oxidase has a great interest within the scientific community.
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Active site of Galactose Oxidase
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