It has been concluded therefore that tunichrome or

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It has been concluded, therefore, that tunichrome or similar ligands cannot reduce the vanadium(IV) complex; so the
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29 OH HO OH OH HO'~~ N~~)Q(H HOy 0 0 OH Tunichrome b-l H01: HO "-0 HN CfJr:: 0 ~ OH OH o ~~N NKg) OH O~ 0 OH OH HO~ HO "-0 HN « 0 ~ OH OH HO 0 ~~N N~ OH O~ 0 OH OH 3,4,5-TRENPAMH 6 OH HO~OH HN HO~O ~ OH O N~N----------Nk9t HO H 0 O~ . OH OH OH TRENCAMH 6 2,3,4-TRENPAMH 9 Figure 1.20 Structures of tunichrome b-l and synthetic analogues. 43
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30 1 / TRANSITION-METAL STORAGE, TRANSPORT, AND BIOMINERALIZATION highly reducing vanadium(III) complex of tunichrome must be generated in some other way. 47 Although a detailed presentation of examples of the known transport prop- erties of essential transition-metal ions into various biological systems could be the subject of a large book, the examples that we have given show how the underlying inorganic chemistry of the elements is used in the biological trans- port systems that are specific for them. The regulation of metal-ion concentra- tions, including their specific concentration when necessary from relatively low concentrations of surrounding solution, is probably one of the first biochemical problems that was solved in the course of the evolution of life. Iron is transported in forms in which it is tightly complexed to small che- lators called siderophores (microorganisms) or to proteins called transferrins (an- imals) or to citrate or mugeneic acid (plants). The problem of how the iron is released in a controlled fashion is largely unresolved. The process of mineral formation, called biomineralization, is a subject of active investigation. Vana- dium and molybdenum are transported as stable anions. Zinc and copper appear to be transported loosely associated with peptides or proteins (plants) and pos- sibly mugeneic acid in plants. Much remains to be learned about the biological transport of nonferrous metal ions. C. Iron Biomineralization Many structures formed by living organisms are minerals. Examples include apatite [Ca2(OH)P04] in bone and teeth, calcite or aragonite (CaC0 3 ) in the shells of marine organisms and in the otoconia (gravity device) of the mamma- lian ear, silica (Si0 2 ) in grasses and in the shells of small invertebrates such as radiolara, and iron oxides, such as magnetite (Fe304) in birds and bacteria (nav- igational devices) and ferrihydrite FeO(OH) in ferritin of mammals, plants, and bacteria. Biomineralization is the formation of such minerals by the influence of organic macromolecules, e.g., proteins, carbohydrates, and lipids, on the pre- cipitation of amorphous phases, on the initiation of nucleation, on the growth of crystalline phases, and on the volume of the inorganic material. Iron oxides, as one of the best-studied classes of biominerals containing transition metals, provide good examples for discussion. One of the most re- markable recent characterizations of such processes is the continual deposition of single-crystal ferric oxide in the teeth of chiton.
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