10. Playing with a Full Deck:
       The Periodic Table
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       d Orbitals in Bonding

The antibiotic valinomycin and several other antibiotics work in the same way as EDTA. Valinomycin is known to make it easier for potassium ions to get through biological membranes, and x-ray crystallographers found out why recently when they solved the structure of the potassium salt of the antibiotic. The valinomycin molecule is a closed ring of 36 atoms, with 12 carbonyl (C=O) groups. It wraps around the potassium ion with six of the carbonyl oxygens as octahedral ligands, and effectively gives the ion an organic molecule "overcoat." The coated ion can slip through a membrane more easily because its charge is disguised. Part of the toxicity of this antibiotic to microorganisms is believed to lie in its upsetting of the natural balance of potassum ions within the cell membranes.

The iron atom in the oxidation-reduction protein cytochrome c is octahedrally coordinated, with four of the six ligands coming from one large, planar porphyrin molecule, as shown below. This ironporphyrin complex is called a heme group. In cytochrome c the fifth and sixth octahedral ligands, above and below the plane of the heme group, are provided by a nitrogen and a sulfur atom from the protein wrapped around the heme. The delocalized electrons in the porphyrin ring are shared with the iron, thereby changing its oxidation--reduction behavior. Hemoglobin is another combination of heme groups and protein, in which the fifth octahedral ligand is a nitrogen atom from the protein, and the sixth position is left open for binding the molecule that each hemoglobin molecule carries in the bloodstream.

Four of the six octahedral ligand positions of are occupied by four nitrogen lone pairs from the flat porphyrin ring, and the fifth and sixth positions are filled by other chemical groups. The double-bond electrons in the porphyrin ring actually are delocalized over the entire ring system.
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