MANGANESE

JOAN SELVERSTONE VALENTINE, UNIVERSITY OF CALIFORNIA, LOS ANGELES

My first impression of manganese was decidedly negative, the result of an unfortunate encounter with a table of reduction potentials in high school. To my eye, that table contained a bizarre array of manganese compounds with no apparent logic to their oxidation states or to the number of oxide ions that each contained. How was a naive student to make sense of the fact that the oxidation states of manganese vary so widely, from +7 to +2?

I still recall puzzling over Mn(VII)O₄^–, Mn(VI)O₄^2–, Mn(V)O₄^3–, Mn(IV)O₂, Mn(III)PO₄, and Mn(II)Cl₂. I'd have been even more distressed had I known then that the oxidation states actually keep going down to –3, e.g., Mn(-III)(CO)₄^3–. I might have found manganese more appealing if I had been aware of the beautiful colors of its various forms: violet Mn(VII), green Mn(VI), and blue Mn (V). However, at the time I was familiar only with manganese dioxide--dark brown, ugly, and insoluble.

And it was more than disturbing to me how easy it was to confuse the words manganese and magnesium. Enlightenment came in the shape of the "Oxford English Dictionary," where I learned that the words magnesium and manganese can be traced back to the same root, magnesia, the postclassical Latin name for a type of mineral ore from the ancient city of Magnesia. This ore consisted mainly of oxides and carbonates of magnesium and manganese. If, as some suppose, the ore also contained talc, Mg₃Si₄O₁₀(OH)₂, then a silvery luster may have been imparted to the ore, which could account for its attraction for alchemists.

"Magnesia alba," or white magnesia, apparently was mainly hydrated magnesium carbonate and oxide. "Magnesia niger," or black magnesia, was manganese dioxide. For a while, manganese was actually named "magnesium." Carl W. Scheele proposed in 1774 that black magnesia contained a new metallic element, and Torbern Bergman proposed the next year that it be named "magnesium." It wasn't until 1780 that the name settled down to manganese.

Magnesia was of particular interest to alchemists, who believed it to be an important component of the Philosopher's Stone, "a legendary substance with astonishing powers. The stone will transform any metal into pure gold. It also produces the Elixir of Life, which will make the drinker immortal." (J. K. Rowling, "Harry Potter and the Philosopher's Stone," London: Bloomsbury, 1997, page 161.)

	OLD SCIENCE An early illustrated edition of the alchemical tracts of Jabir Ibn Hayyan, who, among other things, described the use of manganese dioxide in glassmaking. UNIVERSITY OF ILLINOIS, URBANA-CHAMPAIGN

And, not surprisingly, it was an alchemist, Jabir Ibn Hayyan, who first described the use of manganese dioxide in glassmaking. This extraordinary scientist, who lived in the late 8th or early 9th century, not only made many fundamental discoveries in inorganic chemistry but very likely was also the originator of the methods of modern chemical research.

Fortunately my attitude about manganese changed: I am an admirer. I especially appreciate the rich complexity of its biological inorganic chemistry. But not everything about manganese is positive. It has a dark side: It is toxic. Manganese at high levels is, for example, a serious hazard to steelworkers, miners, and welders, and the symptoms of manganese toxicity can mimic Parkinson's disease. Moreover, there is some indication that low-level manganese exposure may accelerate Parkinson's in some susceptible people.

Evidence of the element's biological inorganic chemistry is to be found far from the laboratory, namely on the ocean floor, much of which is strewn with manganese nodules, especially in the Pacific. These nodules, apparently of biogenic origin, are dark brown, slightly flattened spheres about 5–10 cm across. When split open, a core surrounded by concentric rings is revealed. Often the core is a bit of another nodule, a piece of rock, or even a shark's tooth. Apparently, the nodules do not become buried by sediments due to "bioturbation," motion caused by animals. Are marine creatures perchance playing pool at the bottom of the sea?

Although others might say that the most important transition metal in biology is iron, I think a good case could be made for manganese, which seems to be required by all forms of life. Perhaps there is an organism that has no need for it, but if so, I have yet to hear about it. No one could say this about iron: Lactobacillus plantarum, a lactic acid bacterium, and Borrelia burgdorferi, the bacterial pathogen that causes Lyme disease, apparently have no need for iron at all. Instead, they require manganese, which to my thinking implies that manganese can do everything that those bacteria would otherwise need iron to do. That raises an interesting question: Would multicellular forms of life based on manganese have evolved if iron didn't exist? Maybe so, but then what form and what colors would manganese-based, hemeless multicellular organisms take? Perhaps an alchemist could tell us.

Joan Selverstone Valentine is professor of chemistry and biochemistry at University of California, Los Angeles, and editor of the ACS journal Accounts of Chemical Research. She and her research group explore mechanisms of oxidative stress and the link between copper-zinc superoxide dismutase and ALS (Lou Gehrig's disease).

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