Wednesday, April 4, 2012

Molybdenum Fixes Nitrogen

Molybdenum's odd-sounding name comes from its resemblance to lead ores. That makes three elements: plumbum (lead), carbon (plumbago), and now molybdenum named after lead. Perhaps it's not so much the color as the texture:

Native ore: MoS2

Native plumbago (graphite)

Native lead (Galena, PbS)
Molybdenum's role in life is anything but leaden. Mo is in nitrogenase, an enzyme absolutely essential for life. Nitrogenase-containing bacteria "fix" nitrogen, turning thin air into solid plant food (ammonium), a feat no less remarkable than photosynthesis. The bacteria in turn get first dibs on the sugars made by the plants in perfect symbiosis.

If artificial photosynthesis is cool--making sugars and fuels from CO2--nitrogen fixation is even more exciting because we're about a third of the way there.*  Bacteria make about 2/3 of the world's fixed nitrogen, and the ancient Haber-Bosch process does the rest.  The Haber-Bosch process, first developed in 1913, enabled Germany to fight the First World War. Today it helps feed about a third of the world's population but it also consumes about 1-2 % of the world's annual energy supply. We can do even better, perhaps by learning from nature.

Nitrogenase manages to fix nitrogen under much milder conditions than Haber-Bosch, though like many natural processes, the yield is diffuse and unusable industrially. Molybdenum's connection to nitrogen fixation was first noted in 1930 when bacteria raised on ammonia showed no need for the element while those raised on nitrogen did (before the enzyme nitrogenase had even been identified). When nitrogenase was found, the next step was to crystallize it and determine its structure. This happened in 1992, at a time when molybdenum and the heavier tungsten were already famous for loosely binding things like H2 and N2 in synthetic tungsten compounds. Some thought perhaps nature had gotten there first. Alas, the structure gave few clues as to where and how nitrogen is activated, except that it happened stepwise, using protons and electrons instead of H2.

Another really cool thing about the nitrogenase X-ray crystal structure was the revelation of a mystery element "X" which could be either a C, an N, or an O atom. Several contact points surround the mystery element X which makes that a "Texas" atom.  More recent work, published last year in Science, says it is carbon (link). More on that later.
*There are vanadium-dependent nitrogenases which convert CO to alkanes--just like Fischer-Tropsch catalysts do: link  That would be so cool if the X-factor were a Texas carbide.

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