Nitrogen is all around us, making up 78% of the air we breathe, yet we do not notice its presence; an atmosphere of 100% nitrogen, although nontoxic, is fatal. Leguminous bacteria, living in the root nodules of plants such as clover, have an advantage over other living things, as they can convert atmospheric nitrogen to nitrate, providing this essential element for the growth of legumes. This feat of biology was not matched by chemistry until 1914, when Fritz Haber established an industrial process for the manufacture of ammonia from atmospheric nitrogen, for which he received the Nobel Prize in Chemistry in 1918.
Although nitrogen was initially thought to be unreactive, this simple diatomic molecule does have interesting chemical properties. Some elements can "burn" in nitrogen, among them magnesium at 300 °C and lithium even at room temperature, producing crystalline metal nitrides. Complexes of molecular nitrogen with transition metals had been predicted for many years, but the first organometallic compound of dinitrogen, [Ru(NH3)5N2]Cl2, was only reported in 1965. Most of the fascinating chemistry of nitrogen is, however, reserved for its inorganic and especially organic compounds, notably amines, nitro compounds, and their more complex relatives.
Amino acids, the building blocks of the cell's protein factory, occur in only 20 different types in mammalian proteins, yet from these 20 simple molecules an almost infinite variety of structural proteins and enzymes can be built, and they can catalyze chemical processes far more efficiently and selectively than our chemical efforts. The natural amino acids are also remarkable in that they all are constructed by nature in the l form in proteins. What event early in the history of life on Earth led to the chirality of amino acids is a question that has always intrigued me. It may be that catalysis on a primitive inorganic surface favored the l form over the d, or the selection of l amino acids could have been an effect of polarized light.
There is also a theory that life on Earth was seeded by complex molecules from the interstellar void. Indeed, a variety of simple nitrogen compounds, such as cyanides, isocyanides, and formamide, have been found in interstellar gas. More evidence has recently emerged to show that the simplest amino acid, glycine, is also present in outer space.
Another vital property of nitrogen as a basic element of life is its ability to form hydrogen bonds. Hydrogen bonds between amino acids control the folding of proteins into a-helices and -sheets. In DNA, the hydrogen bonds to nitrogen are of even more importance. The realization that adenine only paired with thymine and that cytosine only paired with guanine (Watson-Crick pairing) was one of the keys to deciphering the structure of DNA. Base pairing through the hydrogen bonds of the nitrogen bases is also essential for the transmission of genetic information from DNA to messenger RNA and hence to instruct the protein synthesis factory.
Nitrogen is essential to life, but we must always remember that nitrogen can be destructive as well. Haber's work on ammonia synthesis was not undertaken to manufacture cheap fertilizer, although this was one of the outcomes of his discovery. Rather, it was the need for nitric acid to manufacture explosives when wartime blockades prevented importation of natural potassium nitrate from Chile that drove Haber to find an alternative process. This Janus-like aspect of nitrogen to be an element for good and for evil had already been recognized by Alfred Nobel. Having made his fortune from the manufacture of dynamite from nitroglycerine and kieselguhr, he left it to found the prizes which bear his name, and which a number of researchers in the chemistry of nitrogen compounds have received. The search for ever more potent explosives for both peaceful and military purposes continued after Nobel's death, and led in 2000 to the preparation by Philip Eaton of octanitrocubane, perhaps the most powerful explosive ever to be made. Both the good and the dark side of the chemistry of nitrogen continue to fascinate chemists today, and this essential element will surprise researchers studying its compounds well into the future.
Peter Nagler is head of the fine chemicals business unit at Degussa AG. After many years working with amino acids, he is, like nitrogen, generally noncombustible and nontoxic.
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