Download Prebiotic Formation of Ammonia

Angew. Chem. Int. Ed. 2003, 42, 1540 – 1543
No. 13/2003
Ammonia for the Primordial Stew
Newly discovered nitrogen-fixation reaction
could have played a role in the emergence of life
Primeval fogs drift over the earth; the first tiny life forms develop. In order for amino and
nucleic acids to form, the element nitrogen had to be present in the ”primordial stew”. Molecular
nitrogen (N2) in the atmosphere is certainly not a good source of nitrogen atoms because it is
much too unreactive. A significantly more reactive form of nitrogen, ammonia (NH3) must have
taken part in the emergence of life. But how was ammonia generated from the molecular nitrogen
in air? Researchers from Jena have a new theory.
Because the bond between the two atoms in the nitrogen molecule is very difficult to break,
the industrial synthesis of ammonia only works in the presence of catalysts and at very high
temperatures and pressures. However, a gentler reaction is possible; nitrogen-fixing bacteria
convert atmospheric nitrogen into ammonia. Up to 200 million tons are produced in this manner
annually. The details of this reaction mechanism are still undetermined. It is known that the
enzyme involved, nitrogenase, requires a cofactor that contains several iron- and sulfurcontaining units. Both of these elements were plentiful on earth in the distant past and could have
taken part in the primeval formation of ammonia.
A research team at the University of Jena and the Max Planck Institute for biogeochemistry
in Jena has now worked out in the lab how such a nitrogen-fixation reaction without an enzyme
and at room temperature may have worked. The researchers, headed by Günter Kreisel and
Wolfgang Weigand, blow nitrogen (N2) into an aqueous suspension of iron sulfide (FeS), where
it reacts with dissolved hydrogen sulfide (H2S) to form ammonia (NH3). The driving force is the
reaction of iron sulfide to form iron disulfide (FeS2, pyrite). However, the whole thing only
works if the iron sulfide used is freshly generated. With commercial grade or old iron sulfide, not
a trace of ammonia is formed. It is clear that successful reaction depends on the special, cleft
surface of the fresh iron sulfide. It hosts various iron-sulfur structures, some of which are able to
bind molecular nitrogen. This weakens the bond between the two nitrogen atoms so that
positively charged hydrogen atoms can attack and bind to them.