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8. Nitrogenmonoxide

N O N O

NO forms in the troposphere by electric discharges, and under stratospheric conditions
under the influence of cosmic rays and high-energy UV. Further oxidation to NO 2 readily
occurs:
N 2 → 2 N; N + O 2 → NO + O
2NO + O 2 → 2NO 2
With additional oxygen and moisture, nitric acid is formed, one of the constituents of ”acid
rain“:
2NO 2 + H 2O + ½O 2 → 2HNO 3
Industrially, NO is obtained by passing a mixture of ammonia and oxygen through a platinum
-3
net (contact time 10 s, temperature 1000 °C), which is further processed to form nitric acid
(Ostwald process):
2NH 3 + 2½O 2 → 2NO + 3H 2O
2NO + 1½O 2 + H 2O → 2HNO 3

A large amount of HNO 3 goes into the production of ammonium nitrate for artificial fertiliser.
NO is also contained in the exhaust gases of automobiles (along with water and CO 2 as the
main components, and fuel constituents), as well as in industrial and domestic exhaust, and
rapidly is oxidised to NO 2. Under the influence of UV, i.e. on sunny days, NO 2 is split into NO
and oxygen atoms, which oxidise alkanes to alkylhydroperoxides, and form ozone with
molecular oxygen (“summer smog”):
NO 2 + hν → NO + O
O + O 2 → O 3
O + C 2H 6 + NO 2· → C 2H 5O 2H + NO·
In the stratosphere, NO catalyses ozone depletion (see also the tutorial ‘oxygen’ on p. 11):
NO + O 3 → NO 2 + O 2
NO 2 + O → NO + O 2
__________________
O 3 + O → 2 O 2 (kinetically hindered without catalyst)

In organisms, NO is an important multi-
functional messenger and neuro-
transmitter, targeting, inter alia, metal
centres in haeme-type proteins, and cyclic
guanosine-monophosphatase (cGMPase).
Biosynthesis of NO is achieved by
oxidation of one of the NH 2 groups of
arginine, catalysed by a NO-synthase
(NOS), thereby converting Arg via
hydroxyarginine to citrulline; see the
illustration to the right. Three functionally
different NOS are known: (1) nNOS, in
the neurons, initiates signal transduction
and thus takes part in mnemonic functions;
(2) iNOS, in macrophages, induces the
liberation of NO in case of infections and