nickel-cadmium cell(nicad battery) A dry cell in
which the anode is cadmium (Cd), the cathode is NiO 2 ,
and the electrolyte is basic. This “old” rechargeable
battery technology is now being replaced by newer
forms such as nickel-metal hydride.
nif A set of about 20 GENESrequired for the assembly
of the NITROGENASE ENZYMEcomplex.
NIH shift The INTRAMOLECULARhydrogen MIGRA-
TIONthat can be observed in enzymatic and chemical
hydroxylations of aromatic rings. It is evidenced by
appropriate deuterium labeling, for example:
In enzymatic reactions, the NIH shift is generally
thought to derive from the rearrangement of arene
oxide intermediates, but other pathways have been sug-
gested. NIH is the acronym of the National Institutes
of Health, Bethesda, Maryland, where the shift was
discovered.
nitrate reductase A METALLOENZYME containing
molybdenum that reduces nitrate to nitrite.
nitrene Generic name for HN: and substitution
derivatives thereof, containing an electrically neutral
univalent nitrogen atom with four nonbonding elec-
trons. Two of these are paired; the other two may have
parallel spins (triplet state) or antiparallel spins (singlet
state). The name is the strict analog of CARBENEand, as
a generic name, it is preferred to a number of alterna-
tives proposed (imene, imine radical, azene, azylene,
azacarbene, imin, imidogen).
nitrenium ion The cation H 2 N+and its N-hydrocar-
byl derivatives R 2 N+, in which the nitrogen has a posi-
tive charge, and two unshared electrons. A synonymous
term is aminylium ion.
nitrite reductase A METALLOENZYME that reduces
nitrite. DISSIMILATORYnitrite reductases contain copper
and reduce nitrite to nitrogen monoxide. ASSIMILATORY
nitrite reductases contain SIROHEMEand IRON-SULFUR
CLUSTERs and reduce nitrite to ammonia.nitrogenase An ENZYMEcomplex from bacteria that
catalyzes the reduction of dinitrogen to ammonia: N 2 +
8e– +10H+ → 2 +NH 4 + H 2 with the simultaneous
HYDROLYSISof at least 16 ATPmolecules. The electron
donor is reduced ferredoxin or flavodoxin. Dihydrogen
is always a coproduct of the reaction. Ethyne (acety-
lene) can also be reduced to ethene (ethylene) and in
some cases ethane. All nitrogenases are IRON-SULFUR
PROTEINs. Three different types, which differ in the
type of COFACTORpresent, have been identified: molyb-
denum-nitrogenase (the most common, which contains
the iron-molybdenum cofactor), vanadium-nitrogenase,
and iron-only nitrogenase.
See alsoFEMO COFACTOR; REDUCTION.nitrogen cycle A biochemical cycle in which occurs
the transformation of nitrogen from an atmospheric gas
to organic compounds in the soil, then to compounds in
plants, and eventually back to the atmospheres as gas.nitrogen fixation The natural process where atmo-
spheric nitrogen, N 2 , is converted to compounds that
can be easily utilized by plants.
All organisms require nitrogen compounds, but
few are able to utilize N 2 , a relatively inert and unreac-
tive form and, unfortunately, the most readily avail-
able. Most organisms require fixed forms such as NH 3 ,
NO 3 –, NO 2 –, or organic-N. Bacteria perform nitrogen
fixation by combining the nitrogen with hydrogen to
form ammonia (NH 3 ) in the soil, which plants can then
use. Cyanobacteria (blue-green algae) and bacteria
(e.g., Rhizobium spp.; Azotobacter spp.) associated
with legumes, like peas, can fix N 2 by reducing it to
ammoniacal (ammonialike) N, mostly in the form of
amino acids.NMR See NUCLEAR MAGNETIC RESONANCE SPEC-
TROSCOPY.NMR 193