water-soluble organic compounds
that possess both a carboxyl (–COOH)
and an amino (–NH 2 ) group attached
to the same carbon atom, called the
α-carbon atom. Amino acids can be
represented by the general formula
R–CH(NH 2 )COOH. R may be hydrogen
or an organic group and determines
the properties of any particular
amino acid. Through the formation
of peptide bonds, amino acids join to-
gether to form short chains (pep-
tides) or much longer chains
(polypeptides). Proteins are com-
posed of various proportions of about
20 commonly occurring amino acids
(see table on pp 30–31). The sequence
of these amino acids in the protein
polypeptides determines the shape,
properties, and hence biological role
of the protein. Some amino acids
that never occur in proteins are nev-
ertheless important, e.g. ornithine
and citrulline, which are intermedi-
ates in the urea cycle.
Plants and many microorganisms
can synthesize amino acids from sim-
ple inorganic compounds, but ani-
mals rely on adequate supplies in
their diet. The *essential amino acids
must be present in the diet whereas
others can be manufactured from
them.
A
- Information about IUPAC nomenclature
amino acid racemization (AAR)
A dating technique used in archaeol-
ogy based on the relative amounts of
the optical isomers of an amino acid
in a sample. In most organisms, the
l-isomer of the amino acid is the one
produced by metabolism. When the
organism dies, this isomer slowly
converts into the d-form, and eventu-
ally an equilibrium is reached in
which the two forms are present in
equal amounts. Measuring the pro-
portions of the l- and d-forms in a
sample can, in principle, give an esti-
mate of the time since death. Not all
amino acids racemize at the same
rate, and the rate of the process de-
pends on other factors such as mois-
ture and temperature. Most work has
been done using leucine or aspartic
acid.
A particular application in forensic
science involves measuring the d/l
ratio of aspartic acid in the dentine
of teeth. Once a tooth has fully
formed, the dentine is isolated by the
enamel and then racemization takes
place in the living subject at a fairly
constant temperature and moisture
level. Measuring the ratio gives a
fairly good estimate of the age of the
subject (rather than the time since
death).aminobenzeneSee phenylamine.amino group See amines.
aminophenolAny of various or-
ganic compounds used as reducing
agents, especially as photographic
developers, and for making dyes. Ex-
amples include amidol(the dihydro-
chloride of 2,4-diaminophenol),
metol(the hemisulphate of 4-methyl-
aminophenol) and rhodinol(4-methyl-
aminophenol).α-aminotolueneSee benzylamine.ammineA coordination *complex
in which the ligands are ammonia
molecules. An example of an am-
mine is the tetraamminecopper(II)
ion [Cu(NH 3 ) 4 ]2+.ammoniaA colourless gas, NH 3 ,
with a strong pungent odour; r.d.
0.59 (relative to air); m.p. –77.7°C;
b.p. –33.35°C. It is very soluble in
water and soluble in alcohol. The
compound may be prepared in the
laboratory by reacting ammonium
salts with bases such as calcium hy-
droxide, or by the hydrolysis of a ni-
tride. Industrially it is made by the
*Haber process and over 80 million29 ammonia
a