by a
--NH--) group. They may be regarded as ammonia, NH_{3}, with one of its
hydrogen atoms replaced by an acid radical; or as the acid with one of its
hydrogens replaced by the NH_{2} group. For example, an amino-acid derived
from acetic acid, CH_{3}.COOH, is glycine, or amino-acetic acid,
CH_{2}NH_{2}.COOH; from propionic acid, CH_{3}.CH_{2}.COOH, there may be
obtained either [alpha]-amino-propionic acid, CH_{3}.CHNH_{2}.COOH, or
[beta]-amino-propionic acid, CH_{2}NH_{2}.CH_{2}.COOH, etc.

All of the amino-acids which result from the hydrolysis of proteins are
[alpha]-amino-acids, that is to say, the NH_{2} group is attached to the
[alpha]-carbon atom, i.e., the one nearest to the COOH group. Hence, the
general formula for all the amino-acids which are found in plants is
R.CHNH_{2}.COOH.

These amino-acids contain both the basic NH_{2} group and the acid COOH
group. For this reason, they very easily unite together, in the same way
that all acids and bases unite, to form larger molecules, the linkage
taking place between the basic NH_{2} group of one molecule and the acid
COOH group of the other, as indicated by the following equation:

R R R R
| ---------- | | |
HOOC.C.N-|H + HO|OC.C.NH_{2} = HOOC.C.N-OC.C.NH_{2} + H_{2}O
| | ---------- | | | |
H H H H H H

It is obvious that the compound thus formed still contains a free NH_{2}
group and a free COOH group, and is, therefore, capable of linking to
another amino-acid molecule in exactly the same way; and so on
indefinitely. In actual laboratory experiments, as many as eighteen of
these amino-acid units have been caused to unite together in this way, and
the resulting compounds thus artificially prepared have been found to
possess the characteristic properties of natural proteins.

These artificially prepared, protein-like, substances have been called
"polypeptides," and the individual amino-acids which unite together to form
them are called "peptides." Thus, a compound which contains three such
units linked together is called a "tripeptid"; one which contains four, a
"tetrapeptid." The use of the term "peptid" was suggested by the fact that
these amino-acids are produced from the hydrolysis of proteins by the
digestive enzyme _pepsin_.

The peptid units of any such complex as those which have been referred to
in the