preceding paragraphs may be linked together in a great variety of
ways. Thus, in a tetrapeptid containing units which may be designated by
the letters _a_, _b_, _c_, and _d_, the arrangement may be in the orders
_abcd_, _bacd_, _acbd_, _dbca_, etc., etc. Similarly, the same peptid unit
may appear in the molecule in two or more different places. Hence, the
number of possible combinations of amino-acids into protein molecules is
very great. Further, it is possible that the peptid units in natural
proteins may be united together through other linkages than the one
illustrated above, as they often contain alcoholic OH groups in addition to
the basic NH_{2} groups, and these OH groups may form ester-linkages with
the acid (COOH) groups of other units. Still other acid and basic groups
are present in some of the amino-acids which have been found in natural
proteins, so that the possibility of variation in the polypeptid linkages
is almost limitless.

INDIVIDUAL AMINO-ACIDS FROM PROTEINS

About twenty different amino-acids have been isolated from the products of
hydrolysis of natural proteins, and this number is being added to from time
to time, as the methods of isolation and identification of these compounds
are improved. Many of these same amino-acids have been found in free form
in plant tissues, particularly in rapidly growing buds, or shoots, or in
germinating seeds, where they undoubtedly exist as intermediate products in
the transformation of proteins into other types of compounds.

These amino-acids, grouped according to the characteristic groups which
they contain, are as follows:

A. Monoamino-monocarboxylic acids:

Glycine, C_{2}H_{5}NO_{2}, CH_{2}NH_{2}.COOH, amino-acetic acid.

Alanine, C_{3}H_{7}NO_{2}, CH_{3}.CHNH_{2}.COOH, amino-propionic acid.

Serine, C_{3}H_{7}NO_{3}, CH_{2}OH.CHNH_{2}.COOH, oxy-amino-propionic
acid.

CH_{3}
\
Valine, C_{5}H_{11}NO_{2}, CH.CHNH_{2}.COOH,
/
CH_{3}
amino-isovalerianic acid.

CH_{3}
\
Leucine, C_{6}H_{13}NO_{2}, CH.CH_{2}.CHNH_{2}.COOH,
/
CH_{3}
amino-isocapr