It is obvious that the compound thus formed still contains a free NH2 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 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 NH2 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:

  1. Monoamino-monocarboxylic acids:
    1. Glycine, C2H5NO2, CH2NH2·COOH, amino-acetic acid.
    2. Alanine, C3H7NO2, CH3·CHNH2·COOH, amino-propionic acid.
    3. Serine, C3H7NO3, CH2OH·CHNH2·COOH, oxy-amino-propionic acid.
    4. Valine, C5H11NO2,
    5. , amino-isovalerianic acid.
    6. Leucine, C6H13NO2,
    7. , amino-isocaproic acid.
    8. Isoleucine, C6H13NO2,
    9. , amino-methylethyl-propionic acid.
    10. Phenylalanine, C9H11NO2,
    11. , phenyl-amino-propionic acid.
    12. Tyrosine, C9H11NO3,
    13. , paraoxy-phenylalanine.
    14. Cystine, C6H12N2O4S2, HOOC·CHNH2·CH2S—SH2C·CHNH2·COOH, di(thio-amino-propionic acid).
  2. Monoamino-dicarboxylic acids:
    1. Aspartic acid, C4H7NO4, HOOC·CH2·CHNH2·COOH, mino-succinic acid.
    2. Glutamic acid, C5H9NO4, HOOC·CH2·CH2·CHNH2·COOH, amino-glutaric acid.
    1. Ornithine, C5H12N2O2, H2N·CH2·CH2·CH2·CHNH2·COOH, di-amino-valerianic acid.
    2. Lysine, C6H14N2O2, H2N·CH2·CH2·CH2·CH2·CHNH2·COOH, di-amino-caproic acid.
    3. Arginine, C6H14N4O2,
    4. , guanidine-amino-valerianic acid.
    5. Di-amino-oxysebacic acid, C11H12N2O3.
    6. Di-amino-trioxydodecanic acid, C12H26N2O3.
  4. Diamino-monocarboxylic acids:
    1. Proline, C5H9NO2,
    2. , pyrrolidine-carboxylic acid.
    3. Oxyproline, C5H9NO3, proline with one (OH) group.
  5. Monoimido-monocarboxylic acids:
    1. Histidine, C6H9N3O2,
    2. , imidazole-amino-propionic acid.
    3. Tryptophane, C11H12N2O2,
    4. , indole-amino-propionic acid.
  6. Monoimido-monoamino-monocarboxylic acids:

As has been said, other amino-acids are being found, from time to time, as additional proteins are examined, or as better methods of examination of the cleavage products of the natural proteins are devised.

COMPOSITION OF PLANT PROTEINS

The distribution of the different amino-acids in some of the different plant proteins which have been examined in this way is shown in the following table: