The Chemistry of Plant Life

by Roscoe Wilfred Thatcher

In addition to the proteins which constitute the active protoplasm, plants also contain large amounts of reserve, or stored, proteins, especially in the seeds. In the early stages of growth, the proteins are present in largest proportions in the vegetative portions of the plant; but as maturity approaches, a considerable proportion of the protein material is transferred to the seeds.

GENERAL COMPOSITION OF PROTEINS

The plant proteins are fairly uniform in their percentage composition. The analyses of some sixteen different plant proteins show the following maximum limits of percentages of the different chemical elements which they contain: Carbon, 50.72-54.29; hydrogen, 6.80-7.03; nitrogen, 15.84-19.03; oxygen, 20.86-24.29; sulfur, 0.17-1.09. Animal proteins vary more widely, both in percentage composition and in properties, than do those of plant origin.

Protein molecules are very large and, in the case of the so-called "conjugated proteins" in particular, their structure is very complex. The molecular weight of some of the proteins has been determined directly, in the case of those particular ones which can be prepared in proper form for the usual determination of molecular weight by the osmotic pressure method; and has been computed for various others, from the percentage of sulfur found on analysis, or (in the case of the hæmoglobin of the blood) from the proportion by weight of oxygen absorbed. From these determinations and computations, the following formulas for certain typical proteins have been calculated: for zein (from Indian corn), C₇₃₆H₁₁₆₁N₁₈₄O₂₀₈S₃; for gliadin (from wheat), C₆₈₅H₁₀₆₈N₁₉₆O₂₁₁S₅; for casein (from milk), C₇₀₈H₁₁₃₀N₁₈₀O₂₂₄S₄P₄; for egg-albumin, C₆₉₆H₁₁₂₅N₁₇₅O₂₂₀S₈. These few examples will serve to illustrate the enormous size and complexity of the protein molecule. The conjugated proteins are still more complex than the simple proteins whose formulas are here presented.

Fortunately for the purposes of the study of the chemistry of the proteins, however, it has been found that most of the common plant proteins, known as the "simple proteins," can easily be hydrolyzed into their constituent unit groups, which are the comparatively simple amino-acids, whose composition and properties are well understood. A study of the results of the hydrolysis of some twenty common plant proteins has shown that it is rarely possible to recover the amino-acids in sufficient quantities to account for a full 100 per cent of the material used, the actual percentage of amino-acids recovered usually totaling from 60 to 80 per cent. The remaining material is supposed to be also composed of amino-acids which are linked together in some arrangement which is not broken apart by any method of hydrolysis which has yet been devised. This view is borne out by the fact that substances which exhibit all the characteristic properties of proteins have been artificially synthetized, by using only amino-acid compounds. Animal proteins often show a much larger proportion of unhydrolyzable material than do plant proteins.

AMINO-ACIDS AND PEPTID UNITS

The products of hydrolysis of the common simple proteins are all amino-acids. These are ordinary organic acids with one (or more) of the hydrogen atoms of the alkyl group replaced by a —NH₂ (or sometimes by a —NH—) group. They may be regarded as ammonia, NH₃, with one of its hydrogen atoms replaced by an acid radical; or as the acid with one of its hydrogens replaced by the NH₂ group. For example, an amino-acid derived from acetic acid, CH₃·COOH, is glycine, or amino-acetic acid, CH₂NH₂·COOH; from propionic acid, CH₃·CH₂·COOH, there may be obtained either α-amino-propionic acid, CH₃·CHNH₂·COOH, or β-amino-propionic acid, CH₂NH₂·CH₂·COOH, etc.

All of the amino-acids which result from the hydrolysis of proteins are α-amino-acids, that is to say, the NH₂ group is attached to the α-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·CHNH₂·COOH.

These amino-acids contain both the basic NH₂ 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 NH₂ group of one molecule and the acid COOH group of the other, as indicated by the following equation: