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 --NH_{2} (or sometimes 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