Chapter II: Introduction

Proteins are organic compounds of natural origin, being found in plants and in animals, though much more plentifully in the latter. They are compounds of great complexity of composition, and of very high molecular weight. The constitution of none of them is fully understood, but although there are a great number of different individual proteids, they present typical resemblances and divergences which serve to differentiate them from other groups of organic bodies, and also from one another.

Proteins resemble one another in both proximate and ultimate analysis. They contain the usual elements in organic compounds, but in proportions which do not vary over very wide limits. This range of variation is given approximately below:--

Element. Per cent.

Carbon 49 to 55
Hydrogen 6.4 to 7.3
Oxygen 17 to 26
Nitrogen 13 to 19
Sulphur 0.3 to 3.0

The most characteristic feature of the protein group is the amount of nitrogen usually present. This is generally nearer the higher limit, seldom falling below 15 per cent. This range for the nitrogen content is determined largely by the nature of constituent groups which go to form the proteid molecule. Roughly speaking, proteins consist of chains of amido-acids and acid amides with smaller proportions of aromatic groups, carbohydrate groups and thio compounds attached. In these chains an acid radical may combine with the amido group of another amido acid, the acid group of the latter combining with an amido group of another amido acid, and so on. Hydrogen may be substituted in these chains by alkyl or aromatic groups. There is obviously infinite possibility of variation in constitution for compounds of this character, the general nature of which varies very little. Practically all of the proteins are found in the colloid state, and this makes them very difficult to purify and renders the ultimate analysis in many cases doubtful. It is, for example, often difficult to ascertain their moisture content, for many are easily hydrolyzed with water only, and many part easily with the elements of water, whilst on the other hand many are lyophile colloids and practically cannot be dehydrated or dried. A few, such as gelatin and some albumins, have been crystallized.

The constituent groups have been investigated chiefly by hydrolytic methods. The chains of amido acids are split up during hydrolysis, and individual amido acids may thus be separated. The hydrolysis may be assisted either by acids, alkalies or ferments, but follows a different course according to the nature of the assistant. Under approximately constant conditions of hydrolysis, the products obtained are in approximately constant proportions, and this fact has been utilized by Van Slyke in devising a method of proximate analysis. It is not possible in this volume to enter deeply into the constitution of the different proteids. Reference must be made to works on pure chemistry, especially to those on advanced organic chemistry. It will be interesting, however, to mention some of the amido acids and groups commonly occurring in proteids. These comprise ornithine (1:4 diamido valeric acid), lysine (1:5 diamido-caproic acid), arginine (1 amido, 4 guanidine valeric acid), histidine, glycine (amidoacetic acid), alanine (amido propionic acid), amido-valeric acid (amido-iso-caproic acid), liacine, pyrollidine carboxylic acid, aspartic acid, glutamic acid (amido-glutaric acid), phenyl-alanine, serine (hydroxy-amido propionic acid), purine derivatives (_e.g._ guanine), indol derivatives (_e.g._ tryptophane and skatol acetic acid), cystine (a thioserine anhydride), glucosamine, and urea.

There are a few general reactions which are typical of all proteins, and which can usually be traced to definite groupings in the molecule. Amongst these is the biuret reaction: a pink colour obtained by adding a trace of copper sulphate and an excess of caustic soda. This is caused by the biuret, NH(CONH{2}){2} radical or by similar diacidamide groups, _e.g._ malonamide, oxamide, glycine amide. Another general reaction is with "Millon's reagent," a solution of mercuric nitrate containing nitrous fumes. On warming the proteid with this reagent, a curdy pink precipitate or a red colour is obtained. This reaction is caused by the tyrosine group (p. oxy [alpha] amido phenyl-propionic acid). Another general reaction is to boil the protein with 1:2 nitric acid for some days. A yellow flocculent precipitate of "xanthoproteic acid" is obtained, and this dissolves in ammonia and caustic alkalies with a brown or orange-red colour. Another characteristic of proteins is that on dry distillation they yield mixtures of pyridine C{5}H{5}N, pyrrol C{4}H{5}N, and their derivatives.

On the subdivision, classification and nomenclature of the proteins much ink has been spilled, and it is impossible in this volume to go into the various systems which have been suggested. It should be noted, however, that some writers habitually use the terms "proteid" or "albuminoid" as synonyms for protein. The classification of proteins adopted in this work is used because it is the most suitable for a volume on industrial chemistry and has the additional merits that it is simple and is already used in several standard works on industrial chemistry. It is based upon the behaviour of the proteins towards water, a matter of obvious moment in manufacturing processes. On this basis proteins may be divided into albumins, keratins and gelatins.

Cold water dissolves the albumins, does not affect the keratins, and only swells the gelatins. The behaviour in hot water confirms and elaborates the classification. When heated in water, the albumins coagulate at temperatures of 70°-75° C., the gelatins (if swollen) dissolve readily, whilst the keratins only dissolve at temperatures above 100° C. Albumins and keratins may be distinguished also from gelatins by adding acetic acid and potassium ferrocyanide to their aqueous solutions. Albumins and keratins give a precipitate, gelatins do not. Another distinguishing reaction is to boil with alcohol, wash with ether, and heat with hydrochloric acid (S.G. 1.2). Albumins give a violet colour, keratins and gelatins do not.

=Albumins= may be first discussed. They are typified by the casein of milk and by white of egg. Their solutions in water are faintly alkaline, optically active, and lævorotatory. They are coagulated by heat and also by mineral acids, alcohol, and by many poisons. The temperature of coagulation (usually about 72° C.) is affected by mineral salts, the effect being in lyotrope order (see Part V.,