The shell of the seed is built up of four different layers; the prevailing layer consists of very long, simply cylindrical cells, densely packed so as to form only one radial row. Tison[760] has endeavoured to ascertain in what region of the seed the active principle is lodged; and he has arrived at the conclusion that its seat is the granular protoplasmic particles, which alone acquire an orange tint by the action of weak caustic alkalis.
Chemical Composition—Jobst and Hesse[761] proved in 1863 that the poisonous nature of Calabar bean depends upon an alkaloid, to which they gave the name Physostigmine. It is obtained by the method generally adopted for extracting analogous substances, that is, by precipitating one of its salts from an aqueous solution by bicarbonate of sodium, and dissolving out the base with ether or benzol. As extracted by these chemists, physostigmine is an amorphous mass of decidedly alkaline reaction, soluble in much water and in acids. On exposure to the air the solution soon becomes red, or sometimes intensely blue, a partial decomposition of the alkaloid taking place. The red coloration may even be observed in the aqueous infusion of a few cotyledons. It disappears by sulphuretted hydrogen or sulphurous acid, but returns if these reducing agents are allowed to evaporate.
Hesse[762] ascertained (1867) that physostigmine consists of C₃₀H₂₁N₃O₄; he now obtained it perfectly colourless and tasteless, softening at 40° C., fusing at 45°, but not supporting a heat of 100° C., without decomposition, which is manifested by a red coloration.
In 1865 Vée and Leven,[763] by treating the powdered unpeeled seed in nearly the same way, prepared an alkaloid which they called Eserine. It differs from Hesse’s physostigmine in that it forms colourless, rhomboidal, tabular crystals of a bitter taste, melting at 90° C. It dissolves easily in ether, alcohol, or chloroform, but very sparingly in water. The last named solution is alkaline, and reddens by exposure to the air.
It is assumed by some writers, as Tison,[764] that eserine is only the pure form of physostigmine; but at present we feel hardly warranted in admitting the identity of the two substances.
Harnack and Witkowski in 1876 ascertained the presence of another alkaloid in the seed, which they called Calabarine. It is nearly insoluble in ether and also very different from physostigmine in its physiological action, but somewhat similar to strychnine. Calabarine is consequently not to be found in those preparations of calabar bean which have been obtained or purified by means of ether.
Hesse (1878) exhausted the cotyledons of Physostigma with petroleum ether, and obtained crystals of a new indifferent substance C₂₆H₄₄+OH₂, which he called Phytosterin. It is closely allied to Cholesterin, but, in its solution in chloroform, devoid of rotatory power and melting at 133°. Cholesterin melts at 145°, and deviates, in its ethereal solution, the ray of polarized light to the left. Phytosterin also occurs in peas; Hesse suggests that the crystallized appearance of alkaloids as prepared by former observers was perhaps due to phytosterin.
From the cotyledons per se, cold water extracts mucilage, precipitable by neutral acetate of lead. The watery infusion contains also albumin, which may be coagulated by heat or by alcohol. The infusion is colourless, does not redden litmus, nor does it contain sugar in appreciable proportion; a few drops of solution of potash cause it to assume an orange colour. An infusion of the shell of the seed is already of this colour, but the tint is intensified by caustic alkali.
The cotyledons yield to boiling ether ½ to ⅓ per cent. of fatty oil, and after exhaustion by ether and alcohol, afford to cold water 12 per cent. of albuminous and mucilaginous constituents. The proportion of starch according to Teich[765] amounts to 48 per cent., the albuminous matter to 23 per cent. The entire seed furnishes 3 per cent. of ash, chiefly phosphate of potash. These constituents do not widely differ in proportion from those found in the common bean, which yields 23 to 25 per cent. of albuminous matters, and 32 to 38 per cent. of starch, besides 1 to 3 per cent. of oil.
The shells of Calabar bean are stated by Fraser to be by no means devoid of active principle.