Its molecular formula is C40H56. It is, therefore, a hydrocarbon of a very high degree of unsaturation. On exposure to dry air, it absorbs 34.3 per cent of its own weight of oxygen, which corresponds to 11-1/2 atoms of oxygen, computed on the basis of the molecular formula C40H56, and would indicate a formula of (C40H56)2O23 for the oxygenated compound; this being three oxygen atoms less than would be required to bring the compound to the theoretical stage of saturation represented by the unimolecular formula CnH2n+2. In moist air, two more oxygen atoms are absorbed, probably forming two OH groups in the molecule. Moreover, carotin absorbs iodine. When the calculated amount of iodine is used, a definite compound having the formula C40H56I2 is produced; but in the presence of an excess of iodine another compound having the apparent formula C40H56I3 (or 2C40H56I2+I2) is obtained. (Note that 2 atoms of iodine plus 12 atoms of oxygen, or 3 of iodine plus 11-1/2 of oxygen, produce the degree of saturation required by the formula CnH2n+2.) It is evident from these experimental data, that a part of the unsaturated linkage in the carotin molecule is of a type which can easily be saturated by direct addition of oxygen, while the remainder may be saturated by iodine.
The reaction of carotin toward bromine is peculiar. With this element, it forms a compound having the formula C40H36Br22, indicating the direct addition of two atoms of bromine and the substitution of twenty atoms of this element for the same number of hydrogen atoms.
The oxygenated carotins are colorless substances, while the iodide crystallizes in beautiful dark-violet prisms, having a coppery red fluorescence.
Xanthophyll is closely related to carotin. It has the molecular formula C40H56O2. It absorbs 36.55 per cent of oxygen (corresponding to 13 atoms, which would indicate the formation of two OH groups in addition to the saturation required by the CnH2n+2 formula); and an iodine addition product having the formula C40H56O2I2, which crystallizes in dark-violet needles.
Xanthophyll differs markedly from carotin in its solubilities, being insoluble in petroleum ether and only sparingly soluble in carbon disulfide. It may be fairly easily reduced to carotin. This transformation is reversible, and suggests a similarity to the change from hæmoglobin to oxyhæmoglobin, and the reverse, in the blood of animals, as a part of their respiration process.
Separation of the Chlorophylls, Carotin, and Xanthophyll.—These pigments, which exist together in most plant tissues, may easily be separated from each other by taking advantage of the differences in their solubilities, according to the following procedure. Grind up a small quantity of the fresh tissue (leaves of the stinging nettle furnish a conveniently large supply of each of these pigments) with fine sand in a mortar. Cover with acetone, let stand a few moments and then filter on a Büchner funnel. Pour the filtrate into a separatory funnel, add an equal volume of ether and two volumes of water. Shake up once and then allow the ether layer to separate; the pigments will be in this layer. Drain off the water-acetone layer. Now to the etherial solution, add about half its volume of a concentrated solution of potassium hydroxide in methyl alcohol. Shake well and allow to stand until the mixture becomes permanently green. Now add an equal volume of water and a little more ether, until the mixture separates sharply into two layers. The chlorophylls will now be in the lower dilute alcohol layer, and the carotinoids in the upper ether, and may be separated by draining of each layer separately. To separate the carotin from xanthophyll place the ether solution in a small open dish and evaporate to a small volume. Now add about ten volumes of petroleum spirit and an equal volume of methyl alcohol, stir up well, transfer to a separatory funnel and allow the two layers to separate. The carotin will now be in the upper layer of petroleum ether, and the xanthophyll in the lower alcohol layer; these layers may be drained off separately and the solvents evaporated in order to recover the pigments in dry form.
Lycopersicin (or lycopin) is a hydrocarbon pigment having the same formula as carotin. It is, however, brilliantly red in color, and crystallizes in a different form and has a different adsorption spectrum from carotin. It is the characteristic pigment of red tomatoes, and is found also in red peppers. Yellow tomatoes have only carotin as their skin-pigment, while lycopersicin is usually present in the flesh of the ripe fruits of all varieties and in the skin of red ones. It has been shown, however, that if varieties of tomatoes which are normally red when ripe, are ripened at high temperatures, 90° F. or above, their skins will be yellow instead of red when fully ripe. Hence, the occurrence of carotin, or of lycopersicin, as the skin pigment is determined in part by the varietal character (being different in different varieties when ripened at normal temperatures) and in part by the temperature at which the fruit ripens. The two pigments are, of course, isomers; but the difference in their structural arrangement is not known.
Fucoxanthin, C40H54O6, is a brownish-red pigment, found in fresh brown algæ, and in some brown sea-weeds. Its formula indicates that it is an oxidized carotin. With iodine, it forms a compound having the formula C40H54O6I4. It is unlike carotin and xanthophyll in that it has basic properties, forming salts with acids, which are blue in color.
PHYCOERYTHRIN AND PHYCOPHÆIN
These are the principal pigments of red and brown seaweeds, respectively. Their most characteristic difference from the pigments of non-aquatic plants is that they are easily soluble in water, and insoluble in most organic solvents, such as alcohol, ether, etc. At first thought, this would appear to be impossible, since the plants grow in water and it would seem that their water-soluble pigments would be continuously dissolved out of the tissues. The reason why this does not occur lies in the fact that these pigments exist in the cells of the seaweeds in colloidal form (see [Chapter XV]), and, hence, cannot diffuse out through the cell-wails. The only way in which they can be extracted from the tissues is by rupturing the cells, by grinding with sharp sand, etc., after which the pigments can readily be dissolved out by water.