Phycoerythrin is the red pigment. It is a colloidal, nitrogenous substance, allied to the proteins (see [Chapter XIII]) but not a true protein compound. Hydrolysis by acids indicates that it contains leucine and tyrosine, two amino-acids which are constituents of proteins, along with other bodies of unknown composition.
The colloidal solution of phycoerythrin in water has a brilliant rose-red color, with an orange fluorescence. It readily sets to a gel (see [Chapter XV]), so that the solution is almost impossible to filter. On this account, purified solutions of this pigment are very difficult to secure, and no satisfactory analysis to indicate its composition has yet been obtained.
Actinically, it is a complementary pigment to chlorophyll, that is, it absorbs the blue and green rays and permits the passage of light which is of the wave length that is absorbed by chlorophyll.
Phycophæin.—Still less is known of the composition of this pigment than of that of phycoerythrin. It is the characteristic pigment of brown seaweeds. It is supposed to exist in the cells of algæ, chiefly as a colorless chromogen, which becomes first yellow and then brown on exposure to air. Associated with it are other pigments, which have been variously reported as carotin, phycoxanthin, etc.
THE ANTHOCYANS
These are a group of pigments of red, blue, or violet color, which occur in the flowers, fruits, or leaves of many species of plants. They are essentially ornamental pigments, and constitute a large proportion of the brilliant colors of flowers, etc. They occur not only dissolved in the cell-sap, but also as deposits of definite crystals or amorphous compounds in the cell protoplasm.
They are all glucosides. When the anthocyans are hydrolyzed, the sugar molecules are split off and the characteristic hydroxy-derivatives of the three-ring anthocyan nucleus (figured on [page 83]), known as "anthocyanidins," remain. These anthocyanidins are themselves pigments. They have been shown to be all derivatives of the anthocyan nucleus. The oxygen atom in this nucleus is very strongly basic and exhibits its quadrivalent property by forming stable salts by direct addition of acid radicles. The variation of color of the anthocyanins has been explained by Willstätter, as follows; the red is the acid salt, the blue is a neutral metallic salt, and the violet is the anhydride of the anthocyanidin in question, thus
All of the natural anthocyanin pigments appear to contain a chlorine atom attached directly to the ring oxygen, as shown in the above partial formulas. In addition, they have four, five, or six hydroxyl (OH), or methoxy (OCH3), groups attached at various points around the three rings. The following formula for œnidin, one of the most complex of these anthocyanidins, will illustrate their structural arrangement.
