It is understood, of course, that the mineral element does not furnish the definite means of holding the structural units together as otherwise it would not be possible to remove the iron, or magnesium, without breaking down the molecule, as is done in the case of the porphyrins. The actual binding linkage is undoubtedly between carbon atoms, as indicated in Willstätter's formulas for ætiophyllin and ætioporphyrin (see [page 109]). The attachment of the magnesium to each one of the four nitrogen atoms in the skeleton formula assumes the existence of subsidiary valences of 2-4 for magnesium (and of 3-5 for iron), or of possible oscillating valences similar to those supposed to be exhibited by carbon in its closed-ring arrangements.
PROPERTIES OF THE CHLOROPHYLLS
The phytyl esters, or natural chlorophylls, are amorphous solids; while the methylethyl esters (chlorophyllins) and the free acids (phyllins) are crystalline compounds. All of these compounds are easily soluble in ether and alcohol, but insoluble in water. The chlorophylls and chlorophyllins are practically insoluble in petroleum ether and chloroform; but the monobasic acids (pyrrophyllin and phyllophyllin) and the neutral ætiophyllin dissolve easily in chloroform.
Solutions of the chlorophylls are fluorescent, being green by transmitted, and red by reflected light.
Chlorophyll a is a blue-black solid, which gives dark green solutions in all of its solvents. Chlorophyll b is a dark-green solid, which yields brilliant green solutions. Solutions in ether of glaucophyllin and of cyanophyllin are blue; of rhodophyllin, deep violet; of rubiphyllin, light violet; of erythrophyllin, red; and of pyrrophyllin and phyllophyllin, bluish-red. Solutions of the porphyrins are all red, the di-basic ones being usually a bluish-red, and the simpler ones a brilliant red to deep brownish-red in color.
The several chlorophyll derivatives are further distinguished by characteristic differences in their absorption spectra. These differences have been pictured by Willstätter in his book dealing with the results of his investigations concerning the chlorophylls, and reproduced in one or two other texts which treat in detail with the physical-chemical properties of these pigments, but need not be presented in such detail here.
THE CAROTINOIDS
The characteristic brilliant green of healthy plant tissues is due to the fact that there are always associated with the dark bluish-green chlorophylls two (or more) yellow pigments. These are known as the "carotinoids." This group includes the two brilliant yellow pigments, carotin and xanthophyll, and the reddish brown fucoxanthin and the brilliant red lycopersicin, which are similar in their chemical composition. The first two are found universally distributed in plants, associated with the chlorophylls, and may be regarded as vegetative pigments, although the characteristic ornamental yellow and orange colors of many flowers and fruits, as well as that of the roots of carrots, etc., due to these pigments.
Carotin.—This pigment occurs in various forms in plants, both amorphous and crystalline. It crystallizes out of solution in flat plates, which are orange-red by transmitted light, and greenish-blue by reflected light, and have a melting point of 168°. Carotin is insoluble in water, only very slightly soluble in acetone or cold alcohol, readily soluble in petroleum ether, ether, chloroform, and carbon disulfide. Its solutions are strongly fluorescent.