Chlorophyll

[Editor’s note: In observance of Earth Week, we are reprising this molecule, which originally appeared on January 21, 2019.]

Chlorophyll¹ is the green pigment in plants, algae, and cyanobacteria that is essential for photosynthesis. Its central structure is an aromatic porphyrin or chlorin (reduced porphyrin) ring system with a sequestered magnesium atom. A fifth ring is fused to the porphyrin.

Chlorophyll is not a single molecule: There are six main varieties that have various side groups on their rings. In most chlorophylls, one of the substituents is a 20–carbon atom phytyl ester chain. Chlorophyll a, shown here, is called the “universal” chlorophyll because it is present in all photosynthetic organisms.

Chlorophyll has been known since 1817, but scientists did not realize that it contains magnesium until 1906. An early report in the chemical literature appeared in 1884, when agricultural chemist Robert Sachsse at the University of Leipzig (Germany) described the discovery of a new coloring matter in chlorophyll in addition to three that he had previously reported. In a follow-up article, he made several critical comments about British chemist Edward Schunk’s work on chlorophyll.

But the pioneering research into chlorophyll was performed by German chemist Richard Willstätter, who won the 1915 Nobel Prize for Chemistry for his work. In 1911, in the 16th of at least 18 articles about the molecule, he and Max Utzinger at ETH Zurich published an opus on the primary transformations of chlorophyll. In it, they went into great detail about the reactions of chlorophyll with numerous reagents, identifying the products and describing their properties. For a full account, see the SciFinder entry on the article.

By far the most important “application” of chlorophyll is photosynthesis; but it has also been used as a green coloring agent in foods, cosmetics, soaps, and alcoholic beverages. Its ester side chain can be cleaved to obtain phytol², an alcohol used in the synthesis of vitamins E and K₁. It has even been tried as an antiknock additive for gasoline.

Why is chlorophyll green? The bonding in many metal–organic coordination compounds causes them to absorb some wavelengths of white light while reflecting others. In the case of chlorophyll, wavelengths in the blue and red regions of the visible light spectrum are required for the pigment to do its business. Chlorophyll absorbs those wavelengths and reflects the intense green colors of leaves.

This year’s ACS Chemists Celebrate Earth Week topic is trees—the largest organisms to make use of chlorophyll.

1. SciFinder name: magnesium, [(2E,7R,11R)-3,7,11,15-tetramethyl-2-hexadecenyl (3S,4S,21R)-9-ethenyl-14-ethyl-21-(methoxycarbonyl)-4,8,13,18-tetramethyl-20-oxo-3-phorbinepropanoato(2–)-κN23,κN24,κN25,κN26]-, (SP-4-2)-.
2. CAS Reg. No. 150-86-7.