Chemistry Chat

Though blue is the color of ocean and sky, it is for some reason a rare color in the biological world. The rarity of blue is perhaps reflected in our traditional images such as bluebirds being a symbol of happiness and blue rose being a synonym of improbability. As for naturally occurring organic compounds having a bluish color, only a few are known. Indigo and delphinidin (the pigment of the Delphinium flowers) are well-known examples but there aren’t too many others.

A famous hydrocarbon compound appearing blue is azulene, which is composed of fused five- and seven-membered aromatic rings and is an isomer of naphthalene. Azul means blue and is a part of such words as Côte d'Azur (the French Riviera) and the Azzurri, the nickname of Italian national football team.

There are natural products possessing an azulene skeleton. For example, the unforgettably bright blue color of the mushroom species known as rurihatsutake or blue milk mushroom originates from azulene derivatives. Also, some essential flower oils turn bluish under high thermal conditions during distillation. This phenomenon is the result of dehydration and oxidation of certain terpenoids containing fused five- and seven-membered rings, producing aromatic compounds of the likes of guaiazulene. These compounds are known for their anti-inflammatory effects and are used as medicines even today. If you see any stomach medicines or mouthwash liquids colored blue, those products very likely contain an azulene derivative.

The total synthesis of vitamin B₁₂ led by the late R. B. Woodward was a monumental achievement in the history of organic synthesis that resulted in the development of various spin-off chemistries. One of the compounds synthesized as byproduct was the macrocyclic compound composed of five pyrrole units. This compound was named sapphyrin after its bright blue color and the same suffix to porphyrin. The synthesis of sapphyrin sparked the following development of macrocyclic porphyrinoid chemistry.

Many porphyrinoids were synthesized afterwards and were named after jewels, including rubyrin, which was introduced in the previous article. Besides rubyrin, there have been made assortment of jewels of organic synthesis such as green-colored smaragdyrin (named after the Latin smaragdus meaning emerald) and ozaphyrin (based on the Emerald City from the Wizard of Oz), rose red-colored rosarin, orange-colored orangarin, turquoise-colored turcasarin, purple-colored amethyrin (named after amethyst), and bronze-colored bronzaphyrin.

As mentioned so far, a wide variety of porphyrin derivatives have been synthesized that show beautiful colors. The original porphyrin itself is colored purple so dark that even a highly dilute solution of it is brightly colored. Accordingly, its name is derived from porphyra, the Greek term meaning purple.

There is an organic compound called viologen whose name is derived from violet. The structure of viologen contains two N-methylpyridinium cations bonded at the 4 position of each other. The fact that it turns bright violet when it is reduced to form a radical cation led to its name. Upon reoxidation, the radical cation produces reactive peroxides that are known to damage nearby compounds. Viologen and related compounds are used as pesticides (paraquat) for this property, but since it is also toxic to animals its use is banned or strictly restricted in some countries.

In a similar manner, one of the amino acids leucine was named after the Greek leukos that means white because it was first obtained as white crystals. Another amino acid arginine has similar naming background, as it was named after the Greek argyros, meaning silver, because it was first isolated in silvery crystalline form. It is also the origin of the atomic symbol of silver, Ag.

The Greek leukos also became the origin of the name of white blood cells (leukocytes). Branching out from there, names like leukotrienes and interleukins were assigned to relevant important biological substances. There are apparently many other examples containing the prefix leuko in the field of biology.