Scientists harness a rare true blue color from nature — in red cabbage

Chemists gave this natural pigment an extra blue boost with aluminum ions

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Think about last time you were at the grocery store looking around for fresh vegetables. All the items you probably saw were green, red, yellow, or orange. But what about blue?

Blue is quite uncommon in nature and this fact has baffled scientists for several years. Most colors we see in everyday life comes from small molecules called pigments. Pigments absorb a set of wavelengths of light and reflect the remainder, thus producing color. While uncommon, some birds and butterflies, a rare berry type Pollia condensata, and eyes can be blue. But they are all blue for a different reason. Their blue comes from nanosized structures that scatter or reflect the light wavelengths of the color we see.

Red cabbage leaves are colored dark red or purple because of the pigments in them known as anthocyanins. In 2016, researchers at the Ohio State University used an anthocyanin extract from red cabbage and found around eight different types. Remarkably, among these was a tiny amount of a blueish anthocyanin pigment. In April in the journal Science Advances, scientists report turn this pigment into a true blue with a neat chemistry trick.

The international team of researchers described the full molecular structure of this interesting blue pigment, named P2. The pigment molecule is enormous and has several sugar molecules attached to an anthocyanin core. On its own, it is not strongly blue, but P2 can be turned into a “true” blue pigment by adding aluminum ions to it. 

The charged aluminum ions form a stable molecular complex with three big P2s clamped to a central metal aluminum ion in a “Y” shape. And since they revealed the intricate structure of P2, the researchers could also produce more of it than red cabbage normally does. They used enzymes to convert similar, but more abundant, pigments into P2. By using these tricks, this new blue complex pigment can now be produced in larger amounts and be investigated for future use in the food industry.