Thread by @kakape

Last week I wrote a thread on Prussian Blue including a quote from Fontane’s Frau Jenny Treibel:
“What are all the cornflowers in the world compared with a Prussian blue factory?”

So it seems only fair to talk about the blue of the cornflower today...
#TuesdayisBluesday

Blue flowers are beautiful but they are also quite rare.
Gardeners and naturalists had long noticed that a true blue seemed less common in flowers than a vibrant red or yellow. Goethe mentioned this in his “Theory of Colors” published in 1810, for instance:

This fascinated artists.
In German romanticism the blue flower became a symbol of longing, of the unattainable.

Joseph von Eichendorff wrote:
“I seek the flower of blue
Seek yet never can find.
I dream that in its hue
My happiness is enshrined."

You can still see it today. In many stories blue flowers are prized for producing some rare chemical compound.
Take two movie examples:
Batman Begins in which a rare blue flower produces a hallucinogen
A Scanner Darkly in which the drug Substance D comes from blue flowers

Scientists were equally fascinated and wanted to unlock the chemical secrets of blue flowers. But they made little progress.
"Sixty years have passed during which the chemical investigation of the pigment of blue flowers has made no progress”, Richard Willstätter wrote in 1913.

Willstätter was one of the foremost scientists of his generation. He found the chemical structure of cocaine and was interested in plant pigments. After important work on chlorophyll, he turned his attention to the cornflower “because of its beautiful, fairly pure blue color”

Willstätter managed what so many others before him had not accomplished: He found what he said was the chemical structure of the cornflower’s blue, a simple molecule, named cyanidin. He published his findings in 1913.

Two years later, he had another triumph: He isolated the color of the red rose.
The molecule, it turned out was… cyanidin.

This raised a questions, of course: How could the red of the rose and the blue of the cornflower come from the same molecule?
Well, Willstätter had a theory: It was all about the pH. Cornflowers must have a higher pH (be less acidic) in their petals, he hypothesized.

This made sense. Cyanidin is an anthocyanin and they change the color depending on pH:
Cut a red cabbage into small pieces, cook them in water, then drain it off and let the liquid cool. Add bleach and the liquid turns blue, add lemon juice and it turns red.

Still, some researchers were not convinced that cornflowers could change the pH enough.
They proposed other solutions: that the blue form of the molecule is stabilised through metal ions, for instance, or through building a complex with other molecules.

In 2005 researchers using x-ray crystallography produced the answer. They found that the blue of the cornflower is due to an elaborate assembly:
six molecules of cyanidin and six copigment molecules arranged around one atom of iron and one of magnesium.

The same basic building principle is used by several other blue flowers, like Mexican sage or the common dayflower, to produce their hue.
pubs.rsc.org/en/content/art…

Willstätter was awarded the Nobel Prize in Chemistry in 1915 "for his researches on plant pigments”. But his life took a dark turn: His son died the same year and with the First World War raging he stopped his work on plant pigments to work on a new filter for gas masks.

“The pigment solutions from stemless gentian of Lugano, from Dutch tulips, from raspberries and strawberries spoiled, the cultures of colorful blossoms went unharvested at first, then we carried the beautiful flowers in baskets to the military hospitals,” he wrote in his memoirs.

Willstätter never returned to his research on plant pigments. He was Jewish and after the war he faced ever more hostile anti-semitism. In 1939 he had to flee Germany and he died in Switzerland in 1942.

And his theory on blue flowers and pH? That was wrong?
Well, not completely. It turns out that some flowers actually do change the pH in their petals to appear blue.

Researchers found that this beautifully blue morning glory increases its pH while its blossoms are open, pumping protons out of the cell, to appear blue.

ncbi.nlm.nih.gov/pmc/articles/P…

This time-lapse video from Japanese scientists shows a morning glory opening its petals and as it does it turns from red to blue.

It always makes me think of Willstätter

So yes, a Prussian blue factory may be impressive. But it has nothing on the chemistry plants do for blue...

There is a lot more to say about blue in plants, but it shouldn’t surprise anyone at this point that the chapter on plants in my book opens with the picture of …
a cornflower.