The discovery of the structure of the purple color molecule by Paul Friedländer is described in detail.
You can see the whole method, from the extraction out of the Murex shellfishes to the chemical extraction.
Nous décrivons la découverte, par Paul Friedlaänder, de la structure moléculaire de la couleur pourpre.
Vous pouvez voir toute la méthode, depuis l’extraction des coquillages Murex jusqu’à l’extraction chimique.
P.S. It’s no wonder from here why Paul Friedlander would tell R’ Yitzchak Herzog that murex trunculus snails only produce purple and not blue.
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English synopsis:
Purple is an animal-based violet color used since ancient times. It is extracted from a shellfish called the Murex.
We are going to follow the trail of the German chemist Paul Friedländer, who isolated and identified the molecule responsible for this purple dye.
Paul Friedländer was born in 1857 in the city of Königsberg, now called Kaliningrad. In 1878 he earned his doctorate in Munich, in the famous Bayer laboratory, the world specialist in dyes—where the structure of indigo had been discovered.
In 1895 he moved to Vienna, Austria, to continue his research. In 1905 Friedländer discovered a violet dye in the indigo family. This dye was easy to produce and very resistant to light, and it went on to be an industrial success.
That violet reminded Friedländer of the famous purple of the ancients, so prestigious in the Roman Empire. Studies from the 19th century had already suggested that this colorant had a structure close to indigo. Friedländer wondered whether the ancient purple—so similar in shade—might be chemically related to an indigo-type compound.
To confirm his hypothesis, a large quantity of shellfish had to be collected in order to extract the precious dye. The zoological station in Trieste, under the direction of Professor Cori, assisted him. About 12,000 Murex were collected during the summer of 1908.
The dye was extracted from the species once called Murex brandaris and now known as Bolinus brandaris.
We will now extract the purple dye using the same technique that the Romans—and even earlier, the Phoenicians—probably used.
The shells are placed in a mortar, and with a pestle the shell is fractured at the right spot to reveal a slightly yellowish part: the hypobranchial gland. This gland is carefully removed with tweezers.
Next, the glands are spread on filter paper and exposed to light.
Exposure to light triggers a transformation that makes the color appear. In the living shellfish, the purple color is not present as such; it is hidden in a colorless form which, under light, gradually shifts through yellow, green, blue, and finally violet.
The speed of this transformation is directly linked to the intensity of the light and to the thickness of the layer of extracted glands. Generally, the process is complete after about half an hour to an hour. If the same operation were done in darkness, no color would appear.
The odor is very strong, especially in warm conditions, and it attracts flies.
The next step is to purify and concentrate the color deposited on the filter paper. The first stage is to treat the filter paper with a 30% sulfuric acid solution—strong enough to remove the impurities, but not so strong that it destroys the dye. This treatment removes most of the proteins mixed with the color, along with mucus and other impurities.
After this, only the color remains on the paper.
A second, more complex purification step is required: the dye must be extracted, but it is only weakly soluble in solvents like alcohol. Like Friedländer, we use ethyl benzoate (the ethyl ester of benzoic acid). This solvent has a higher boiling point, which helps dissolve the dye.
We place the solvent in a flask and the pieces of colored, acid-treated filter paper into a cellulose cartridge. This is used in an apparatus called a Soxhlet extractor, named after the chemist who developed it. This equipment was not commercially available in Friedländer’s time; he had it made by a glassblower.
For the extraction, we place the cartridge into the Soxhlet body. The flask containing the solvent is heated with a Bunsen burner. The solvent vapors rise, travel through the side tube, cool slightly at the top, and then drip as liquid into the cartridge below.
A clever siphon system returns the solvent to the flask once the liquid reaches the top of the siphon. In this way, the solvent can be used continuously an unlimited number of times, greatly reducing consumption.
In Friedländer’s experiment, the process was repeated until the dye had been completely extracted from the filter paper.
After several hours, all the dye is extracted. The liquid in the flask is cooled, and the small amount of colorant precipitates as tiny crystals. These crystals constitute the purified purple.
We extracted purple from about a dozen shellfish. Friedländer and his collaborators extracted it from 12,000 Murex. One can imagine the amount of work required to obtain about 1.4 grams of a violet, crystalline product.
He analyzed it and determined that it was 6,6′-dibromoindigo. The Deutsches Museum in Munich still preserves an original vial containing about 520 dried Murex glands dating from Friedländer’s 1908 experiments. It also houses a sample of textile dyed with synthetic purple from his laboratory.
In 1909 Friedländer published his results. His analyses showed that natural purple does not contain sulfur—unlike his thioindigo. Despite the similarity in color, his artificial product was therefore not identical to the shellfish dye.
The presence of another element, bromine, was discovered in the natural product. The high bromine content—around 38%—allowed him to calculate a structure corresponding to two bromine atoms per indigo molecule.
Assuming a symmetrical structure, four possibilities remained. Three of them were blue, and only 6,6′-dibromoindigo was violet.
The mystery surrounding this color for millennia was finally solved thanks to Friedländer’s work.
