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Using advanced electron crystallography techniques, researchers at Stockholm University have succeeded in determining the structure of the historically significant red pigment carmine. It turns out that the substance, used today in products such as candy and paint, has a complex porous structure.

The study is in the journal Crystal Growth & Design.

Carmine is a natural red coloring agent produced from cochineal extract. The extract from these insects is rich in carminic acid, which is combined with aluminum (Al) and calcium (Ca) to produce carmine. Analysis using advanced electron microscopy techniques has now revealed that this pigment has an unexpected structure. It is a so-called metal complex, built from two calcium ions, two aluminum ions, and four organic ligand molecules of carminic acid.

Together, these components form a well-defined, three-dimensional porous structure that has intrigued researchers at the Department of Chemistry.

"It was truly surprising that such a long-used pigment made from a naturally occurring molecule had this type of structure," says Erik Svensson Grape, former Ph.D. student at Stockholm University and currently a postdoctoral researcher at the University of Oregon and Uppsala University.

He initiated the study of carmine using the new technique, driven by curiosity about how materials are built, and with the hope of discovering new potential applications.

"Chemists have only recently begun to deliberately design and use materials with this type of porous architecture—for example in catalysis, pollutant capture, and energy storage. The first commercially available nanoporous materials were only developed in the middle of the 20th century," says Grape.

Why has the structure of carmine remained unknown until now?

"Although carmine is a crystalline substance, it hasn't been possible to grow large enough crystals for analysis using traditional methods like single-crystal X-ray diffraction. But with the help of the new electron microscopy techniques available at Stockholm University—specifically 3D electron diffraction (MicroED)—we've now been able to determine the structure of carmine from real pigment samples," explains Grape.

This new knowledge may open new potential applications for carmine, including in environmental applications.

"The well-defined structure of carmine could make it easier to identify the pigment in historical artifacts. And the porous structure could also lead to new applications and inspire the development of new materials, for example as an adsorbent to capture pollutants," says Grape.

An adsorbent is a material that can concentrate another substance, called an adsorbate, on its surface through a process called adsorption, in which molecules adhere to a surface. Adsorbents are often used to purify water and air, or to separate different substances.

Carmine may also become more relevant as a natural alternative to synthetic coloring agents in the food industry.

"As synthetic food dyes are increasingly scrutinized by the public due to health concerns, natural pigments like carmine may soon be used more often in the food industry," Grape explains.