Brilliantly red: the structure of carmine

Carmine is a red pigment made from dried cochineal, a scale insect that has been a source of brilliant scarlet reds in clothing and art for more than two millennia, with records dating back to 700 BC. Since the 16th century, it has been intensely traded all over the world and was one of the most important trade goods for the Spanish empire at its economic peak. Despite still being used on an industrial scale, with hundreds of metric tonnes produced annually, the exact molecular and crystal structure of the dyestuff remains undetermined. Notably, both modern-day commercial carmine and pigments prepared following historical recipes show strikingly similar diffraction patterns, indicating a common crystalline structure. Several model structures of carmine have been proposed, and so far spectroscopic measurements have hinted at a tetrameric complex containing the conjugate base of carminic acid, aluminium ions, and calcium ions. Considering its commercial and historical significance, determining the structure and composition of carmine would aid in both modern-day regulation and identification in historical samples. Here we show that the crystal structure of carmine can at last be determined using three-dimensional electron diffraction measurements. Our investigation reveals that carmine is indeed comprised of a tetrameric complex, that assembles into a nanoporous supramolecular structure with pore diameters of approximately 1.8 nm, held together by intermolecular hydrogen bonding. Our results establish a definite structure of carmine, unveiling a surprisingly complicated arrangement in a product that has been traded around the world for centuries. We anticipate that this study will prompt further investigations of age-old crystalline compounds and formulations using state-of-the-art electron crystallography methods, while also highlighting the serendipitous creation of a man-made supramolecular material that dates back thousands of years.