Scientists at Northwestern University have stumbled serendipitously onto a method to create nanostructures called metal-organic frameworks… which also happen to be edible. Metal-organic frameworks are metals connected in a highly organized crystal structure that allows for the trapping, storage, and controlled release of gas (i.e: hydrogen or carbon dioxide) or other molecules. These structures have been proposed as a possible new targeted drug delivery system that can be seen on imaging. They also have many non-medical uses such as for gas purification and for gas storage.
Usually metal organic frameworks are made from toxic petroleum-based products. The secret to Northwestern’s great gastronomical success is gamma-cyclodextrin, a sugar produced from corn starch. Add in a little alcohol in the form of Everclear (no joke), and the scientists accidentally ended up with a huge breakthrough.
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“They taste kind of bitter, like a Saltine cracker, starchy and bland,” said Ronald A. Smaldone, a postdoctoral fellow at Northwestern.
“The metal-organic framework technology has been around since 1999 and relies on chemicals that come from crude oil,” explained Ross S. Forgan, also a postdoctoral fellow in Stoddart’s lab and co-first author of the paper. “Our main constituent is a starch molecule that is a leftover from corn production.”
For their edible MOFs, the researchers use not ordinary table sugar but gamma-cyclodextrin, an eight-membered sugar ring produced from biorenewable cornstarch. The salts can be potassium chloride, a common salt substitute, or potassium benzoate, a commercial food preservative, and the alcohol is the grain spirit Everclear.
With these ingredients in hand, the researchers actually had set out to make new molecular architectures based on gamma-cyclodextrin. Their work produced crystals. Upon examining the crystals’ structures using X-rays, the researchers were surprised to discover they had created metal-organic frameworks — not an easy feat using natural products.
“Symmetry is very important in metal-organic frameworks,” Stoddart said. “The problem is that natural building blocks are generally not symmetrical, which seems to prevent them from crystallizing as highly ordered, porous frameworks.”
It turns out gamma-cyclodextrin solves the problem: it comprises eight asymmetrical glucose residues arranged in a ring, which is itself symmetrical. The gamma-cyclodextrin and potassium salt are dissolved in water and then crystallized by vapor diffusion with alcohol.
The resulting arrangement — crystals consisting of cubes made from six gamma-cyclodextrin molecules linked in three-dimensions by potassium ions — was previously unknown. The research team believes this strategy of marrying symmetry with asymmetry will carry over to other materials.
Read more here…
Abstract in Angewandte Chemie International Edition: Metal-Organic Frameworks from Edible Natural Products