Zeolite Isomerizes Sugars With Enzymelike Mechanism

Lewis acid zeolites catalyze the isomerization of sugars via reaction pathways similar to those used by metalloenzymes, Mark E. Davis of Caltech and coworkers report (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1206708109). Silica zeolites with a zeolite-beta structure and containing small amounts of Ti⁴⁺ or Sn⁴⁺, known as Ti-beta and Sn-beta, respectively, have previously been shown to catalyze isomerization of glucose to fructose, a reaction carried out industrially with an immobilized enzyme, d-xylose isomerase. A better understanding of the zeolite-catalyzed reaction, which can be carried out at high temperature under acidic conditions, could make it a viable alternative to the enzyme-catalyzed reaction. Using Sn-beta, Davis and coworkers found that glucose in its pyranose form enters the zeolite’s pores where tin-containing Lewis acid sites catalyze the opening of the ring to the acyclic form. The acyclic glucose then isomerizes to the acyclic form of fructose via a hydride shift. The fructose subsequently cyclizes to the furanose form. This mechanism is similar to that facilitated by d-xylose isomerase, the researchers note. They suggest that the most likely active site is an open site without adjacent silanol groups.