Chemical Engineering News reported on a modified synthesis method for producing γ-cyclodextrin which spars steam distillation step (“Taking the steam out of γ-cyclodextrin synthesis”).
C&N explains that the chemists start with maltohexaose—a linear chain of 6 glucose units—and add the enzyme cyclodextrin glucanotransferase. This enzyme can shorten, elongate, cyclize, or open sugary molecules to form different structures, including γ-cyclodextrin. These sugary molecules are in equilibrium until the chemists add the Z isomer of the hydrazone template. Two of these Z-hydrazones bind within γ-cyclodextrin. That binding stabilizes the molecule and pushes the equilibrium to favor γ-cyclodextrin formation. Then the chemists destroy the enzyme and use light to switch to the E isomer of the hydrazone, which no longer fits within γ-cyclodextrin.
The paper of Yang et al. (2021) descibes the use a photoremovable hydrazone template as a proof-of-concept strategy for increasing the efficiency of the enzymatic synthesis of γ-CD and lowering the associated production costs. The results show that while both the Z and E isomers of the hydrazone switch form low affinity (K = 250–725 M−1) complexes with β-CD (1:1), only the Z isomer of the switch can be included in γ-CD (2:1; K2 = 8,970 M−1). This property was utilized to preferably synthesize γ-CD and increase its yield by 6-fold and then taking advantage of the photoremovable nature of the template to isolate the product. Considering the very limited number of photoswitches that can bind to γ-CDs, it is anticipated that this newly discovered host-guest couple will open the way for designing γ-CD-based adaptive materials.
Sirun Yang, Dennis Larsen, Maria Pellegrini, Dale F. Mierke, Sophie R. Beeren, Ivan Aprahamian (2021) Dynamic enzymatic synthesis of γ-cyclodextrin using a photoremovable hydrazone template