Enzyme-based biocatalysts are a key aspect of industrial green-chemistry initiatives, but they often don’t have the required stability to perform in industrial settings. A strategy for allowing enzymes to function longer and under harsher conditions has been to encapsulate them within metal-organic frameworks (MOFs), a class of lightweight porous structures with high surface area. One way to do this is liquid-assisted grinding (LAG), a mechanochemical approach that promotes self-assembly of MOF metals and ligands.
A project by researchers at the University of Missouri (UM; Columbia; www.missouri.edu) has been developing a method for synthesizing a wider and more customizable range of MOF-protected enzymes in a more eco-friendly way. The researchers have developed a technique for using water as a LAG agent, rather than organic solvents like dimethyl formamide (DMF), which can denature enzymes and requires hazardous waste disposal. The technique also improves upon an alternative path to synthesizing MOF-encapsulated enzymes called co-crystallization, where yields are often poor and stability is low.
Source: University of Missouri
Called eco-LAGent, the UM technique was used to immobilize enzymes in 12 MOFs composed of different metal-ion-ligand combinations, while maintaining enzyme activity, the researchers say. “Our collection of MOFs includes a number of low- to non-toxic metal ions, minimizing the environmental concerns. The resultant MOFs display different stabilities in acidic or basic conditions, making it possible to immobilize enzymes according to their optimal pH conditions,” says Zhongyu Yang, associate professor at UM.
Eco-LAGent synthesis reduces the water volume and enhances the contact efficiency between metal ions and ligands through the grinding of stainless-steel beads. The grinding action changes the contact interface of the metals and ligands, in comparison to the random collisions of metal ions and ligands in water-phase co-crystallization, Yang points out.
MOFs synthesized using the eco-LAGent method can be degraded under proper pHs for metal ion/ligand recovery, while the stainless-steel balls can be easily reused.
By expanding the available “library” of enzyme-encapsulated MOF biocatalysts synthesized using the eco-friendly technique, the researchers were able to achieve improved thermal and pH stability and significantly enhanced product yields, according to the Missouri team, while also proving the high chance of eventually scaling up to industrial applications, they say.
Work by the Missouri researchers generalizes the synthesis method to a wider range of MOFs and “opens a new avenue to encapsulate enzymes in high-quality crystals with a customized selection of metal ions and ligands under green and scalable conditions,” Yang states.The researchers are now developing ways to use deep eutectic solvents (DES) to further expand the possibilities.