Probing Interactions between Metal–Organic Frameworks and Freestanding Enzymes in a Hollow Structure

Abstract

It has been reported that the biological functions of enzymes could be altered when they are encapsulated in metal–organic frameworks (MOFs) due to the interactions between them. Herein, we probed the interactions of catalase in solid and hollow ZIF-8 microcrystals. The solid sample with confined catalase is prepared through a reported method, and the hollow sample is generated by hollowing the MOF crystals, sealing freestanding enzymes in the central cavities of hollow ZIF-8. During the hollowing process, the samples were monitored by small-angle X-ray scattering (SAXS) spectroscopy, electron microscopy, powder X-ray diffraction (PXRD), and nitrogen sorption. The interfacial interactions of the two samples were studied by infrared (IR) and fluorescence spectroscopy. IR study shows that freestanding catalase has less chemical interaction with ZIF-8 than confined catalase, and a fluorescence study indicates that the freestanding catalase has lower structural confinement. We have then carried out the hydrogen peroxide degradation activities of catalase at different stages and revealed that the freestanding catalase in hollow ZIF-8 has higher activity.

Hierarchically Porous Biocatalytic MOF Microreactor as a Versatile Platform towards Enhanced Multienzyme and Cofactor‐Dependent Biocatalysis

“…Based on the above evidence, we conclude that there are three factors that collectively contribute to the improved biocatalytic cascade efficiency: (i) the enzyme structure reorientation in their lower energy form which could led to more active site exposure; [23] (ii) the enhanced interfacial contact between the enzymes and ZIF‐L after TA etching results in lower diffusional limitations as compared to the more aggregated enzymes in the small micropores; [43, 44] and (iii) the significantly improved substrate diffusion in the large mesopores. A Knudsen diffusion model [45] was further employed to quantify the enhanced substrate diffusion in the newly created mesopores.…”

“…Consequently,c onsistent results from FTIR and solid-state NMR reveal the hierarchically porous MOF enables the enzymes to reorientate and spread in their lower surface energy form, forming stronger coordination with the MOF framework. Based on the above evidence,weconclude that there are three factors that collectively contribute to the improved biocatalytic cascade efficiency:( i) the enzyme structure reorientation in their lower energy form which could led to more active site exposure; [23] (ii)the enhanced interfacial contact between the enzymes and ZIF-L after TA etching results in lower diffusional limitations as compared to the more aggregated enzymes in the small micropores; [43,44] and (iii)the significantly improved substrate diffusion in the large mesopores.AKnudsen diffusion model [45] was further employed to quantify the enhanced substrate diffusion in the newly created mesopores.C alculations showed that the substrate diffusion rate was improved by c.a. 14 times in the optimized TA etched ZIF-L than in the pristine,u netched ZIF-L (Supplementary Note).…”

“…[25] Their results suggested that the encapsulated enzyme presented al ess constrained conformation within hollow MOFs,a sd etermined from infrared (IR) spectroscopy. [23] Nevertheless,e nzyme aggregation could still occur inevitably in the overly expanded space. [39,40] Although IR technique could provide some useful information for the conformation changes of enzymes in the confined space, [5,20,23] the understanding of the interfacial interactions between organic ligands,m etal ions,e nzymes and the etching reagent during and after the etching process from atomic level is required, which is important for the design of ideal hierarchically porous MOFs as functional matrices for the multienzyme and cofactor-dependent enzyme biocatalysis.…”

“…Creating meso‐/macro voids inside the nanoporous MOFs enables the encapsulation of large‐sized guest species in its relatively free state in the large pores while improving the substrate diffusion [23] . So far, various bottom‐up and top‐down approaches have been reported to generate large voids or defects in pristine nanoporous MOFs as hierarchically porous supports for enzyme immobilization [24–30] .…”

“…Very recently, Tsung and co‐workers probed the interactions between encapsulated enzyme and hollow‐shell MOF matrices [25] . Their results suggested that the encapsulated enzyme presented a less constrained conformation within hollow MOFs, as determined from infrared (IR) spectroscopy [23] . Nevertheless, enzyme aggregation could still occur inevitably in the overly expanded space [39, 40] .…”