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• Enzymes are extremely complicated biocatalysts in several industrial processes due to their numerous advantages over conventional catalysts, including lower physiological and environmental toxicity, exceptional selectivity, and milder reaction conditions [1]. However, their widespread applications are constrained by the high production costs, low operational stability, and complexity involved in the recovery and reusability of enzymes [2]. During the previous decade, peptide-based biomimetic catalysts have been proven to be more operational and robust than certain advanced enzymes [3]. However, challenges in achieving operational stability, recyclability, and a limited operating range have hindered the widespread implementation of peptides in practical applications [4]. To tackle such stability barriers without compromising the activity of peptide assemblies, one of the favorable approaches is to immobilize them into an inert and robust matrix [5]. Various porous materials, like supramolecules [6], zeolites [7] and metal-organic frameworks [8], have been utilized. Beyond them, covalent organic frameworks (COFs) with atomic level of control render them an optimal selection for armoring the biomimetic catalysts [9,10].

  Recently, Banerjee's group reported a new strategy to stabilize biomimetic catalysts within COF backbones through cladding technique [11]. As shown in Fig. 1, firstly, two different decanoic acid-functionalized peptide-amphiphiles (C₁₀FFVK and C₁₀FFVR) were prepared using the microwave-assisted Fmoc-based solid phase method in 40% acetonitrile/water (0.1% TFA) for 30 days. These two peptide-amphiphiles showed nanotubular morphologies (Fig. 1a). Secondly, COF-armored peptide nanotubes were produced using interfacial crystallization (Fig. 1b). The nanotubular morphology of COF-armored peptide assembly was confirmed by SEM (Figs. 1d and e), TEM (Fig. 1f), and AFM (Fig. 1g) characterizations.

  Figure 1

  

  Figure 1.  Schematic representation of (a) self-assembly of C₁₀FFVK and C₁₀FFVR peptides into nanotubes, (b) synthesis of TpAzo-peptide nanotubes by interfacial crystallization with peptide nanotubes and zoomed-in structural representation of TpAzo COF backbone, and (c) catalysis with TpAzo−C₁₀FFVK nanotubes. (d, e) SEM images, (f) TEM image, and (g) AFM image of TpAzo−C₁₀FFVK nanotubes. Reproduced with permission [11]. Copyright 2023, American Chemical Society.

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  To generalize this cladding technique, Banerjee and his co-workers prepared several biomimetic catalysts with hollow tubular morphology by different peptide-amphiphile-based tubular assemblies (C₁₀FFVK and C₁₀FFVR) and different COFs (TpAzo COF and TpDPP COF). Then, they investigated the growth mechanism of COF on peptide-amphiphile-based nanotubes and proposed that the noncovalent interaction between the surface-exposed imine groups on peptide-amphiphile-based nanotubes and carbonyl groups on COFs facilitate the generation of COF crystallites on the surface of peptide-amphiphiles. The electron microscopy images and TGA data confirmed the presence of peptide-amphiphile-based nanotubes inside the COF matrix.

  Moreover, in order to exam the stability and activity of biomimetic catalysts prepared by the COF cladding technique, Banerjee and his co-workers probed the catalytic potential of these catalysts in carbon−carbon bond cleavage of methodol to produce 6‑methoxy-2-naohthaldehyde (Fig. 1c). Owning to the porous nature and high stability of the coated TpAzo COF, the TpAzo−C₁₀FFVK biomimetic catalyst (yield 7%) displayed competitive performance to C₁₀FFVK (yield 6%) in buffer system (Figs. 2a and b). In addition, the yield of product barely declined after ten continuously reuses of TpAzo−C₁₀FFVK biomimetic catalyst, confirming its excellent recyclability (Fig. 2c). Impressively, TpAzo−C₁₀FFVK biomimetic catalyst promoted catalytic conversion of methodol to 6‑methoxy-2-naohthaldehyde in different organic solvents (acetone, acetonitrile, dichloromethane, and ethyl acetate), where pristine C₁₀FFVK nanotubes were not that efficient in this process (Fig. 2d). After subjecting the TpAzo−C₁₀FFVK biomimetic catalyst to a stability test in organic solvents, specifically acetone, acetonitrile, and dichloromethane, for a duration of three days, it was observed that the C₁₀FFVK nanotubes remained intact within the TpAzo COF backbones even after exposure to the solvents (Figs. 2e-j). These results implied that the peptide-based biomimetic catalyst is highly efficient and stable in buffer and various general organic solvents after COF cladding.

  Figure 2

  

  Figure 2.  (a) TpAzo−C₁₀FFVK nanotube catalysis fluorescence data of the product formation with reaction time. (b) HPLC data of the catalysis using TpAzo−C₁₀FFVK nanotubes, C₁₀FFVK nanotubes, and pristine TpAzo COF. (c) Bar diagrams represent the yields of catalysis recyclability data with TpAzo−C₁₀FFVK nanotubes. (d) Comparable yields of the catalysis in different solvents. (e − g) TGA data and (h − j) SEM images after treatment with acetonitrile, acetone, and DCM solvent, respectively. Reproduced with permission [11]. Copyright 2023, American Chemical Society.

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  In conclusion, this work from Banerjee's group has provided a systematic investigation on the contribution of COF cladding to COF-armored biomimetic catalysts. The strong noncovalent interactions between the peptide nanotubes and the COF backbones, as well as the porous nature and high stability of COF endow the biomimetic catalysts with enhanced stability and preserved activity. This universally applicable stabilization strategy shows immense promise in the advancement of versatile biomimetic catalysts across a range of practical applications, such as antimicrobial coating, tissue engineering, drug delivery, and bioinspired nanotechnology.

  Acknowledgements

  We thank the support from Nanqiang Young Talents Supporting Program of Xiamen University.

  
  
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  Figure 1  Schematic representation of (a) self-assembly of C₁₀FFVK and C₁₀FFVR peptides into nanotubes, (b) synthesis of TpAzo-peptide nanotubes by interfacial crystallization with peptide nanotubes and zoomed-in structural representation of TpAzo COF backbone, and (c) catalysis with TpAzo−C₁₀FFVK nanotubes. (d, e) SEM images, (f) TEM image, and (g) AFM image of TpAzo−C₁₀FFVK nanotubes. Reproduced with permission [11]. Copyright 2023, American Chemical Society.

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  Figure 2  (a) TpAzo−C₁₀FFVK nanotube catalysis fluorescence data of the product formation with reaction time. (b) HPLC data of the catalysis using TpAzo−C₁₀FFVK nanotubes, C₁₀FFVK nanotubes, and pristine TpAzo COF. (c) Bar diagrams represent the yields of catalysis recyclability data with TpAzo−C₁₀FFVK nanotubes. (d) Comparable yields of the catalysis in different solvents. (e − g) TGA data and (h − j) SEM images after treatment with acetonitrile, acetone, and DCM solvent, respectively. Reproduced with permission [11]. Copyright 2023, American Chemical Society.

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