The interaction between enzyme and substrates is usually accompanied by induced-fit conformational changes for better realizing pre-orientation and activation of the substrate and catalytic active center in its binding pocket. Coordination-drive self-assembly strategies have been applied to construct a large number of molecular cages with cavity structure to mimic artificial enzyme systems.
However, current molecule cages often have rigid fixed cavities, resulting in poor substrate binding ability or severe product inhibition problems, and it is difficult to achieve high-efficiency enzyme-mimicking catalytic functions.
In a study published in J. Am. Chem. Soc., the research group led by Prof. SUN Qingfu from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences, reported a new type of dynamic molecular cage system, which integrated the functions of visible light absorption, redox activity, and substrates-adaptive structural transformation.
The researchers synthesized a new molecular capsule by coordination-assembled from three anthracene-bridged bis-TPT [TPT = 2,4,6-tris(4-pyridyl)-1,3,5-triazine] ligands and six (bpy)Pd(NO3)2 (bpy = 2,2′-bipyridine) in aqueous solution. Once substrates bind to its hydrophobic cavity, this capsule would undergo timely and quantitatively capsule-to-bowl transformation. The installation of anthracene units and the charge-transfer absorption in visible region from quintuple π-π stacked interactions on the new-formed host-guest complex efficiently facilitated aerobic photooxidation for the sulfide guests by visible-light irradiation.
Through spectroscopic measurement, the researchers observed an obvious excitation band centered at 465 nm (λem = 525 nm) of a typical inclusion complex, indicating the complex can be effectively excited by the charge-transfer absorption bands. They also performed a series of inhibition experiments by adding a stronger binding guest 1-adamantanecarboxylic acid within the capsule (Pd6L3) or an effective blocker for the bowl (Pd4L2), both the reduced yield indicated that dynamic transformative behavior is crucial to the excellent catalytic cycles and product selectivity. The experiment results suggested O2·- is the primary oxidizing species during the photocatalysis.
Besides, the researchers elucidated that the products can be easily replaced by new substrates due to the hydrophilicity and reduced shape complementarity of the sulfoxides, and the capsule was finally recovered.
This study shows the first example of a coordinated supramolecular system with both substrate-allosteric properties and photocatalytic functions, and offers new insight into the construction of artificial biomimetic catalysis systems.
Schematic illustration of the research (Image by Prof. SUN’s group)
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