Living catalysis: Gold complex imaged with NIR in mice

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  • Published: Mar 1, 2017
  • Author: David Bradley
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thumbnail image: Living catalysis: Gold complex imaged with NIR in mice

NIR imagery

The propargyl ester imaging probe Cy7.5-OProp was used as a near-infrared imaging probe for whole-body imaging to reveal how catalysis with a gold complex can be carried out in live mice, according to research published in the journal Angewandte Chemie

The propargyl ester imaging probe Cy7.5-OProp was used as a near-infrared imaging probe for whole-body imaging to reveal how catalysis with a gold complex can be carried out in live mice, according to research published in the journal Angewandte Chemie.

Kazuki Tsubokura, Kenward Vong, Ambara Pradipta, Akihiro Ogura, Sayaka Urano, and Katsunori Tanaka of the Biofunctional Synthetic Chemistry Laboratory, RIKEN, Saitama, Tsuyoshi Tahara, Satoshi Nozaki, Hirotaka Onoe, and Yasuyoshi Watanabe of Waseda University, Japan, and Yoichi Nakao, Regina Sibgatullina, and Almira Kurbangalieva of Kazan Federal University, Russia, explain how the application of metal complex catalysts in living things has gained significant attention only in the last three or four years. The technique might have applications in carrying out metal-mediated reactions with therapeutic, diagnostic and research applications.

The team points out that so far, metal complex catalysis within biological systems has been limited largely to cellular and bacterial systems; although one example of catalysis in embryo yolks of zebrafish has also been reported. For example, scientists have carried out ruthenium-mediated uncaging of allyl carbamates, protein labelling, and iron-mediated uncaging of azide groups, palladium-mediated uncaging of allenyl, propargyloxycarbonyl, and allyl carbamate groups, as well as palladium-mediated N-dealkylation. Also demonstrated are Suzuki–Miyaura and Sonogashira cross-couplings, and gold-mediated hydroarylative cyclizations.

Low concentration targets

To expand the repertoire of metal catalysts, and further develop metal catalysis for whole organisms, the team has now designed and developed a glycoalbumin–gold(III) complex that can be exploited to carry out localized propargyl ester amidation with organ-specific proteins in live laboratory mice. The reactions can be imaged with Cy7.5- and TAMRA-linked propargyl ester based fluorescent probes.

"This targeting system could enable the exploitation of other metal catalysis strategies for biomedical and clinical applications," the team reports. Crucially, with their approach, the researchers in Japan and Russia have managed to balance reactivity towards water, air, and non-target cellular components with reactivity towards the desired substrates that are present at much lower concentrations.

Adaptive catalysis

"This work explores the adaptation and usage of organ-targeting glycans as biologically compatible metal carriers," the team explains. The team previously developed the targeting method based on N-glycoalbumins synthesized by immobilizing ten N-glycan molecules on to albumin through the "RIKEN click" reaction. Thus the glycoalbumin can deliver the biocompatible metal catalyst, namely the gold complex. The gold complex acts as an efficient organometallic catalyst for the reaction between biologically relevant molecules and organic substrates in this case in the liver or the intestine as target organs. The team carried out whole-body fluorescence imaging of the live animal. It took just two hours following administration of the catalyst and the substrate (the functionalized fluorescent dye) to see strong fluorescence in both target organs thus demonstrating successful in vivo gold catalysis. Key to the development of this technique is that no antibodies are required for the targeting protein rather the targeting is through organic chemistry. Until now, reactions such as those mentioned earlier have not addressed the non-invasive targeting of specific tissues.

The team suggests that applications such as catalytic uncaging of active anticancer enzymes selectively at tumour sites will be possible with this technology. They are now exploring other N-glycan structures that might be used to target different tissues and possibly tumour tissues selectively.

Related Links

Angew Chem Int Edn Engl 2017, online: "In Vivo Gold Complex Catalysis within Live Mice"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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