Assessment of the bioorthogonality of the nitrile imine 1,3-Dipole Mhairi Gibson 1 , Craig Jamieson 1 , Jonathan Pettinger 2 1 University of Strathclyde, Glasgow, UK, 2 GlaxoSmithKline, Stevenage, UK
The nitrile imine (NI) 1,3-dipole is a highly reactive and readily accessible synthetic intermediate generated via the photolysis of 2,5-disubstituted tetrazoles. [1] Its ability to participate in 1,3-dipolar cycloadditions has enabled its application in a variety of synthetic methods. [2] Of note, NIs have found application in medicinal chemistry, [3] materials chemistry, [4] and more recently, bioorthogonal chemistry. [5] NI-mediated photoclick reactions have recently found traction in bioorthogonal labelling techniques due to the light-activated, traceless nature of the system and the formation of stable, fluorescent adducts with biomolecules modified with an appropriate dipolarophile. While NIs are renowned for their proclivity towards cycloadditions, this species exhibits broad reactivity with a range of nucleophilic functionalities. [6-8] Such functionalities are ubiquitous in biomolecules and therefore their promiscuous reactivity with the NI dipole may hinder its application as a true bioorthogonal labelling tool. Previous work in our group has sought to explore the reactivity profile of the NI species through a series of competition experiments utilising a library of nucleophilic model substrates and dipolarophiles. Interestingly, the quantification of NI dipole reactivity with a range of carboxylic acid moieties revealed an enhancement in reactivity with decreasing pK a of the acidic coupling partner. These findings have been expanded to assess the biorthogonality of the dipole through the competitive reaction of an electronically activated dipolarophile versus a highly acidic fluorinated carboxylic acid. A range of NI species were generated through photolysis of a 2,5-disubstituted tetrazole and their reactivity with a model substrate was quantified. The selectivity observed demonstrated that NI reactivity can be tuned via modulation of the pK a of an acidic coupling partner. The next phase of this work sought to exploit this observation by exemplifying the application of highly acidic carboxylic acid moieties as novel bioorthogonal handles for NI-mediated photoclick reactions. A suitable NI precursor has been identified which enhances chemoselectivity for the bioorthogonal handle, suppressing the reactivity of endogenous competing nucleophiles. Current work is ongoing to incorporate a selection of highly acidic bioorthogonal handles into model peptide sequences containing multiple nucleophilic amino acid residues, allowing the chemoselectivity of the NI with these novel bioorthogonal handles to be assessed. References 1. R. Huisgen et al ., J. Org. Chem ., 1959, 24 , 892-893. 2. R. Huisgen et al ., Tetrahedron , 1962, 17 , 3-29. 3. P. Conti et al ., Chemistry and Biodiversity , 2008, 5 , 657-663. 4. Y. Iwakura et al ., Die Makromolekulare Chemie , 1966, 97 , 278-281.
5. Z. Li et al ., Angew. Chem. Int. Ed. , 2006, 55 , 2002-2006. 6. R. Huisgen et al ., Chem. Ber. , 1961, 94 , 2503-2509. 7. A. Herner et al ., J. Am. Chem. Soc. , 2016, 138 , 14609-14615. 8. L. Bruché et al ., J. Chem. Soc. Perkin Trans. 1 , 1981, 2245-2249.
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