4-Oxo-4H-pyridino[1,2-a]pyrimidine–3–diazonium and 1-cyano-4-oxo-4H-quinolizine–3–diazonium tetrafluoroborate were selected as starting reagents for photochemical sulfanylations and arylations, as well as thermal sulfanylations with esters of N-Boc-protected cysteine, cystine and aromatic amino acid. The stability of both starting reagents in aqueous media was investigated to enable further coupling of (aza)quinolizine diazonium salts to proteins. Both diazonium salts were successfully used in bioconjugation reactions. We demonstrated that (aza)quinolizine diazonium tetrafluoroborates can be used for fluorescent labeling. Under photocatalyzed conditions, these diazonium salts bind to thiol and disulfide bonds of cysteine residues, as well as to aryl groups of aromatic amino acid residues. In contrast, the sulfanylation reaction is favored under thermal conditions. Bovine serum albumin (BSA) was selected as a model protein. We also developed a method for quantifying binding and optimized binding conditions.
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