Triazinium Ligation: Bioorthogonal Reaction of N1-Alkyl 1,2,4-Triazinium Salts

The development of reagents that can selectively react in complex biological media is an important challenge. Here we show that N1-alkylation of 1,2,4-triazines yields the corresponding triazinium salts, which are three orders of magnitude more reactive in reactions with strained alkynes than the parent 1,2,4-triazines. This powerful bioorthogonal ligation enables efficient modification of peptides and proteins. The positively charged N1-alkyl triazinium salts exhibit favorable cell permeability, which makes them superior for intracellular fluorescent labeling applications when compared to analogous 1,2,4,5-tetrazines. Due to their high reactivity, stability, synthetic accessibility and improved water solubility, the new ionic heterodienes represent a valuable addition to the repertoire of existing modern bioorthogonal reagents.

Automated summary

The development of reagents selective in complex biological media is a significant challenge. We demonstrate that N1-alkylation of 1,2,4-triazines produces more reactive triazinium salts when reacting with strained alkynes compared to the parent 1,2,4-triazines. These salts facilitate efficient modification of peptides and proteins through a powerful bioorthogonal ligation. The positively charged N1-alkyl triazinium salts possess favorable cell permeability and outperform analogous 1,2,4,5-tetrazines for intracellular fluorescent labeling applications. These new ionic heterodienes exhibit high reactivity, stability, synthetic accessibility, and improved water solubility, making them valuable additions to the repertoire of modern bioorthogonal reagents.