Tunable Heteroaromatic Nitriles for Selective Bioorthogonal Click Reaction with Cysteine

The binucleophilic properties of 1,2-aminothiol and its
rare occurrence
in nature make it a useful reporter for tracking molecules in living
systems. The 1,2-aminothiol moiety is present in cysteine, which is
a substrate for a biocompatible click reaction with heteroaromatic
nitriles. Despite the wide range of applications for this reaction,
the scope of nitrile substrates has been explored only to a limited
extent. In this study, we expand the chemical space of heteroaromatic
nitriles for bioconjugation under physiologically relevant conditions.
We systematically assembled a library of 116 2-cyanobenzimidazoles,
1-methyl-2-cyanobenzimidazoles, 2-cyanobenzothiazoles, and 2-cyanobenzoxazoles
containing electron-donating and electron-withdrawing substituents
at all positions of the benzene ring. The compounds were evaluated
for their stability, reactivity, and selectivity toward the N-terminal
cysteine of model oligopeptides. In comparison to the benchmark 6-hydroxy-2-cyanobenzothiazole
or 6-amino-2-cyanobenzothiazole, we provide highly selective and moderately
reactive nitriles as well as highly reactive yet less selective analogs
with a variety of enabling attachment chemistries to aid future applications
in bioconjugation, chemical biology, and nanomaterial science.