Fast Diazaborine Formation of Semicarbazide Enables Facile Labeling of Bacterial Pathogens

Bioorthogonal conjugation
chemistry has enabled the development
of tools for the interrogation of complex biological systems. Although
a number of bioorthogonal reactions have been documented in literature,
they are less ideal for one or several reasons including slow kinetics,
low stability of the conjugated product, requirement of toxic catalysts,
and side reactions with unintended biomolecules. Herein we report
a fast (>103 M–1 s–1) and bioorthogonal conjugation reaction that joins semicarbazide
to an aryl ketone or aldehyde with an ortho-boronic
acid substituent. The boronic acid moiety greatly accelerates the
initial formation of a semicarbazone conjugate, which rearranges into
a stable diazaborine. The diazaborine formation can be performed in
blood serum or cell lysates with minimal interference from biomolecules.
We further demonstrate that application of this conjugation chemistry
enables facile labeling of bacteria. A synthetic amino acid D-AB3,
which presents a 2-acetylphenylboronic acid moiety as its side chain,
was found to incorporate into several bacterial species through cell
wall remodeling, with particularly high efficiency for Escherichia
coli. Subsequent D-AB3 conjugation to a fluorophore-labeled
semicarbazide allows robust detection of this bacterial pathogen in
blood serum.