Bio-orthogonal chemical reactions between two exogenous
moieties in living beings are sought after tools that have
powerful applications in chemical biology, molecular imaging,
and medicine.[1–4] The exquisite selectivity of these reactions—
the Staudinger ligation and the strain-promoted azidealkyne
cycloaddition—has been exploited for the metabolic
labeling and subsequent tagging and visualization of biomolecules
in mice by using ex vivo detection,[1–3] and in live
zebrafish embryos.[4] However, the reaction kinetics (k2 7.6 102m1 s1) have required a high dose and large excess
of secondary reagent to achieve detectable binding. Applications
in molecular imaging and medicine that require low
doses and semi-equimolar conditions have therefore
remained unrealized. Reactions that retain selectivity but
have faster kinetics could extend in vivo chemistry to
clinically relevant procedures in mammalian disease models,
by allowing the intravenous administration of a small amount
of probe to non-invasively delineate low-abundance species.
The rapid blood clearance and excretion typical of most small
to medium-sized molecules necessitates that the reaction
occurs within minutes. Such
selectivity and effective equimolar
reactivity has never
been demonstrated in a living
animal.