Sequential “Click” – “Photo-Click” Cross-Linker for Catalyst-Free Ligation of Azide-Tagged Substrates

[Image: see text] Heterobifunctional linker allows for selective catalyst-free ligation of two different azide-tagged substrates via strained-promoted azide–alkyne cycloaddition (SPAAC). The linker contains an azadibenzocyclooctyne (ADIBO) moiety on one end and a cyclopropenone-masked dibenzocyclooctyne (photo-DIBO) group on the other. The first azide-derivatized substrate reacts only at the ADIBO end of the linker as the photo-DIBO moiety is azide-inert. After the completion of the first SPAAC step, photo-DIBO is activated by brief exposure to 350 nm light from a fluorescent UV lamp. The unmasked DIBO group then reacts with the second azide-tagged substrate. Both click reactions are fast (k = 0.4 and 0.07 M(–1) s(–1), respectively) and produce quantitative yield of ligation in organic solvents or aqueous solutions. The utility of the new cross-linker has been demonstrated by conjugation of azide functionalized bovine serum albumin (azido-BSA) with azido-fluorescein and by the immobilization of the latter protein on azide-derivatized silica beads. The BSA–bead linker was designed to incorporate hydrolytically labile fragment, which permits release of protein under the action of dilute acid. UV activation of the second click reaction permits spatiotemporal control of the ligation process.