Synthesis of Bioconjugation Reagents for Use in Covalent Cross-Linking of Proteins by Azide–Alkyne Cycloaddition

A series of azido- and cyclooctyne-functionalized N-hydroxysuccinimidyl esters (NHS esters) and benzotriazolides were prepared and used as N-acylation reagents to obtain azido- (BSA-1) and cyclooctyne-functionalized bovine serum albumin proteins (BSA-2), fluorescein derivatives 5 and 6, and homobifunctional linkers 3 and 4. Strain-promoted azide-alkyne cycloaddition (SPAAC) and copper-catalyzed azide-alkyne cycloaddition (CuAAC) of azido- functionalized fluorescent probe 5 and alkyne- functionalized fluorescent probe 6 with complementary functionalized proteins BSA-2 and BSA-1 yielded fluorescent cycloadducts BSA-2-5 and BSA-1-6. These cycloadducts were used to determine the loading of BSA-1 and BSA-2 with the respective azido and cyclooctyne groups based on their molar absorbances and fluorescence intensities. Dimerization through covalent cross-linking of BSA was then performed by SPAAC between azido-functionalized BSA-1 and cyclooctyne-functionalized BSA-2, and by treating BSA-1 and BSA-2 with 0.5 equiv. of com-plementary bis-cyclooctyne linker 4 and bis-azide linker 3. Although the formation of covalent dimers BSA-1-2-BSA, BSA-1-6-1-BSA, and BSA-2-5-2-BSA was detected by SDS-PAGE analysis, this was a minor process, and most of the functionalized BSA did not form covalent dimers.