<i>ortho</i>-Methoxyphenols as Convenient Oxidative Bioconjugation Reagents with Application to Site-Selective Heterobifunctional Cross-Linkers

The synthesis of complex protein-based bioconjugates has been facilitated greatly by recent developments in chemoselective methods for biomolecular modification. The oxidative coupling of <i>o</i>-aminophenols or catechols with aniline functional groups is chemoselective, mild, and rapid; however, the oxidatively sensitive nature of the electron-rich aromatics and the paucity of commercial sources pose some obstacles to the general use of these reactive strategies. Herein, we identify <i>o</i>-methoxyphenols as air-stable, commercially available derivatives that undergo efficient oxidative couplings with anilines in the presence of periodate as oxidant. Mechanistic considerations informed the development of a preoxidation protocol that can greatly reduce the amount of periodate necessary for effective coupling. The stability and versatility of these reagents was demonstrated through the synthesis of complex protein–protein bioconjugates using a site-selective heterobifunctional cross-linker comprising both <i>o</i>-methoxyphenol and 2-pyridinecarboxaldehyde moieties. This compound was used to link epidermal growth factor to genome-free MS2 viral capsids, affording nanoscale delivery vectors that can target a variety of cancer cell types