Chemical modifications' of native proteins can facilitate production of supernatural protein :functions that are not easily accessible by complementary methods relying on genetic manipulations. However, accomplishing precise control over selectivity while maintaining structural:. integrity and homogeneity still represents a fOrmidable, challenge. Herein, we report a transition metal-free method for tryptophan-selective bioconjugation of proteins that is based on an organoradical and operates under ambient conditions. This method exhibits low levels of cross-reactivity and leaves.higher-order structures of the protein and various functional groups therein unaffected: The strategy to,target less abundant amino acids contributes to the formation of structurally homogeneous conjugates, which may even be suitable for protein. crystallography. The absence of toxic metals and biochemi-i cally incompatible conditions allows a rapid functional- modulation of native proteins such as antibodies and pathogenic aggregative proteins, and this method may thus easily find therapeutic applications.