Two birds one stone: -fluoropyrrolyl-cysteine S_NAr chemistry enabling functional porphyrin bioconjugation

Bioconjugation, a synthetic tool that endows small molecules with biocompatibility and target specificity through covalent attachment of a biomolecule, holds promise for next-generation diagnosis or therapy. Besides the establishment of chemical bonding, such chemical modification concurrently allows alteration of the physicochemical properties of small molecules, but this has been paid less attention in designing novel bioconjugates. Here, we report a "two birds one stone" methodology for irreversible porphyrin bioconjugation based on beta-fluoropyrrolyl-cysteine SNAr chemistry, in which the beta-fluorine of porphyrin is selectively replaced by a cysteine in either peptides or proteins to generate novel beta-peptidyl/proteic porphyrins. Notably, due to the distinct electronic nature between fluorine and sulfur, such replacement makes the Q band red-shift to the near-infrared region (NIR, >700 nm). This facilitates intersystem crossing (ISC) to enhance the triplet population and thus singlet oxygen production. This new methodology features water tolerance, a fast reaction time (15 min), good chemo-selectivity, and broad substrate scope, including various peptides and proteins under mild conditions. To demonstrate its potential, we applied porphyrin beta-bioconjugates in several scenarios, including (1) cytosolic delivery of functional proteins, (2) metabolic glycan labeling, (3) caspase-3 detection, and (4) tumor-targeting phototheranostics.