(173h) Tuning C-Phycocyanin Photoactivity Via pH-Mediated Assembly–Disassembly

Environment-triggered protein conformational changes have garnered wide interests in both fundamental research, for deciphering in vivo acclimatory responses, and practical applications, for designing stimuli-responsive probes. Here we propose a protein–chromophore regulatory mechanism that allows manipulating the photoactivities of C-phycocyanin (C-PC) by environmental pH and UV irradiation.

C-phycocyanin (C-PC), a major protein–chromophore complex originating in cyanobacteria and red algae, is characterized by hierarchical protein architecture and unique light-sensing ability. Using small-angle X-ray scattering, a pH-mediated assembly–disassembly pathway of C-PC in solution state was unraveled. With pH changes in the 3.0–9.0 range, the partially unfolded monomers found in both acidic (pH 3.0) and alkaline (pH 9.0) conditions can be converted to the well-folded trimers at pH 7.0 and high assemblies—a mixture of trimers, hexamers, and nonamers—at pH 5.0. Such flexible protein matrices impart tunability to the embedded tetrapyrroles, whose photochemical behaviors were found to be modulated by protein assembly states. 365-nm UV irradiation on C-PC triggers pH-dependent singlet oxygen (¹O₂) generation and conformational changes. Intermolecular photo-crosslinking occurs at pH 5.0 via dityrosine species, which bridges solution-based C-PC oligomers into unprecedented dodecamers and 24-mers. These supramolecular assemblies impart C-PC at pH 5.0 significantly enhanced ¹O₂ yield, fluorescence emission, and photostability relative to those at other pH values.

The identified pH tunability via protein–chromophore ineractions makes C-PC a potential pH-responsive photosensizer for tumor-targeted photodynamic therapy. The enhanced activation of C-PC in weakly acidic environments (pH 5.0) makes C-PC a potential pH-responsive photosensizer for tumor-targeted photodynamic therapy as tumor microenvironments, for example, the lysosome (pH ~4.5−5.0), are characterized by similar acidity. In this sense, C-PC could be utilized in the design of novel pH-responsive fluorescent imaging probes or ¹O₂ generators to boost photodynamic activities at acidic tumor sites while minimizing phototoxicity to non-target healthy tissues. It is envisaged that with further development, the remarkable pH-responsiveness, photoactivity, and conformational flexibility of C-PC demonstrated in this work, along with this protein’s inherent biocompatibility and wide availability, will open new routes for its applications in cancer diagnosis, imaging, and photodynamic therapy (PDT).