(636a) Photoactive C-Phycocyanin Assemblies: From Food Colorants to Tunable Biofunctional Materials

C-phycocyanin (C-PC), a major protein–chromophore complex originating in cyanobacteria and red algae, is characterized by a hierarchical protein architecture and a unique light-sensing ability. Specifically, two different C-PC subunit chains–α-subunit and β-subunit–form an (αβ) monomer, which is then associated into a ring-shaped (αβ)₃ trimer that serves as the building block for further assembly of (αβ)₆ hexamers, (αβ)₁₂ dodecamers, and subsequent supramolecular structures.^1,2 On each subunit, there are one or two open-chain tetrapyrrole chromophores covalently attached to protein cysteine residues via thioether linkages. These tetrapyrrole cofactors display prominent photochemical³ and redox activity⁴ and thus act as photoreceptors during photosynthesis for light absorption and energy transfer.

C-PC’s unique protein–chromophore architecture provides many attractive features with a wide variety of promising applications, particularly in the food industry. Its vivid blue color, imparted by the chromophore cofactors, is gaining attention, as a rare source of natural blue food colorant. We have also found that the protein architecture of C-PC is flexible and can be easily modified by enzymatic and chemical means to impart techno-functional properties for use as a natural texture-modifying food ingredient. In addition, C-PC has shown health-related bioactivity and nutritional value and is being investigated for its anticancer, anti-inflammation, and antioxidant effects.⁵

Our recent research unveiled a protein–chromophore regulatory mechanism in C-PC that can be used to manipulate the photoactivities of the chromophore cofactors by changing environmental conditions such as pH and light. This new discovery opened new application possibilities of C-PC in material and biomedical areas for constructing stimuli-responsive biofunctional materials.

Using small-angle X-ray scattering, a pH-mediated assembly–disassembly pathway of C-PC in the solution state was uncovered. Partially unfolded monomers found in both acidic (pH 3.0) and basic (pH 9.0) conditions can be converted to the well-folded trimers at pH 7.0. At pH 5, the partially unfolded monomers formed higher assemblies including trimers, hexamers and nonamers. These kinds of flexible protein matrices impart a pH mediated tunability to the embedded tetrapyrroles, whose photochemical behaviors changed based upon protein assembly state.

In addition to the pH-triggered protein structural changes, irradiation at 365-nm triggers pH-dependent singlet oxygen (¹O₂) generation in C-PC and additional conformational changes. At pH 5.0, intermolecular photo-crosslinking occurs across tyrosine residues, which bridge the solution-based C-PC oligomers into unprecedented dodecamers and 24-mers. These supramolecular assemblies give show enhanced ¹O₂ yield, fluorescence emission, and photostability. This selective activation of C-PC in weakly acidic environments (pH 5.0) could be used to design pH-responsive fluorescent imaging probes or ¹O₂ generators that get activated at acidic tumor sites while switch off at normal healthy tissues. With further development, the remarkable pH-responsiveness, photoactivity, and conformational flexibility of C-PC that we have demonstrated, along with this protein’s intrinsic biocompatibility and wide availability, will open new routes for its applications in cancer diagnosis, imaging, and photodynamic therapy (PDT).

Our work on C-PC as food additives along with the novel environmentally triggered structural and photochemical responses will be presented.

Reference

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