Metallochelate Coupling of Phosphorescent Pt-Porphyrins to Peptides, Proteins, and Self-Assembling Protein Nanoparticles

Specific and reversible metallochelate coupling via nitrilotriacetate (NTA) moiety is widely used for immobilization, purification, and labeling of oligo­(histidine)-tagged proteins. Here, we evaluated this strategy to label various peptides and proteins with phosphorescent Pt-porphyrin derivatives bearing NTA group(s). Zn²⁺ complexes were shown to have minimal effect on the photophysics of the porphyrin moiety, allowing quenched-phosphorescence sensing of O₂. We complexed the PtTFPP-NTA conjugate with His-containing peptide that can facilitate intracellular loading, and observed efficient accumulation and phosphorescent staining of MEF cells. The more hydrophilic PtCP-NTA conjugate was also seen to form stable complexes with larger polypeptide constructs based on fluorescent proteins, and with subunits of protein nanoparticles, which retained their ability to self-assemble. Testing in phosphorescence lifetime based O₂ sensing assays on a fluorescence reader and PLIM microscope revealed that phosphorescent metallochelate complexes perform similarly to the existing O₂ probes. Thus, metallochelate coupling allows simple preparation of different types of biomaterials labeled with phosphorescent Pt-porphyrins.