Characterization of Metal-Protoporphyrins as Potential Photosensitizers for Engineered Photoresponsive Proteins.

Metallo-protoporphyrin IX (PpIX) offer a versatile platform for engineering photoresponsive proteins (EPrPs), yet their photophysical and photochemical properties upon interaction with proteins remains insufficiently characterized. Here we investigate the spectroscopic, the excited-state properties and laser-induced photochemistry of three PpIX analogues — MgPpIX, ZnPpIX, and MnPpIX — in aqueous solution and when self-assembled with recombinant human serum albumin (rHSA). Using steady-state absorption and fluorescence spectroscopy, time-correlated single-photon counting, and singlet-oxygen sensing, we show that each metalloporphyrin exhibits a distinct photochemical regime. MgPpIX undergoes significant photobleaching and forms photoproducts that are, however, influenced by a minor but spectroscopically significant fraction of metal-free PpIX. ZnPpIX binds rHSA as a monomer and displays substantial photobleaching without detectable photoproducts or enhanced singlet-oxygen generation, indicating a non–Type-II photosensitization pathway. By contrast, MnPpIX behaves as a d-type hyperporphyrin: it interacts only weakly with rHSA and shows negligible photobleaching, consistent with relaxation dominated by intramolecular charge-transfer states. Collectively, these results establish a mechanistic framework linking metal coordination to protein binding, excited-state dynamics, and photochemical outcomes. The distinct reactivity profiles of MgPpIX, ZnPpIX, and MnPpIX highlight their potential to serve as tunable cofactors for EPrPs with tailored photochemical functions.