Porphyrin as a versatile visible-light-activatable organic/metal hybrid photoremovable protecting group

Abstract

Photoremovable protecting groups (PPGs) represent one of the main contemporary implementations of photochemistry in diverse fields of research and practical applications. For the past half century, organic and metal-complex PPGs were considered mutually exclusive classes, each of which provided unique sets of physical and chemical properties thanks to their distinctive structures. Here, we introduce the meso-methylporphyrin group as a prototype hybrid-class PPG that unites traditionally exclusive elements of organic and metal-complex PPGs within a single structure. We show that the porphyrin scaffold allows extensive modularity by functional separation of the metal-binding chromophore and up to four sites of leaving group release. The insertion of metal ions can be used to tune their spectroscopic, photochemical, and biological properties. We provide a detailed description of the photoreaction mechanism studied by steady-state and transient absorption spectroscopies and quantum-chemical calculations. Our approach applied herein could facilitate access to a hitherto untapped chemical space of potential PPG scaffolds.

Fig. 1. Meso-methylporphyrin as a hybrid organic/metal-complex PPG scaffold.

a Contemporary organic and metal-complex PPGs; the photorelease of leaving groups (LG) occurs via a covalent bond scission or ligand exchange process, respectively. b Meso-methylporphyrin as a hybrid organic/metal-complex PPG: diverse metal ions can be incorporated into the chromophore to modulate its properties, and a leaving group is photoreleased via covalent bond scission.

Fig. 2. Meso-methylporphyrin as a versatile PPG scaffold: synthesis and photochemistry.

a Synthesis of meso-methylporphyrin derivatives. i: PNBA, DMAP, DCC, 20 °C. ii: mesylchloride, DMAP, 20 °C. b Absorption spectra of PNBA- and DMAP-caged meso-methylporphyrins (3 µM, aerated DMSO). Inlet: a 10-fold magnification of the Q-band regions. c Photorelease of leaving groups from meso-methylporphyrin derivatives 5–8 (25 µM, degassed DMSO, λ_irr = 410 nm, 40 mW cm⁻²; HPLC-MS analysis). d Photolysis of 6 (25 µM, degassed DMSO) at λ_irr = 410 nm (18 mW cm⁻²); 545 nm (52 mW cm⁻²); and 640 nm (67 mW cm⁻²). Inlet: focus on early time points. e Structures of mono-, di- and tetra_-N_α-Boc-(L)-Trp caged meso-methylporphyrin derivatives (9–11). f Quantification of the photorelease of leaving groups from 9–11 (λ_irr= 410 nm, 40 mW cm⁻², HPLC-MS analyses). Averages from three experiments are shown; error bars represent the standard deviation.

Fig. 3. Meso-methylporphyrin PPG is functional in cellular environment.

a Synthesis of meso-methylporphyrin derivatives bearing anti-cancer drugs. i: mesylchloride, indibulin, 20 °C. ii: methotrexate, DMAP, DCC, 20 °C. b Absorption spectra of anti-cancer-caged meso-methylporphyrins (3 µM, aerated DMSO). Inlet: a 10-fold magnification of the Q-band regions. c Photorelease of leaving groups from meso-methylporphyrin derivatives 12 and 13 (25 µM, degassed DMSO, λ_irr = 410 nm, 40 mW cm⁻² or 545 nm 52 mW cm^−2; HPLC-MS analysis). d Representative fluorescence microscopy images of the cellular distribution of 13 in cultured murine 4T1 mammary carcinoma cells. Red: compound 13, green: LysoTracker Green. e Co-localization parameters of 13 and LysoTracker Green. The middle line represents the mean, and error bars represent the standard deviation. f Calculated IC₅₀ concentrations of 5 and 13 in cultured murine 4T1 mammary carcinoma cells following or not irradiation with 545 nm (5 min) or 640 nm (10 min) light. The experiment was repeated at least three times in triplicates; error bars represent the standard deviation.

Fig. 4. Metal-containing meso-methylporphyrin PPGs: synthesis and photochemistry.

a Synthesis of metal-containing meso-methylporphyrins followed by the installation of DMAP as a model leaving group. b Absorption spectra of DMAP-caged metal-containing meso-methylporphyrin derivatives (3 µM, aerated DMSO); that of an analogous metal-free meso-methylporphyrin (6) is shown for comparison. Inlet: a 10-fold magnification of the Q-bands region. c Photorelease of DMAP from metal-containing meso-methylporphyrin derivatives (25 µM, degassed DMSO, λ_irr = 410 nm, 40 mW cm⁻²; HPLC-MS analysis), the reaction of an analogous metal-free meso-methylporphyrin (6) is shown for comparison. Inlet: focus on early time points.

Fig. 5. Photoreaction kinetics of DMAP-caged porphyrin derivatives.

a Photoproducts formed upon photoexcitation (λ_irr= 410 nm, 18 mW cm⁻²) of 6 (25 µM) in a degassed DMSO solution. b Photochemistry of 6 (3 µM) in aerated or degassed DMSO solutions. c Photochemistry of 6 (3 µM) in aerated or degassed methanol solutions (λ_irr = 420 nm) and in the presence of a singlet oxygen generator (rose bengal, RB, λ_irr = 545 nm, 52 mW cm⁻²) or quencher (furfuryl alcohol, FFA). Inlet: focus on early time points.

Fig. 6. Photochemistry of DMAP-caged porphyrin derivatives.

a The proposed photorelease mechanism for porphyrin PPGs. b An extended Jablonski diagram for free-base, Zn, and Pd porphyrin PPGs. Intensities of the UV/visible transitions are also depicted. c An extended Jablonski diagram for a Cu porphyrin PPG.