Visible Light Triggerable CO Releasing Micelles

Abstract

Carbon monoxide (CO), along with nitric oxide and hydrogen sulfide, is one of a trinity of known gasotransmitters, or endogenously produced gaseous molecules that signal and regulate a panoply of physiological functions. CO releasing molecules (CORMs) are chemical tools that enable the study and application of this ephemeral gas, that, ideally, release CO on-demand when externally stimulated. Surveying the available triggers, photolysis is potentially advantageous: It is contactless and grants practitioners unparalleled spatial and temporal control. However, current phototriggered CORMs are capricious and do not meet current needs. Presented here is a highly efficient platform for the visible light triggered release of CO gas. This platform is built on a unique CO containing functionality, the cyclopropenone, which undergoes facile decarbonylation through visible light (470 nm) mediated photoredox catalysis. Due to the exothermic strain-release that occurs upon formation of CO, this photoreaction is rapid, quantitative, and has tunable release rates. To render this photo-CORM water-soluble, deliverable, and to keep reactants in proximity, necessary components were polymerized into block copolymers that self-assemble into CO releasing micelles (CORMIs). This platform was compared directly to other state-of-the-art CORMs, showing significantly improved CO production efficiency, lower toxicity, tunable release rates, and consistent efficacy in ex vivo and in vitro settings.

Figure 1.

A. Commonly utilized CORMs (CORM-3, top and Flav-1, bottom) that require reaction with external reagents to release CO. B. Diphenylcyclopropenone (DPCP) undergoes facile photodecarbonylation upon exposure to UV light (≤400 nm). C. Proposed platform for the visible light triggered release of CO gas from a self-assembled micelle.

Figure 2.

A. General reaction scheme showing the polymerization of a CO releasing monomer (norDPCP) and a photoredox catalyst (norIr) to form a statistical copolymer (DPCP_n-s-Ir_m). Exposure of this copolymer to visible light (470 nm) results in the rapid and quantitative formation of an alkyne-pendant copolymer (DPA_n-s-Ir_m) and CO gas propelled by strain-release photoredox catalysis. B. Representative NMR and SEC data showing the robustness and tunability of the ROMP approach. a. Prepared by the irradiation (470 nm, ~50 mW/cm², 30 min) of DPCP₁₀₀-s-Ir₁ in DMSO-d₆. b. Independently synthesized by polymerization of norDPA (Supporting Information). C. Overlayed NMRs showing the copolymer (DPCP₁₀₀-s-Ir₁) quantitatively forms the alkyne-pendant copolymer (DPA₁₀₀-s-Ir₁) via irradiation (470 nm, ~50 mW/cm², 30 min). Independently synthesized DPA₁₀₀-s-Ir₁ shown for comparison. D. Solution phase FTIR kinetic plot showing the loss of the cyclopropenone (1856 cm⁻¹) as a function of time using different intensities of visible light. No reaction was noted in the absence of iridium.

Figure 3.

A. A general reaction scheme depicting the polymerization of a CO releasing block copolymer by sequential addition of monomers to a solution of G3 (1. hydrophilic monomer [norPEG], 2. hydrophobic monomers [norDPCP/norIr], and 3. an ene-yne terminator). B. Workflow for preparation of a CO releasing micelle MiDPCP from PEG₅₅-b-DPCP₄₅-s-Ir₁ and the corresponding control MiDPA from PEG₅₅-b-DPA₄₅. C. Dynamic light scattering (DLS) showing the tight dispersity of MiDPCP. D. Electron microscopy (EM) evidencing that MiDPCP forms spherical micelles. E. A profluorescent assay for quantifying the in situ formation of CO. F. Irradiation of MiDPCP under ex vivo conditions (PBS buffer) leads to a ~10-fold turn on of fluorescence. G. Varying light intensity modulates the kinetics of CO release from MiDPCP under ex vivo conditions.

Figure 4.

A. Irradiation of MiDPCP under in vitro conditions with A549 cells leads to a ~10-fold turn on of fluorescence of the CO-probe CODP-102. B. Confocal images of irradiated A549 cells pre-exposed to 50 μM CO units of MiDPCP leads to CO probe turn-on within cells.