Bright, "Clickable" Porphyrins for the Visualization of Oxygenation under Ambient Light

Abstract

A new group of "clickable" and brightly emissive metalloporphyrins has been developed for the visualization of oxygenation under ambient light with the naked eye. These alkynyl-terminated compounds permit the rapid and facile synthesis of oxygen-sensing dendrimers through azide-alkyne click chemistry. With absorption maxima overlapping with the wavelengths of common commercial laser sources, they are readily applicable to biomedical imaging of tissue oxygenation. An efficient synthetic methodology, featuring the stable trimethylacetyl (pivaloyl) protecting group, is described for their preparation. A paint-on liquid bandage containing a new, click-synthesized porphyrin dendrimer has been used to map oxygenation across an ex vivo porcine skin burn model.

Keywords: biotechnology; dendrimers; luminescence; porphyrins; sensors.

Figure 1

Absorption (top) and emission (bottom) spectra of pivaloyl-protected porphyrins 7, 8 (in DMF as solvent) and benzoporphyrins 9, 10 (in dichloromethane). Spectral profiles have been normalized to display similar absorbances at the Soret bands and similar emission intensities.

Figure 2

Sensor response to different oxygen levels. a) A solution of platinum porphyrin 7 in dichloromethane under room air (left) and in the absence of oxygen (right); the porphyrin was excited with a handheld LED flashlight. Image was acquired using a smartphone camera. b) Palladium porphyrin 8 was embedded in a liquid bandage formulation along with a green-emitting, oxygen-insensitive fluorophore, and the film’s emission was recorded with a commodity color camera equipped with filters, under room air (left) and in the absence of oxygen (right).

Figure 3

Oxygen consumption maps, expressed as normalized percent oxygen consumption, obtained with Clickaphor Red G2 in burned tissue on freshly excised porcine skin. Images shown for (a,b) circular burn and control skin imaged using the intensity-based ratiometric approach; (c,d) circular burn and control skin imaged using the lifetime approach; R/G: red-to-green ratio; %Con: Percentage oxygen consumption. The information presented on the images were obtained from Matlab analysis of the raw data acquired with a custom-modified camera.^[18]

Scheme 1

Synthesis of alkynyl-terminated porphyrins. Reagents and conditions: (a) K₂OsO₄, K₃Fe(CN)₆, K₂CO₃, CH₃SO₂NH₂, tBuOH/H₂O (1:1), r.t., overnight, 78%; (b) (CH₃)₃CCOCl, 4-Dimethylaminopyridine, pyridine, CH₂Cl₂, r.t., 18 h, 85%; (c) CNCH₂CO₂tBu, tBuOK, THF, r.t., 4h, 81%; (d) TFA, r.t. 30min, 52%; (e) p-TsOH(monohydr.), formaldehyde (37% in water), benzene, reflux (Dean-Stark condenser), 8 h, 69%; (f) PtCl₂, PhCN, reflux, 6–8 h, 65%; (g) PdCl₂, benzonitrile, reflux, 15–20 min, 85%; (h) DDQ, dry 1,4-Dioxane, reflux, 8–10 h, 60%; (i) DIBAL-H, dry CH₂Cl₂, −78°C-r.t., 18 h; (j) NaH (60%), BrCH₂CCH, dry DMF, 18 h, 0°C-r.t., 35% (2-step yield).