Bioconjugatable, PEGylated hydroporphyrins for photochemistry and photomedicine. Narrow-band, near-infrared-emitting bacteriochlorins

Abstract

Synthetic bacteriochlorins absorb in the near-infrared (NIR) region and are versatile analogues of natural bacteriochlorophylls. The utilization of these chromophores in energy sciences and photomedicine requires the ability to tailor their physicochemical properties, including the incorporation of units to impart water solubility. Herein, we report the synthesis, from two common bacteriochlorin building blocks, of five wavelength-tunable, bioconjugatable and water-soluble bacteriochlorins along with two non-bioconjugatable benchmarks. Each bacteriochlorin bears short polyethylene glycol (PEG) units as the water-solubilizing motif. The PEG groups are located at the 3,5-positions of aryl groups at the pyrrolic β-positions to suppress aggregation in aqueous media. A handle containing a single carboxylic acid is incorporated to allow bioconjugation. The seven water-soluble bacteriochlorins in water display Qy absorption into the NIR range (679–819 nm), sharp emission (21–36 nm full-width-at-half-maximum) and modest fluorescence quantum yield (0.017–0.13). Each bacteriochlorin is neutral (non-ionic) yet soluble in organic (e.g., CH2Cl2, DMF) and aqueous solutions. Water solubility was assessed using absorption spectroscopy by changing the concentration ∼1000-fold (190–690 µM to 0.19–0.69 µM) with a reciprocal change in pathlength (0.1–10 cm). All bacteriochlorins showed excellent solubility in water, except for a bacteriochlorin–imide that gave slight aggregation at higher concentrations. One bacteriochlorin was conjugated to a mouse polyclonal IgG antibody for use in flow cytometry with compensation beads for proof-of-principle. The antibody conjugate of B2-NHS displayed a sharp signal upon ultraviolet laser excitation (355 nm) with NIR emission measured with a 730/45 nm bandpass filter. Overall, the study gives access to a set of water-soluble bacteriochlorins with desirable photophysical properties for use in multiple fields.

Modifications of Porphyrins and Hydroporphyrins for their Solubilization in Aqueous Media

“…Unwanted reactivity to form oligomeric species is also supressed by using heterobifunctional PEG chains, in which one end of the PEG chain often has a methyl cap. PEGylated porphyrins are also much more amenable to traditional aqueous work-up conditions, as they may partition favorably in organic solvents such as CH2Cl2 and EtOAc [127,128]. Often times PEGylated porphyrins can be purified by normal silica-gel chromatography using polar solvents [14,[129][130][131].…”

“…Another representative example for the introduction of PEG groups via Suzuki coupling of dibromobacteriochlorin 19 with a Boc-protected amine-derivatized boronic ester, followed by Bocdeprotection under standard conditions, afforded diaryl bacteriochlorin 175 carrying two amine groups. The amines were PEGylated by amide formation with a PEG-NHS ester to afford bacteriochlorin 176 (Scheme 54) [127]. Derivatives bearing bioconjugtable tethers (177) and (178) were also prepared using a regioselective meso-bromination strategy, followed by similar coupling sequences [127].…”

“…The amines were PEGylated by amide formation with a PEG-NHS ester to afford bacteriochlorin 176 (Scheme 54) [127]. Derivatives bearing bioconjugtable tethers (177) and (178) were also prepared using a regioselective meso-bromination strategy, followed by similar coupling sequences [127].…”

“…The PEGylated derivatives were also shown to display very little aggregation, as demonstrated by the sharp absorbance and emission peaks in their optical spectra over a 1000-fold concentration range. With the PEGylated derivatives appearing to be among the most promising motifs for solubilization, Lindsey and co-workers set out to prepare a series of PEGylated chlorins and bacteriochlorins (with options for bioconjugation) using a range of synthetic methodologies [127,128]. One such example for the direct, late-stage PEGylation of chlorin derivatives was the azide-alkyne Huisgen-cycloaddition (click) Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 6 June 2017 doi:10.20944/preprints201706.0032.v1…”

“…Lindsey and co-workers also used an amide coupling of an alkyl-amine substituted bacteriochlorin with a PEG-NHS ester to produce PEGylated bacteriochlorin 186 (Scheme 57) [127]. The β-functionalized bacteriochlorin 185 was prepared by Sonagashira coupling and subsequent reduction of the alkynes.…”