Bioconjugatable, PEGylated hydroporphyrins for photochemistry and photomedicine. Narrow-band, red-emitting chlorins

Abstract

Chromophores that absorb and emit in the red spectral region (600–700 nm), are water soluble, and bear a bioconjugatable tether are relatively rare yet would fulfill many applications in photochemistry and photomedicine. Here, three molecular designs have been developed wherein stable synthetic chlorins – analogues of chlorophylls – have been tailored with PEG groups for use in aqueous solution. The designs differ with regard to order of the installation (pre/post-formation of the chlorin macrocycle) and position of the PEG groups. Six PEGylated synthetic chlorins (three free bases, three zinc chelates) have been prepared, of which four are equipped with a bioconjugatable (carboxylic acid) tether. The most effective design for aqueous solubilization entails facial encumbrance where PEG groups project above and below the plane of the hydrophobic disk-like chlorin macrocycle. The chlorins possess strong absorption at ~400 nm (B band) and in the red region (Qy band); regardless of wavelength of excitation, emission occurs in the red region. Excitation in the ~400 nm region thus provides an effective Stokes shift of >200 nm. The four bioconjugatable water-soluble chlorins exhibit Qy absorption/emission in water at 613/614, 636/638, 698/700 and 706/710 nm. The spectral properties are essentially unchanged in DMF and water for the facially encumbered chlorins, which also exhibit narrow Qy absorption and emission bands (full-width-at-half maximum of each <25 nm). The water-solubility was assessed by absorption spectroscopy over the concentration range ~0.4 μM – 0.4 mM. One chlorin was conjugated to a mouse polyclonal IgG antibody for use in flow cytometry with compensation beads for proof-of-principle. The conjugate displayed a sharp signal when excited by a violet laser (405 nm) with emission in the 620–660 nm range. Taken together, the designs described herein augur well for development of a set of spectrally distinct chlorins with relatively sharp bands in the red region.

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].…”

“…PEG-substituted dipyrromethanes also found use in the early introduction of solubilizing groups. The group of Lindsey was able to synthesize chlorins from a PEGylated dipyrromethane precursor (Scheme 38) [128]. This strategy also incorporated a bioconjugatable tether at another meso-position, the precursor of which was also introduced at the dipyrromethane stage.…”

“…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…”

“…This strategy was employed to perform a direct PEGylation of trans-AB porphyrin 179 (Scheme 55) to afford the PEGylated porphyrin 180 [157]. Along analogous routes, synthetic chlorins 181 and 182 were prepared from alkyne-substituted meso-aryl groups and subsequent reactions with PEG-azides [128].…”

A Study of the Reaction between Methylpyropheophorbide a and Hydrazine Hydrate

“…The position and intensity of the redmost Q y maximum is related to the molecular transition dipole moment along the y-axis, which in turn affected by the π-conjugated functional groups located in the C3-and C13-positions of the macrocycle. [9][10][11] A number of synthetic approaches has been reported for the C3-substituents modification, involving different methodologies such as condensation and olefination reactions, Pd-catalyzed cross-couplings, 1,3-cycloaddition etc. [12][13][14] Chemical modification of the 13-carbonyl group has been also reported.…”