Amphiphilic, hydrophilic, or hydrophobic synthetic bacteriochlorins in biohybrid light-harvesting architectures: consideration of molecular designs

Abstract

Biohybrid light-harvesting architectures can be constructed that employ native-like bacterial photosynthetic antenna peptides as a scaffold to which synthetic chromophores are attached to augment overall spectral coverage. Synthetic bacteriochlorins are attractive to enhance capture of solar radiation in the photon-rich near-infrared spectral region. The effect of the polarity of the bacteriochlorin substituents on the antenna self-assembly process was explored by the preparation of a bacteriochlorin-peptide conjugate using a synthetic amphiphilic bacteriochlorin (B1) to complement prior studies using hydrophilic (B2, four carboxylic acids) or hydrophobic (B3) bacteriochlorins. The amphiphilic bioconjugatable bacteriochlorin B1 with a polar ammonium-terminated tail was synthesized by sequential Pd-mediated reactions of a 3,13-dibromo-5-methoxybacteriochlorin. Each bacteriochlorin bears a maleimido-terminated tether for attachment to a cysteine-containing analog of the Rhodobacter sphaeroides antenna β-peptide to give conjugates β-B1, β-B2, and β-B3. Given the hydrophobic nature of the β-peptide, the polarity of B1 and B2 facilitated purification of the respective conjugate compared to the hydrophobic B3. Bacteriochlorophyll a (BChl a) associates with each conjugate in aqueous micellar media to form a dyad containing two β-peptides, two covalently attached synthetic bacteriochlorins, and a datively bonded BChl-a pair, albeit to a limited extent for β-B2. The reversible assembly/disassembly of dyad (β-B2/BChl)2 was examined in aqueous detergent (octyl glucoside) solution by temperature variation (15-35 °C). The energy-transfer efficiency from the synthetic bacteriochlorin to the BChl-a dimer was found to be 0.85 for (β-B1/BChl)2, 0.40 for (β-B2/BChl)2, and 0.85 for (β-B3/BChl)2. Thus, in terms of handling, assembly and energy-transfer efficiency taken together, the amphiphilic design examined herein is more attractive than the prior hydrophilic or hydrophobic designs.

Enhanced Light‐Harvesting Capacity by Micellar Assembly of Free Accessory Chromophores and LH1‐like Antennas

“…Förster energy‐transfer calculations were also performed. In our prior studies such calculations revealed that trends in Φ EET are consistent with Förster (through‐space) energy transfer and reflect differences in the relevant photophysical properties of the donor and acceptor molecules . (The alternative is Dexter (through‐bond) energy transfer; the chromophores in the antenna systems described herein are generally expected to be too far apart for a through‐bond mechanism to make a substantial contribution.)…”

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

“…One area entails utilization of the PEGylated molecules as light-harvesting constituents in biohybrid antenna complexes via attachment at hydrophilic terminal regions of membrane-spanning photosynthetic peptides. 53 A second research area focuses on the utilization of the PEGylated (bacterio)chlorins as fluorophores in clinical diagnostics, particularly flow cytometry, where antibodies are to be multiply labeled. While diverse porphyrins have been used in bioconjugation studies, 24,54 few PEGylated synthetic chlorins have heretofore been reported, 55–58 and only one was equipped (but not tested) with a bioconjugatable tether.…”

Synthetic bacteriochlorins bearing polar motifs (carboxylate, phosphonate, ammonium and a short PEG). Water-solubilization, bioconjugation, and photophysical properties

“…25,26,49,[59][60][61][62][63] The design, synthesis, and physicochemical characterization of the substituted bacteriochlorins described herein provide a small step toward this goal. Ultimately one goal is not only to solubilize bacteriochlorin monomers but to be able to create multipigment architectures containing bacteriochlorins that are water-soluble for use in diverse scientific applications.…”

“…25,26 The tetrapyrroles of present interest are bacteriochlorins, which absorb strongly in the near-infrared (NIR) region. Herein, a set of bacteriochlorins bearing distinct polar motifs is reported.…”