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

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 ß-peptide to give conjugates ß-B1, ß-B2, and ß-B3. Given the hydrophobic nature of the ß-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 ß-peptides, two covalently attached synthetic bacteriochlorins, and a datively bonded BChl-a pair, albeit to a limited extent for ß-B2. The reversible assembly/disassembly of dyad (ß-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 (ß-B1/BChl)2, 0.40 for (ß-B2/BChl)2, and 0.85 for (ß-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.

Hydrophilic tetracarboxy bacteriochlorins for photonics applications.

Bacteriochlorins absorb strongly in the near-infrared (NIR, 700-900 nm) region and hence are well suited for photophysical studies and photomedical applications, yet such endeavors heretofore have been largely limited by the intrinsic lipophilicity of the bacteriochlorin macrocycle. Here, a new molecular design is investigated wherein 3,5-dicarboxyphenyl units are appended to the ß-pyrrolic positions of the bacteriochlorin. Use of the 3,5-aryl substitution motif places the carboxylic acid groups, which are anionic at neutral pH, above and below the plane of the bacteriochlorin macrocycle. A de novo synthesis has been employed to create five such bacteriochlorins, which uses as intermediates two new 2,12-dibromobacteriochlorin building blocks and a known 3,13-dibromobacteriochlorin. The aryl groups with protected carboxylate moieties were introduced by Suzuki coupling; subsequent deprotection afforded the hydrophilic bacteriochlorins. The latter were characterized by absorption and fluorescence spectroscopy in DMF and in aqueous phosphate buffer (pH 7). In most cases, comparable sharp emission (FWHM of ∼25 nm) and modest fluorescence yields (0.060-0.11) were observed in aqueous phosphate buffer medium and in DMF. Aqueous solubility was examined by absorption spectral interrogation of samples over a 1000-fold concentration range with reciprocal change in pathlength (∼0.5, 5, 50, and 500 µM; 10, 1, 0.1, and 0.01 cm pathlength cuvettes). One hydrophilic bacteriochlorin was prepared that contains a single maleimido-terminated tether for bioconjugation; the tether was installed by the sequence of 15-bromination of the bacteriochlorin, Suzuki coupling, and DCC-mediated amide formation. The maleimido-bacteriochlorin was conjugated to a 48-residue cysteine-containing peptide analogue of a constituent from a bacterial photosynthetic light-harvesting complex. Taken together, the results show a new molecular design and facile de novo synthetic route for obtaining hydrophilic bacteriochlorins including a bioconjugatable group if desired.