Browsing by Author "Holten, D."

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    De novo synthesis and properties of analogues of the self-assembling chlorosomal bacteriochlorophylls
    (New Journal of Chemistry, 2011) Mass, O.; Pandithavidana, D.R.; Ptaszek, M.; Santiago, K.; Springer, J.W.; Jiao, J.; Tang, Q.; Kirmaier, C.; Bocian, D.; Holten, D.; Lindsey, J.S.
    Natural photosynthetic pigments bacteriochlorophyllsc, d and e in green bacteria undergo self-assembly to create an organized antenna system known as the chlorosome, which collects photons and funnels the resulting excitation energy toward the reaction centers. Mimicry of chlorosome function is a central problem in supramolecular chemistry and artificial photosynthesis, and may have relevance for the design of photosynthesis-inspired solar cells. The main challenge in preparing artificial chlorosomes remains the synthesis of the appropriate pigment (chlorin) equipped with a set of functional groups suitable to direct the assembly and assure efficient energy transfer. Prior approaches have entailed derivatization of porphyrins or semisynthesis beginning with chlorophylls. This paper reports a third approach, the de novo synthesis of macrocycles that contain the same hydrocarbon skeleton as chlorosomal bacteriochlorophylls. The synthesis here of Zn(II) 3-(1-hydroxyethyl)-10-aryl-131-oxophorbines (the aryl group consists of phenyl, mesityl, or pentafluorophenyl) entails selective bromination of a 3,13-diacetyl-10-arylchlorin, palladium-catalyzed 131-oxophorbine formation, and selective reduction of the 3-acetyl group using BH3�tBuNH2. Each macrocycle contains a geminal dimethyl group in the pyrroline ring to provide stability toward adventitious dehydrogenation. A Zn(II) 7-(1-hydroxyethyl)-10-phenyl-17-oxochlorin also has been prepared. Altogether, 30 new hydroporphyrins were synthesized. The UV-Vis absorption spectra of the new chlorosomal bacteriochlorophyll mimics reveal a bathochromic shift of [similar]1800 cm?1 of the Qy band in nonpolar solvent, indicating extensive assembly in solution. The Zn(II) 3-(1-hydroxyethyl)-10-aryl-131-oxophorbines differ in the propensity to form assemblies based on the 10-substituent in the following order: mesityl
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    Palette of lipophilic bioconjugatable bacteriochlorins for construction of biohybrid light-harvesting architectures
    (Chemical Science, 2013) Reddy, K.R.; Pandithavidana, D.R.; Parkes-Loach, P.S.; Loach, P.A.; Bocian, D.F.; Holten, D.; Lindsey, J.S.
    The challenge of creating both pigment building blocks and scaffolding to organize a large number of such pigments has long constituted a central impediment to the construction of artificial light-harvesting architectures. Light-harvesting (LH) antennas in photosynthetic bacteria are formed in a two-tiered self-assembly process wherein (1) a peptide dyad containing two bacteriochlorophyll a molecules forms, and (2) the dyads associate to form cyclic oligomers composed of 8 or 9 dyads in LH2 and 15 or 16 in LH1 of purple photosynthetic bacteria. While such antenna systems generally have near-quantitative transfer of excitation energy among pigments, only a fraction of the solar spectrum is typically absorbed. A platform architecture for study of light-harvesting phenomena has been developed that employs native photosynthetic peptide analogs, native bacteriochlorophyll a, and synthetic near-infrared-absorbing bacteriochlorins. Herein, the syntheses of 10 lipophilic bacteriochlorins are reported, of which 7 contain bioconjugatable handles (maleimide, iodoacetamide, formyl, carboxylic acid) for attachment to the peptide chassis. The bioconjugatable bacteriochlorins typically exhibit a long-wavelength absorption band in the range 710 to 820 nm, fluorescence yield of 0.1?0.2, and lifetime of the lowest singlet excited state of 2?5 ns. The ?-helical structure of the native-like peptide is retained upon conjugation with a synthetic bacteriochlorin, as judged by single-reflection infrared studies. Static and time-resolved optical studies of the oligomeric biohybrid architectures in aqueous detergent solution reveal efficient ([similar]90%) excitation energy transfer from the attached bacteriochlorin to the native-like bacteriochlorophyll a sites. The biohybrid light-harvesting architectures thus exploit the self-constituting features of the natural systems yet enable versatile incorporation of members from a palette of synthetic chromophores, thereby opening the door to a wide variety of studies in artificial photosynthesis.