Synthesis and Self-Aggregation of π‑Expanded Chlorophyll Derivatives to Construct Light-Harvesting Antenna Models

Chlorosomes are one of the elegant light-harvesting antenna systems in anoxygenic photosynthetic bacteria, whose core is constructed from <i>J</i>-type self-aggregation of bacteriochlorophyll-<i>c</i>, bacteriochlorophyll-<i>d</i>, bacteriochloro­phyll-<i>e</i>, and bacteriochlorophyll-<i>f</i> molecules without the influence of polypeptides. Chlorosomal supramolecular models were built up using synthetic porphyrin-type bacteriochloro­phyll-<i>d</i> analogues with a methoxycarbonylethenyl, formyl, vinyl, or ethyl group at the 8-position. Their chlorosomal self-aggregates in an aqueous micelle solution showed relatively intense absorption bands around 500–600 nm where antennas of natural oxygenic phototrophs, as well as green sulfur bacteria possessing bacteriochlorophylls-<i>c</i>/<i>d</i>, absorb light less efficiently; this observation is called the “green gap”. Furthermore, the functional chlorosomal models were constructed by simple addition of a small amount of an energy acceptor model bearing a bacteriochlorin moiety to the pigment self-assemblies in an aqueous micelle. The resulting excited energy donor–acceptor supramolecules played the roles of chlorosomal light-harvesting and energy-transfer antenna systems and were efficient at light absorption in the “green gap” region

Synthesis, Structure, and Optical and Redox Properties of Chlorophyll Derivatives Directly Coordinating Ruthenium Bisbipyridine at the Peripheral β‑Diketonate Moiety

The diketonate group of the peripheral position in chlorophyll derivatives <b>1</b> and <b>2</b> coordinated ruthenium bisbipyridine to give direct linkages <b>3</b>–<b>5</b> of the chlorin ring with the Ru­(II) complex. Zinc metalation of the central position in the chlorin ring of free base <b>3</b> afforded the Ru–Zn binuclear complex <b>3-Zn</b>. Because the diketonate group at the C3 position of chlorophyll derivatives coordinated to bulky Ru­(bpy)₂²⁺, the plane of the diketonate group was twisted from the chlorin π ring in synthetic <b>3</b>–<b>5</b> and <b>3-Zn</b> to lead to a partial deconjugation and a slight blue shift of the longest wavelength electronic absorption band in dichloromethane. A broad metal-to-ligand charge-transfer absorption band derived from the Ru complex was observed around 500 nm, in addition to visible absorption bands from the chlorophyll moiety. Chlorophyll derivatives <b>3</b>–<b>5</b> and <b>3-Zn</b> directly coordinating the ruthenium complex were less fluorescent in dichloromethane than chlorophyll–diketonate ligands <b>1</b>, <b>2</b>, and <b>1-Zn</b> due to the heavy atom effect of the ruthenium in a molecule. The coordination to the ruthenium complex moiety at the peripheral position shifted the electrochemical reduction of the chlorin part in acetonitrile to a negative potential, and the coordination to zinc at the central position decreased the redox potentials. Chemical modification of the bipyridine and diketonate ligands of the ruthenium complexes greatly affected the redox potentials of Ru­(II)/(III) and/or Ru­(II)/(I) but minimally the redox properties of the chlorin moiety. Substitution with electron-donating groups shifted the former to a negative potential but only barely shifted the latter. The zinc metalation caused no apparent shifts for the redox potentials of the Ru center

Modification of 3-Substituents in (Bacterio)Chlorophyll Derivatives to Prepare 3-Ethylated, Methylated, and Unsubstituted (Nickel) Pyropheophorbides and Their Optical Properties

Methyl mesopyropheophorbide-<i>a</i> possessing an ethyl group at the 3-position, its 3¹-demethyl analogue (3-methyl homologue), and its 3¹-deethyl analogue (3-unsubstituted chlorin) were prepared by modifying naturally occurring (bacterio)­chlorophylls bearing 3-vinyl, formyl, acetyl, and 1-hydroxyethyl groups. These synthetic 3-(un)­substituted chlorophyll derivatives and their nickel complexes are probable intermediates during degradation of (bacterio)­chlorophylls to chemically stable porphyrinoids. The optical properties (visible absorption, circular dichroism, and fluorescence emission) of the catabolic candidates in a solution were measured, and the substitution effect was investigated

Coherent Oscillations in Chlorosome Elucidated by Two-Dimensional Electronic Spectroscopy

Chlorosomes are the most efficient photosynthetic light-harvesting complexes found in nature and consist of many bacteriochlorophyll (BChl) molecules self-assembled into supramolecular aggregates. Here we elucidate the presence and the origin of coherent oscillations in chlorosome at cryogenic temperature using 2D electronic spectroscopy. We observe coherent oscillations of multiple frequencies superimposed on the ultrafast amplitude decay of 2D spectra. Comparison of oscillatory features in the rephasing and nonrephasing 2D spectra suggests that an oscillation of 620 cm^–1 frequency arises from electronic coherence. However, this coherent oscillation can be enhanced by vibronic coupling with intermolecular vibrations of BChl aggregate, and thus it might originate from vibronic coherence rather than pure electronic coherence. Although the 620 cm^–1 oscillation dephases rapidly, the electronic (or vibronic) coherence may still take part in the initial step of energy transfer in chlorosome, which is comparably fast

Coherent Oscillations in Chlorosome Elucidated by Two-Dimensional Electronic Spectroscopy

Chlorosomes are the most efficient photosynthetic light-harvesting complexes found in nature and consist of many bacteriochlorophyll (BChl) molecules self-assembled into supramolecular aggregates. Here we elucidate the presence and the origin of coherent oscillations in chlorosome at cryogenic temperature using 2D electronic spectroscopy. We observe coherent oscillations of multiple frequencies superimposed on the ultrafast amplitude decay of 2D spectra. Comparison of oscillatory features in the rephasing and nonrephasing 2D spectra suggests that an oscillation of 620 cm^–1 frequency arises from electronic coherence. However, this coherent oscillation can be enhanced by vibronic coupling with intermolecular vibrations of BChl aggregate, and thus it might originate from vibronic coherence rather than pure electronic coherence. Although the 620 cm^–1 oscillation dephases rapidly, the electronic (or vibronic) coherence may still take part in the initial step of energy transfer in chlorosome, which is comparably fast

Nanotubes of Biomimetic Supramolecules Constructed by Synthetic Metal Chlorophyll Derivatives

Various supramolecular nanotubes have recently been built up by lipids, peptides, and other organic molecules. Major light-harvesting (LH) antenna systems in a filamentous anoxygenic phototroph, <i>Chloroflexus</i> (<i>Cfl.</i>) <i>aurantiacus</i>, are called chlorosomes and contain photofunctional single-wall supramolecular nanotubes with approximately 5 nm in their diameter. Chlorosomal supramolecular nanotubes of <i>Cfl. aurantiacus</i> are constructed by a large amount of bacteriochlorophyll­(BChl)-<i>c</i> molecules. Such a pigment self-assembles in a chlorosome without any assistance from the peptides, which is in sharp contrast to the other natural photosynthetic LH antennas. To mimic chlorosomal supramolecular nanotubes, synthetic models were prepared by the modification of naturally occurring chlorophyll­(Chl)-<i>a</i> molecule. Metal complexes (magnesium, zinc, and cadmium) of the Chl derivative were synthesized as models of natural chlorosomal BChls. These metal Chl derivatives self-assembled in hydrophobic environments, and their supramolecules were analyzed by spectroscopic and microscopic techniques. Cryo-transmission electron microscopic images showed that the zinc and cadmium Chl derivatives could form single-wall supramolecular nanotubes and their outer and inner diameters were approximately 5 and 3 nm, respectively. Atomic force microscopic images suggested that the magnesium Chl derivative formed similar nanotubes to those of the corresponding zinc and cadmium complexes. Three chlorosomal single-wall supramolecular nanotubes of the metal Chl derivatives were prepared in the solid state and would be useful as photofunctional materials

Specific Gene <i>bciD</i> for C7-Methyl Oxidation in Bacteriochlorophyll <i>e</i> Biosynthesis of Brown-Colored Green Sulfur Bacteria

<div><p>The gene named <i>bciD</i>, which encodes the enzyme involved in C7-formylation in bacteriochlorophyll <i>e</i> biosynthesis, was found and investigated by insertional inactivation in the brown-colored green sulfur bacterium <i>Chlorobaculum limnaeum</i> (previously called <i>Chlorobium phaeobacteroides</i>). The <i>bciD</i> mutant cells were green in color, and accumulated bacteriochlorophyll <i>c</i> homologs bearing the 7-methyl group, compared to C7-formylated BChl <i>e</i> homologs in the wild type. BChl-<i>c</i> homolog compositions in the mutant were further different from those in <i>Chlorobaculum tepidum</i> which originally produced BChl <i>c</i>: (3¹<i>S</i>)-8-isobutyl-12-ethyl-BChl <i>c</i> was unusually predominant.</p> </div

UV-Vis-NIR absorption spectra of whole cells (A) and extracted pigments (B) of the <i>Cba. limnaeum</i> wild type (broken lines) and <i>bciD</i> mutant (solid lines); measured using a Hitachi UV-2550 (Shimadzu, Japan) spectrophotometer.

<p>(A) Wild type and mutant cells in stationary phase were collected and suspended in 50 mM Tris-HCl (pH 7.8) containing 150 mM NaCl; normalized at 660 nm. (B) Pigments were extracted with a mixture of acetone and methanol (7∶2, v/v), and used for the measurements; normalized at Soret maxima. The dotted line shows absorption spectrum of pigments from <i>Cba. tepidum</i> as control for BChl <i>c</i>.</p

Construction of <i>Cba. limnaeum bciD</i> gene inactivated mutant.

<p>(A) Schematic map of genes arrangement around <i>bciD</i> gene in the genome of <i>Cba. limnaeum</i> RK-j-1, and its insertional inactivation. The <i>aadA1</i> gene, conferring resistance to streptomycin and spectinomycin, was inserted in <i>bciD</i>. Arrows represent the primers bciD-F (i), bciD-R (ii), bciD-inf-F (iii), bciD-inf-R (iv), bciD-comf-F (v), and bciD-comf-R (vi). (B) PCR confirmation of gene interruption. The <i>bciD</i> gene was amplified from genomic DNA extracted from the wild type (lanes 1 and 2) and a mutant (lanes 3 and 4) of <i>Cba. limnaeum</i>, using above bciD-comf-F and -R primers. The products in lanes 2 and 4 were then digested by restriction enzyme <i>Eco</i>RV, and the fragments yielded from wild type and the mutant were 1.36 and 0.78, and 2.22 kbp, respectively. Lane M, molecular size marker (the sizes of bands are indicated at left).</p

HPLC elution profiles of extracted pigments from GSB.

<p>(A) From <i>Cba. tepidum</i> as control for BChl <i>c</i>, recorded at 435 nm. (B) From <i>Cba. limnaeum bciD</i> mutant recorded at 435 nm. (C) From <i>Cba. limnaeum</i> wild type recorded at 465 nm. Peak 1, R[E,M]BChl <i>c</i>; peak 2, R[E,E]BChl <i>c</i>; peak 3, S[E,E]BChl <i>c</i>; peak 4, R[P,E]BChl <i>c</i>; peak 5, S[P,E]BChl <i>c</i>; peak 6, R[I,E]BChl <i>c</i>; peak 7, S[I,E]BChl <i>c</i>; peak 8, R[E,E]BChl <i>e</i>; peak 9, S[E,E]BChl <i>e</i>; peak 10, R[P,E]BChl <i>e</i>; peak 11, S[P,E]BChl <i>e</i>; peak 12, R[I,E]BChl <i>e</i>; peak 13, S[I,E]BChl <i>e</i>. Peaks at the asterisk in panel (B) indicate impurities produced during handling of the sample.</p