Machiko Akiyama

Spectroscopy and Structure Determination

Summary Analysis by a combination of absorption-, fl uorescence-, circular dichroism-, mass- and nuclear magnetic resonance-spectrometry is often used to investigate the structure of chlorophylls. Here, we show several examples of spectroscopic determination of molecular structure of the recently-discovered chlorophylls and compare them with the well-known chlorophylls, chlorophyll a and bacteriochlorophyll a.

Identification of the primary electron donor in PS II of the Chl d-dominated cyanobacterium Acaryochloris marina

The primary electron donor of photosystem (PS) II in the chlorophyll (Chl) d‐dominated cyanobacterium Acaryochloris marina was confirmed by delayed fluorescence (DF) and further proved by pigment contents of cells grown under several light intensities. The DF was found only in the Chl a region, identical to Synechocystis sp. PCC 6803, and disappeared following heat treatment. Pigment analyses indicated that at least two Chl a molecules were present per each two pheophytin a molecules, and these Chl a molecules are assigned to PD1 and PD2. These findings clearly indicate that Chl a is required for water oxidation in PS II.

Composition and optical properties of reaction centre core complexes from the green sulfur bacteria Prosthecochloris aestuarii and Chlorobium tepidum.

Photosynthetically active reaction centre core (RCC) complexes were isolated from two species of green sulfur bacteria, Prosthecochloris (Ptc.) aestuarii strain 2K and Chlorobium (Chl.) tepidum, using the same isolation procedure. Both complexes contained the main reaction centre protein PscA and the iron–sulfur protein PscB, but were devoid of Fenna–Matthews–Olson (FMO) protein. The Chl. tepidum RCC preparation contained in addition PscC (cytochrome c). In order to allow accurate determination of the pigment content of the RCC complexes, the extinction coefficients of bacteriochlorophyll (BChl) a in several solvents were redetermined with high precision. They varied between 54.8 mM−1 cm−1 for methanol and 97.0 mM−1 cm−1 for diethylether in the QY maximum. Both preparations appeared to contain 16 BChls a of which two are probably the 132-epimers, 4 chlorophylls (Chls) a 670 and 2 carotenoids per RCC. The latter were of at least two different types. Quinones were virtually absent. The absorption spectra were similar for the two species, but not identical. Eight bands were present at 6 K in the BChl a QY region, with positions varying from 777 to 837 nm. The linear dichroism spectra showed that the orientation of the BChl a QY transitions is roughly parallel to the membrane plane; most nearly parallel were transitions at 800 and 806 nm. For both species, the circular dichroism spectra were dominated by a strong band at 807–809 nm, indicating strong interactions between at least some of the BChls. The absorption, CD and LD spectra of the four Chls a 670 were virtually identical for both RCC complexes, indicating that their binding sites are highly conserved and that they are an essential part of the RCC complexes, possibly as components of the electron transfer chain. Low temperature absorption spectroscopy indicated that typical FMO–RCC complexes of Ptc. aestuarii and Chl. tepidum contain two FMO trimers per reaction centre.

The primary electron acceptor of green sulfur bacteria, bacteriochlorophyll 663, is chlorophyll a esterified with Δ 2,6-phytadienol

The primary electron acceptor of green sulfur bacteria, bacteriochlorophyll (BChl) 663, was isolated at high purity by an improved purification procedure from a crude reaction center complex, and the molecular structure was determined by fast atom bombardment mass spectroscopy (FAB-mass), 1H- and 13C-NMR spectrometry, double quantum filtered correlation spectroscopy (DQF-COSY), heteronuclear multiple-quantum coherence (HMQC) and heteronuclear multiple-bond correlation (HMBC) spectral measurements. BChl 663 was 2.0 mass units smaller than plant Chl a. The NMR spectra showed that the macrocycle was identical to that of Chl a. In the esterifying alcohol, a singlet P71 signal was observed at the high-field side of the singlet P31 signal in BChl 663, while a doublet peak of P71 overlapped that of P111 in Chl a. A signal of P7-proton, seen in Chl a, was lacking, and the P6-proton appeared as a triplet signal near the triplet P2-proton signal in BChl 663. These results indicate the presence in BChl 663 of a C=C double bond between P6 and P7 in addition to that between P2 and P3. The structure of BChl 663 was hence concluded to be Chl a esterified with 2,6-phytadienol instead of phytol. In addition to BChl 663, two molecules of the 132-epimer of BChl a, BChl a′, were found to be present per reaction center, which may constitute the primary electron donor.

Quest for minor but key chlorophyll molecules in photosynthetic reaction centers – unusual pigment composition in the reaction centers of the chlorophyll d-dominated cyanobacterium Acaryochloris marina

A short overview, based on our own findings, is given of the minor pigments that function as key components in photosynthesis. Recently, we found the presence of chlorophyll a, chlorophyll d′ and pheophytin a as minor pigments in the chlorophyll d-dominated cyanobacterium Acaryochloris marina.

Light-independent isomerization of bacteriochlorophyll g to chlorophyll a catalyzed by weak acid in vitro

Abstract Rapid conversion of bacteriochlorophyll g (BChl g) to chlorophyll a (Chl a) was observed in acetone on addition of acid in the dark. The product, Chl a esterified with farnesol (Chl aF), was identified by liquid chromatography and fast atom bombardment mass spectrometry. Acid-catalyzed formation of 81-OH-Chl aF, a primary electron acceptor in the heliobacterial reaction center, was also observed in diethyl ether in the dark. These results suggest that acid-catalyzed isomerization is a candidate for the chemical evolution of BChl g to the more stable Chl a and that 81-OH-Chl aF can easily be synthesized from BChl g under weakly acidic conditions in the dark.

Serendipitous discovery of Chl d formation from Chl a with papain

Abstract Cancer photodynamic therapy (PDT) requires the availability of photosensitizers which have a high efficiency and selectivity for the destruction of tumor cells. Chlorophyll (Chl) a is one of the favorable photosensitizers, because it has a high extinction coefficient in the red light region, where light transmission through the human tissues is very high. However, Chl a had a serious problem that it cannot be dissolved in water, so we tried to prepare water-soluble chlorophyllide a from Chl a by several enzymes, and serendipitously came across a unique formation of Chl d from Chl a when papain was used in aqueous acetone. Similar oxidation was observed in Chl b and pheophytin a, although the reactions were very slow. Our finding will provide insight into the unsolved key question as to the biosynthetic pathway of Chl d via Chl a in a recently found novel cyanobacterium Acaryochloris marina.

Acidiphilium Rubrum and Zinc-Bacteriochlorophyll Part 1: Molecular Structure of the Zinc-Containing Bacteriochlorophyll

The essential pigments of photo-synthesis are Chlorophylls (Chls) and bacterlochlorophylls (BChls). Chls and BChls contain magnesium (Mg) as a central metal of substituted porphyrin, chlorin or bacteriochlorin macrocycles, wlth a sole exception of metal-free Chi a, BChl a and BChl b, called pheophytln (Pheo) a, bacteriopheophytin (BPheo) a (Flg. 1) and BPheo b, that act as the prlmary electron acceptors In the reactlon centers of photosystem 2 of higher plants and algae [1] and the photosynthetic purple bacteria [2,3].

Acidiphilium Rubrum and Zinc-Bacteriochlorophyll Part 3: High Resistance of Zinc-Bacteriochlorophyll A to Acid

The chemical structure of the novel Zn-containing bacteriochlorophyll found in A. rubrum [1] has been confirmed to be Zn-BChl a (Fig. 1) [2,3]. This incites us to compare the physicochemical properties of Zn-BChl a with those of MgBChl a (Fig. 1). Easy pheophytinization (demetalation) of Mg-(B)Chls is empirically known, and hence more stable Zn-complexes are widely used in studies on artificial photosynthesis [4,5]. Though this may be a clue for the choice of Zn, quantitative comparison has been done in a limited number of works on the pheophytinization of Mg-(B)Chls [6-12].

Acidiphilium Rubrum and Zinc-Bacteriochlorophyll Part 2: Physicochemical Comparison of Zinc-Type Chlorophylls and Other Metallochlorophylls

The chemical structure of the novel Zn-containing bacteriochlorophyll (BChl) found in A. rubrum [1] has been confirmed to be Zn-BChl a [2,3]. This incites us to compare the physicochemical properties of ZnBChl a with those of BChl a (Mg-BChl a) as well as metallobacteriochlorophylls (M-BChls) a and metallochlorophylls (M-Chls) a. Here we discuss the reason for the use of Zn as the central metal in A. rubrum, referring to such physicochemical behaviors of M-BChls a and M-Chls a as the absorption, fluorescence and redox potentials.