Toru Oba

Which side of the π-macrocycle plane of (bacterio)chlorophylls is favored for binding of the fifth ligand?

Reported crystallographic data and calculated molecular models indicated that chlorophyll (Chl) a and bacteriochlorophyll (BChl) a tend to bind the fifth ligand on the side of the macrocycle where the C132-(R)-methoxycarbonyl moiety protrudes (denoting the ‘back’ side). The crystal structures of 34 photosynthetic proteins possessing (B)Chl cofactors revealed that most of Chl a and BChl a (and b) are coordinated by any peptidyl residue (e.g., histydyl-imidazolyl group), peptidyl backbone or water from the ‘back’ side. Almost all the cofactors that bind a water molecule as the fifth ligand in these proteins have a ‘back’ configuration. Theoretical model calculations for methyl chlorophyllide a (MeChlid a) and methyl bacteriochlorophyllide a (MeBChlid a) bound to an imidazole molecule indicated that the ‘back’ side is energetically favored for the ligand binding. These results are consistent with the fact that ethyl chlorophyllide a (EtChlid a) dihydrate crystal consists of the ‘back’ complex. The modeling also showed that both removal and stereochemical inverse of the C132-methoxycarbonyl group affect the relative stability between the ‘back’ and ‘face’ complexes. The effect of the C132-moiety on the choice of the macrocycle side for the ligand binding is discussed in relation to the function of P700.

Non-enzymatic conversion of chlorophyll-a into chlorophyll-d in vitro: A model oxidation pathway for chlorophyll-d biosynthesis

Chlorophyll‐a (Chl‐a) was readily converted into Chl‐d under mild conditions without any enzymes. Treatment of Chl‐a dissolved in dry tetrahydrofuran (THF) with thiophenol and acetic acid at room temperature successfully produced Chl‐d in 31% yield. During the acidic oxidation, removal of the central magnesium, pheophytinization, was sufficiently suppressed. This mild pathway can give insights into the yet unidentified Chl‐d biosynthesis.

Asymmetry of chlorophylls in photosynthetic proteins: from the viewpoint of coordination chemistry

We conducted a meta-analysis of (bacterio)chlorophyll [(B)Chl] molecules in photosynthetic pigment-protein complexes from the viewpoint of coordination chemistry. We surveyed the ligand species and site in the axial coordination of 146 Chl and 21 BChl molecules in 42 reported crystal structures of 12-type proteins. The imidazolyl moiety of histidine (His) is the most abundant ligand, and the second is water, a much weaker ligand. We focused on the positions, the circumstances, and the macrocycle sides for the coordination of the 31 hydrated (B)Chl molecules found in these proteins. A ligand water molecule of a hydrated (B)Chl is not necessarily hydrogen-bonded to the surrounding protein residues. A hydrated (B)Chl seems to occupy the redundant space where more strongly coupled His-Chl complexes cannot be formed. It is noted that 28 of 31 hydrated (B)Chl molecules (90) were coordinated from the α-side of the (bacterio)chlorin macrocycle, the opposite side from which the C17-propionic ester protrudes. Among them, all five hydrated Chl molecules at the edges of the proteins were coordinated from the α-side, suggesting that (B)Chl molecules prefer this side for the coordination bondings to the β-side. The analysis also revealed that each (B)Chl binding site was composed of both the protein residues and the neighboring pigment molecules contributing roughly equally. It can be safely said that the cofactor pigments aggregated even in the proteins. Penta-coordination is advantageous to flexible adjustment of intermolecular orientations of (B)Chl molecules in the aggregates.

Ene-thiol reaction of C3-vinylated chlorophyll derivatives in the presence of oxygen: synthesis of C3-formyl-chlorins under mild conditions

Reactions of thiol with the C3-vinyl group of various chlorophyll (Chl) derivatives were examined. The reactions resemble thiol-olefin co-oxidation, except that the vinyl C=C double bond was cleaved to afford a formyl group without any transition metal catalyst, and that the simple anti-Markovnikov adduct of thiol to olefin was obtained as a minor product. Peripheral substituents of Chl derivatives little affected the reaction, while the central metal atom of the chlorin macrocycle influenced the composition of the products. Oxygen and acid dissolved in the reaction mixture can facilitate the oxidation. Sufficiently mild conditions in this regioselective oxidation at the C31-position are significant in bioorganic chemistry.

Synthesis of tripodands with multiple hydroxyl and amide groups exhibiting fluorescent anion sensing

Novel tripodal receptors (T2 and T3) having multiple hydroxyl and amide groups with long alkyl chains (N-dodecylamino and N,N-didodecylamino groups, respectively) were prepared. The broadening of proton signals observed in the 1H NMR spectrum of compounds T2 and T3 in a non-polar solvent (CDCl3) was attributed to intermolecular and/or intramolecular hydrogen bonding. In the fluorescence spectrum, the addition of Bu4NF to the solution of receptors T2 and T3 caused the drastic increase in fluorescence intensity compared with the addition of other anion species.

Alternative synthesis of 3-acetyl, 3-epoxy, and 3-formyl chlorins from a 3-vinyl chlorin, methyl pyropheophorbide-a, via iodination.

We developed novel methods to convert the C3-vinyl group of a chlorophyll derivative, methyl pyropheophorbide-a, into an acetyl group, an epoxy group, and a formyl group via iodination with I2 and phenyliodine(III) bis(trifluoroacetate). Reaction of the iodinated intermediate with ethylene glycol and subsequent treatment with base led to formation of the C3-acetyl chlorin. Reaction of the iodinated intermediate with ethylenediamine afforded the C3-oxiranyl chlorin. The C3-formyl chlorin was readily derived from the epoxide without hazardous reagents such as OsO4. These reactions were facile and useful alternatives to the previous methods.

Thermal-induced Immuno-nephelometry Using Gold Nanoparticles Conjugated with a Thermoresponsive Polymer for the Detection of Avidin

Thermoresponsive immunonephelometry was achieved with biotinylated poly(acrylate) and thermoresponsive gold nanocomposites composed of 13-nm gold nanoparticles and thermoresponsive polymers containing triethylenetetramine and biotin groups. The avidin-biotin interaction was used to model an immunoreaction in order to demonstrate thermoresponsive immunonephelometry. In the absence of avidin, positively charged gold nanocomposites electrostatically interacted with biotinylated poly(acrylate) to form binary complexes, in which the charges canceled each other out. The charge cancelation resulted in the binary complexes precipitating when the solution was heated above the phase-transition temperature. However, adding avidin formed ternary sandwich complexes through the avidin-biotin interaction. The ternary complexes remained sufficiently soluble above the phase-transition temperature because of the spatial isolation of the positive and negative charges. The transmittance of the solution containing the thermoresponsive gold nanocomposites and biotinylated poly(acrylate) at 37°C increased as the avidin concentration increased. A sigmoidal profile was observed from 10(-6.5) to 10(-5.5) mol/L. The concentration of avidin spiked in bovine serum was determined by our method.

Demetalation kinetics of the zinc chlorophyll derivative possessing two formyl groups: effects of formyl groups conjugated to the chlorin macrocycle on physicochemical properties of photosynthetic pigments

Demetalation kinetics of zinc chlorophyll derivative 1 possessing two formyl groups directly linked to the A- and B-rings of the chlorin macrocycle, which was synthesized from chlorophyll b, was examined under acidic conditions and compared with those of Zn chlorins 2 and 3 possessing a single formyl group in the A- and B-ring, respectively, as well as Zn chlorin 4 lacking any formyl group to unravel the substitution effects on demetalation properties of chlorophyllous pigments. Demetalation kinetics of diformylated Zn chlorin 1 was slower than those of monoformylated Zn chlorins 2 and 3, indicating that the effect of the electron-withdrawing formyl group on demetalation kinetics was amplified by introduction of two formyl groups to the chlorin macrocycle. High correlations were observed between demetalation rate constants of Zn chlorins 1–4 and the sum of Hammett σ parameters of the 3- and 7-substituents on the chlorin macrocycle, indicating that the combination of electron-withdrawing abilities of the substituents linked directly to the cyclic tetrapyrrole was responsible for demetalation properties of zinc chlorophyll derivatives.

Regulation of Antenna Functions in Chlorophyll Assemblies

We synthesized 7-substituted chlorophyll (Chl) derivatives with different chain lengths and lithocholic esters as spacer molecules in the Chl nanoparticles. The Chl derivatives formed selfaggregate nanoparticles in water, and the particle size and the fluorescence intensity of the nanoparticles depended on the chain length at the 7-position. Chl derivatives and lithocholic esters coexisted in each nanoparticle without microscopic phase separation. The lithocholic ester behaves as a spacer molecule to dilute the Chl concentration in each nanoparticle. The self-aggregates are new model systems to study the pigment-matrix interactions in antenna complexes, as well as potent photoelectronic and clinical nanodevices.