PROPERTIES OF ZINC AND MAGNESIUM METHYL BACTERIOPHEOPHORBIDE d AND THEIR AGGREGATES

Stereoselective self-aggregation of synthetic zinc 31-epimeric bacteriochlorophyll-d analogs possessing a methylene group at the 132-position as models of green photosynthetic bacterial chlorosomes

Zinc bacteriochlorophyll-d analogs possessing a methylene group at the 132-position were prepared by chemical modification of naturally occurring chlorophyll-a. The synthetic 31-epimers were successfully separated by reverse phase HPLC to give diastereomerically pure samples. The stereochemistry of the chiral C31-center in the separated bacteriochlorophyll-d analogs was determined by HPLC analysis of the authentic stereoisomers prepared stereospecifically. Both the epimers were monomeric in tetrahydrofuran to give sharp absorption bands, while they self-aggregated to form chlorosomal oligomers with red-shifted bands in an aqueous Triton X-100 micelle solution. The resulting large oligomers deaggregated by addition of Triton X-100 to give monomeric species. Their aggregation and deaggregation were dependent on the 31-stereochemistry, indicating that each epimer produced self-aggregates that were supramolecularly different. The substitution with the 132-methylene group enhanced their self-aggregation abilities and the stability of their resulting self-aggregates.

Stereoselective Self-Aggregation of 31 -Epimerically Pure Amino Analogs of Zinc Bacteriochlorophyll-d in an Aqueous Micelle Solution

Zinc bacteriochlorophyll-d analogs possessing an amino group instead of the original hydroxy group at the C3¹ position were prepared by chemical modification of naturally occurring chlorophyll-a. The synthetic 3¹ -epimers were successfully separated by reverse phase HPLC to give diastereomerically pure samples. The stereochemistry of the chiral C3¹ -center in the separated amines was determined by NMR analysis of their diastereomeric amides as well as by their asymmetric synthesis from authentic stereoisomers. Both the epimers were monomeric in tetrahydrofuran to give sharp electronic absorption bands, while they self-aggregated to form chlorosomal oligomers with the redshifted bands in an aqueous Triton X-100 micelle solution (pH = 6.9). The resulting oligomers deaggregated by addition of p-toluenesulfonic acid to give monomeric N-protonated ammonium species. The aggregation and deaggregation were dependent on the 3¹ -stereochemistry, indicating that each epimer produced supramolecularly different self-aggregates.

Facile Synthesis of 131-Oxo-porphyrins Possessing Reactive 3-Vinyl or 3-Formyl Group, Protochlorophyll-a/dDerivatives by 17,18-Dehydrogenation of Chlorins

Methyl protopyropheophorbides-a/d were prepared by modification of readily available chlorophyll-a. The key step was oxidation of 131-oxo-chlorin to the corresponding porphyrin, and 2,3-dichloro-5,6-dicyano-p-benzoquinone was effective for 17,18-dehydrogenation. A reduction in oxidation potential of the chlorin was necessary for the oxidation. Facile oxidation was achieved by insertion of zinc as the central metal, reduction of the 13-carbonyl group and/or aggregation by high concentration. The synthetic 131-oxo-porphyrins had vinyl and formyl groups at the 3-position. These groups were very reactive in the molecule and selectively converted to other functional groups.

Efficient Energy Transfer from Peripheral Chromophores to the Self-Assembled Zinc Chlorin Rod Antenna: A Bioinspired Light-Harvesting System to Bridge the “Green Gap”

An artificial light-harvesting rod aggregate based on zinc chlorin and covalently linked naphthalene bisimide chromophore has been realized by self-assembly. Efficient energy transfer (phiET >/= 0.99) takes place upon excitation at 620 nm from peripheral naphthalene bisimides to the zinc chlorin rod aggregate backbone. The appended naphthalene bisimide dyes improve the total LH efficiency of the rod aggregate by 26%. Thus, the present bioinspired antenna system is promising for application in nanodevices for the effective utilization of solar energy by bridging the "green gap".

Stereochemical determination of chlorophyll-d molecule from Acaryochloris marina and its modification to a self-aggregative chlorophyll as a model of green photosynthetic bacterial antennae

Acaryochloris marina is a unique photosynthetic prokaryote containing chlorophyll(Chl)-d as a major photoactive pigment (over 95%). The molecular structure of Chl-d is proposed as the 3-formyl analog of Chl-a. However, the stereochemistry of Chl-d at the 13(2)-, 17- and 18-positions has not yet been established unambiguously. In the first part of this paper, we describe the determination of their stereochemistries to be 13(2)-(R)-, 17-(S)- and 18-(S)-configurations by using 1H-1H NOE correlations in 1H-NMR and circular dichroism spectra as well as chemical modification of Chl-a to produce stereochemically defined Chl derivatives. In the second part of the paper, we report a facile synthesis of a self-aggregative Chl by modifying isolated Chl-d. Since Chl-d was characterized by its reactive 3-formyl group, the formyl group was reduced with t-BuNH2BH3 to afford the desirable Chl, 3-deformyl-3-hydroxymethyl-pyrochlorophyll-d (3(1)-OH-pyroChl-d). The synthetic 3(1)-OH-pyroChl-d molecules spontaneously self-organized to form well-ordered aggregates in a non-polar organic solvent. The self-aggregates are a good model of major light-harvesting antenna systems of green photosynthetic bacteria, chlorosomes, in terms of the following three findings. (1) Both the red-shifted electronic absorption band above 750 nm and its induced reverse S-shape CD signal around 750 nm were observed in 0.5% (v/v) THF-cyclohexane. (2) The stretching mode of the 13-carbonyl group was downshifted by about 35 cm(-1) from the wavenumber of its free carbonyl. (3) The self-aggregates were quite stable on titration of pyridine to the suspension, in comparison with those of natural chlorosomal bacteriochlorophyll-d possessing the 3-(1-hydroxyethyl) group.

Bacteriochlorophyll e monomers, but not aggregates, sensitize singlet oxygen: implications for a self-photoprotection mechanism in chlorosomes

Sensitization of singlet delta oxygen (O2(1delta(g))) by bacteriochlorophyll e (BChle) has been investigated to gain a better understanding of the photoprotection mechanism(s) operating in chlorosomes of green photosynthetic bacteria. The sensitization process has been studied in media where BChle forms monomers (acetone and aqueous solutions containing 0.5% Triton X-100 [TX]) and in systems where BChle aggregates, namely, aqueous solutions containing 0.003% monogalactosyl diglyceride (MGDG) and chlorosomes(control as well as hexanol perturbed) from Chlorobium phaeobacteroides strain CL1401. In Ar-purged acetone, BChle triplets (BChle triplets) have a lifetime of a few tens of microseconds; however, in air-saturated acetone, quenching of BChle triplets by ground-state oxygen (O2(3sigma(-)g)) and formation of O2(1delta(g)) take place. The O2(1delta(g)) so formed is susceptible to quenching by BChle0, a ground-state BChle molecule. A Stern-Volmer analysis reveals a linear fit between the decay rate of O2(1delta(g)) and the BChle concentration. The rate constants for the quenching of O2(1delta(g)) by BChle0 and for the deactivation of O2(1delta(g)) by the solvent come out to be kq = (1.4 +/- 0.1) x 10(9) M(-1) s(-1) and k0 = (18.5 +/- 0.7) x 10(3) s(-1), respectively. The absolute quantum yield of O2(1delta(g)) sensitization by BChle monomers is 0.65 +/- 0.15 in air-saturated acetone. In aqueous phase, the triplet lifetime of BChle aggregates in native or hexanol-perturbed chlorosomes shortens by more than two orders of magnitude when compared with the triplet lifetime of BChle monomers in 0.5% TX solution (a few hundreds of microseconds). Quenching by carotenoids (Car) makes only a minor contribution to the decay of BChle triplets in aggregates. Because O2(1delta(g)) sensitization by BChle triplets could be detected neither in MGDG aggregates nor in chlorosomes (control as well as hexanol perturbed), it is concluded that (1) this process is highly likely when BChle is present as a monomer but not when it is tightly packed in artificial aggregates or in chlorosomes; and (2) Car, though vital for the baseplate BChla, are dispensable for BChle.

Regio- and stereoisomeric control of the aggregation of zinc-chlorins possessing inverted interactive hydroxyl and carbonyl groups

As models for a self-aggregative, naturally occurring magnesium-chlorin bacteriochlorophyll-d possessing 3(1)-secondary alcoholic hydroxyl and 13(1)-oxo groups, zinc-chlorins were synthesized with 3(1)-oxo and 13(1)-secondary (1) or tertiary hydroxyl groups (2). Compared to the monomers in a tetrahydrofuran solution, diastereomers 13(1)R-1R and 13(1)S-1S gave red-shifted absorption maxima (643 --> 674 nm in 1R and 708 nm in 1S) in 1 v/v% CH(2)Cl(2)-hexane solution, indicating their self-aggregation. Therefore, the positioning of the two groups at 3(1)/13(1) or 13(1)/3(1) on the N21-N23 molecular (Q(y)) axis is not necessarily important for the self-aggregation. The (1)H NMR and CD spectroscopic studies showed that the 674 nm absorbing species of 1R was characterized as a face-to-face "closed" dimer, while the 708 nm absorbing species of 1S was a large oligomer constructed with aggregation of head-to-tail "open" dimers. This diastereomeric control over the aggregation of 1R and 1S is more pronounced than that observed in the regioisomerically 3(1)-secondary alcoholic R/S-diastereomers 3R and 3S. The difference is ascribable to the conformational fixation of the 13(1)-hydroxyl group of the exo five-membered ring in 1. In contrast to self-aggregative 3(1)-tertiary alcoholic 4, both 13(1)-epimers of 13(1)-tertiary alcoholic 2 were monomeric even in nonpolar organic media: the additional 13(1)-methyl group (1 --> 2) drastically suppressed the self-aggregation due to the interference of the methyl group in intermolecular pi-pi interaction.

The photophysics of monomeric bacteriochlorophylls c and d and their derivatives: properties of the triplet state and singlet oxygen photogeneration and quenching

Measurements of pigment triplet-triplet absorption, pigment phosphorescence and photosensitized singlet oxygen luminescence were carried out on solutions containing monomeric bacteriochlorophylls (Bchl) c and d, isolated from green photosynthetic bacteria, and their magnesium-free and farnesyl-free analogs. The energies of the pigment triplet states fell in the range 1.29-1.34 eV. The triplet lifetimes in aerobic solutions were 200-250 ns; they increased to 280 +/- 70 microseconds after nitrogen purging in liquid solutions and to 0.7-2.1 ms in a solid matrix at ambient or liquid nitrogen temperatures. Rate constants for quenching of the pigment triplet state by oxygen were (2.0-2.5) x 10(9) M-1 s-1, which is close to 1/9 of the rate constant for diffusion-controlled reactions. This quenching was accompanied by singlet oxygen formation. The quantum yields for the triplet state formation and singlet oxygen production were 55-75% in air-saturated solutions. Singlet oxygen quenching by ground-state pigment molecules was observed. Quenching was the most efficient for magnesium-containing pigments, kq = (0.31-1.2) x 10(9) M-1 s-1. It is caused mainly by a physical process of singlet oxygen (1O2) deactivation. Thus, Bchl c and d and their derivatives, as well as chlorophyll and Bchl a, combine a high efficiency of singlet oxygen production with the ability to protect photochemical and photobiological systems against damage by singlet oxygen.

Qy-excitation resonance Raman spectra of chlorophyll a and bacteriochlorophyll c/d aggregates. Effects of peripheral substituents on the low-frequency vibrational characteristics

Low-frequency (80-700 cm-1) Qy-excitation resonance Raman (RR) spectra are reported for thin-solid-film aggregates of several chlorophyll (Chl) a and bacteriochlorophyll (BChl) c/d pigments. The pigments include Chl a, pyrochlorophyll a (PChl a), methylpyrochloropyllide a (MPChl a), methylbacteriochloropyllide d (MBChl d), [E,M] BChl cS, [E,E] BChl cF, and [P,E] BChl cF. The BChl c/d's are the principal constituents of the chlorosomal light-harvesting apparatus of green photosynthetic bacteria. Together, the various Chl a's and BChl c/d's represent a series in which the peripheral substituent groups on the chlorin macrocycle are varied in systematic fashion. All of the Chl a and BChl c/d aggregates exhibit rich low-frequency vibrational patterns. In the case of the BChl c/d's, certain modes in the very low-frequency region (100-200 cm-1) experience exceptionally strong Raman intensity enhancements. The frequencies of these modes are qualitatively similar to those of oscillations observed in femtosecond optical experiments on chlorosomes. The RR data indicate that the low-frequency vibrations are best characterized as intramolecular out-of-plane deformations of the chlorin macrocycle rather than intermolecular modes. The coupling of the out-of-plane modes in turn implies that the Qy electronic transition(s) of the aggregate have out-of-plane character. The RR spectra of the BChl c/d's also reveal that the nature of the alkyl substituents at the 8 and 12 positions of the macrocycle plays an important role in determining the detailed features of the low-frequency vibrational patterns. The frequencies of the modes are particularly sensitive to larger substituent groups whose conformations may be more easily perturbed in the tightly packed aggregates. These factors also make aggregates of pigments containing larger substituents more susceptible to structural, electronic, and vibrational inhomgeneities. Collectively, the RR studies of the various pigments delineate the factors which influence the low-frequency vibrational characteristics of chlorosomal aggregates.

Self quenching of chlorosome chlorophylls in water and hexanol-saturated water

The optical properties of a methyl ester homolog of bacteriochlorophylld (BChld M ) and bacteriochlorophyllc (BChlc) in H2O, hexanol-saturated H2O and methanol were studied by absorption, fluorescence emission, and circular dichroism (CD). In H2O, BChld M spontaneously forms an aggregate similar to that formed in hexane, with absorption maximum at 730 nm and fluorescence emission at 748 nm. For the pigment sample in hexanol-saturated H2O, while the absorption peaks at 661 nm, only slightly red-shifted compared to the monomer, the fluorescence emission is highly quenched. When diluted 2-3 fold with H2O, the absorption returns to around 720 nm, characteristic of an aggregate. The CD spectrum of the H2O aggregate exhibits a derivative-shaped feature with positive and negative peaks, while the amplitude is lower than that of chlorosomes. The Fourier transform infrared spectra of BChld M aggregates in H2O and hexane were measured. A 1644 cm(-1) band, indicative of a bonded 13(1)-keto group, is detected for both samples. A marker band for 5-coordinated Mg was observed at 1611 cm(-1) for the two samples as well. To study the kinetic behavior of the samples, both single-photon counting (SPC) fluorescence and transient absorption difference spectroscopic measurements were performed. For BChld M in hexanol-saturated H2O, a fast decay component with a lifetime of 10 to 14 ps was detected using the two different techniques. The fast decay could be explained by the concentration quenching phenomenon due to a high local pigment concentration. For the pigment sample in H2O, SPC gave a 16 ps component, whereas global analysis of transient absorption data generated two fast components: 3.5 and 26 ps. The difference may arise from the different excitation intensities. With a much higher excitation in the latter measurements, other quenching processes, e.g. annihilation, might be introduced, giving the 3.5 ps component. Finally, atomic force microscopy was used to examine the ultrastructure of BChld M in H2O and hexanol-saturated H2O. Pigment clusters with diameters ranging from 15 to 45 nm were observed in both samples.

Dimerization of synthetic zinc aminochlorins in non-polar organic solvents

UV-visible spectra of synthetic zinc aminochlorins were measured in 99:1 (v/v) cyclohexane-dichloromethane solution. The compounds formed anti-parallel dimers with mutual coordination of the central zinc in one molecule to the amino nitrogen in the other (Qy band red-shift of about 500 cm(-1)). Such a dimer arrangement appears to be too stable to form far-red (> 1500 cm(-1)) shifted oligomers which have been observed with bacteriochlorophylls-c (possessing a hydroxy group and a central magnesium) and with their model compounds (with a hydroxy group and a central zinc) in non-polar organic solvents.

Generation and quenching of singlet molecular oxygen by aggregated bacteriochlorophyll d in model systems and chlorosomes

Both photogeneration and quenching of singlet oxygen by monomeric and aggregated (dimeric and oligomeric) molecules of bacteriochlorophyll (BChl) d have been studied in solution and in chlorosomes isolated from the green photosynthetic bacterium Chlorobium vibrioforme f. thiosulfatophilum. The yield of singlet-oxygen photogeneration by pigment dimers was about 6 times less than for monomers. Singlet oxygen formation was not observed in oligomer-containing solutions or in chlorosomes. To estimate the efficiency of singlet oxygen quenching an effective rate constant for (1)O2 quenching by BChl molecules (kq (M)) was determined using the Stern-Volmer equation and the total concentration of BChl d in the samples. In solutions containing only monomeric BChl, the kq (M) values coincide with the real values for (1)O2 quenching rate constants by BChl molecules. Aggregation weakly influenced the kq (M) values in pigment solutions. In chlorosomes (which contain both BChl and carotenoids) the kq (M) value was less than in solutions of BChl alone and much less than in acetone extracts from chlorosomes. Thus (1)O2 quenching by BChl and carotenoids is much less efficient in chlorosomes than in solution and is likely caused primarily by BChl molecules which are close to the surface of the large chlorosome particles. The data allow a general conclusion that monomeric and dimeric chlorophyll molecules are the most likely sources of (1)O2 formation in photosynthetic systems and excitation energy trapping by the long wavelength aggregates as well as (1)O2 physical quenching by monomeric and aggregated chlorophyll can be considered as parts of the protective system against singlet oxygen formation.