Synthetic Zinc and Magnesium Chlorin Aggregates as Models for Supramolecular Antenna Complexes in Chlorosomes of Green Photosynthetic Bacteria

Abstract— A comparison of the spectra of in vitro (3-hydroxymethyl-13¹-oxometallochlorin) and in vivo chlorosomal (bacterio-chlorophyll- c ) aggregates suggests a similar supramolecular structure for the artificial oligomers and the bacte-riochlorophyll- c aggregates in the extramembranous antenna complexes (chlorosomes) of green photosynthetic bacteria. Synthetic zinc and magnesium chlorins have been found to aggregate in 1 % (vol/vol) tetrahydrofuran and hexane solutions and in thin films to form oligomers with the Q_y absorption bands shifted to longer wavelengths by about 1900 (Zn chlorins) and 2100 cm⁻¹ (Mg) relative to the corresponding monomer bands. Visible absorption and circular dichroism spectra of various zinc chlorins establish that a central metal, a 3¹-hydroxy and a 13¹-keto group are functional prerequisites for the aggregation. Vibrational bands measured by IR spectroscopy of solid films reveal two characteristic structural features of the oligomers: (1) a five-coordinated metallochlorin macrocycle with an axial ligand (bands at 1500-1630 cm⁻¹), and (2) a hydrogen bond between the keto oxygen of one chlorin and the hydroxy group of a second chlorin, the oxygen of which is chelated to the metal atom of a third molecule, i.e . C=O…H-O…M (=Zn or Mg).     

Examination of chlorin-bacteriochlorin energy-transfer dyads as prototypes for near-infrared molecular imaging probes

Abstract New classes of synthetic chlorin and bacteriochlorin macrocycles are characterized by narrow spectral widths, tunable absorption and fluorescence features across the red and near-infrared (NIR) regions, tunable excited-state lifetimes (<1 to >10 ns) and chemical stability. Such properties make dyad constructs based on synthetic chlorin and bacteriochlorin units intriguing candidates for the development of NIR molecular imaging probes. In this study, two such dyads (FbC-FbB and ZnC-FbB) were investigated. The dyads contain either a free base (Fb) or zinc (Zn) chlorin (C) as the energy donor and a free base bacteriochlorin (B) as the energy acceptor. In both constructs, energy transfer from the chlorin to bacteriochlorin occurs with a rate constant of approximately (5 ps)(-1) and a yield of >99%. Thus, each dyad effectively behaves as a single chromophore with an exceptionally large Stokes shift (85 nm for FbC-FbB and 110 nm for ZnC-FbB) between the red-region absorption of the chlorin and the NIR fluorescence of the bacteriochlorin (lambda(f) = 760 nm, Phi(f) = 0.19, tau approximately 5.5 ns in toluene). The long-wavelength transitions (absorption, emission) of each constituent of each dyad exhibit narrow (相似文献   

Molecular Requirement of Chlorosomal Chlorophylls. Self-Organization of a Chlorophyll Derivative Possessing a Hydroxyl Group at Ring II

Aggregation of zinc 7¹-hydroxyl-13²-demethoxycarbonyl-pheophytin a (Zn-7¹-OH-Chl) was examined in relation to the structure and function of the self-aggregates of 3¹-OH-type chlorophylls (Chi) in chlorosomes of green photosynthetic bacteria. The Zn-7¹-OH-Chl aggregates yielded a Q_y absorption band at 712 nm with a 1.2-fold larger width (full width at half maximum, 500 cm⁻¹) than the monomer's (420 cm⁻¹). Infrared and NMR spectroscopies revealed that each molecule in the aggregate links together with simultaneous coordination (C7¹-OH…Zn) and hydrogen bonding (C7¹-OH … O=C13¹). A nonlinear alignment of the constituent molecules in the oligomeric structure was assumed. Despite the similar molecular linkages, linearly aligned Q_y, moments in the Zn-3¹-OH-Chl aggregate gave a chlorosome-like broader, more redshifted Q_y band (740 nm; 670 cm⁻¹, 2.1-fold larger than the monomer's). Because it is advantageous for efficient light harvesting and energy transfer to have several Q_y, spectral components, spread over a wide spectral range, that can act as the energy gradient, it is concluded that not only the intermolecular linkages but the linear locations of OH, C=0 and Mg in the molecule are crucial for photosynthetic antenna of the self-assembled chiorosomal Chl.     

Self-Assembly of Synthetic Zinc Chlorins in Aqueous Microheterogeneous Media to an Artificial Supramolecular Light-Harvesting Device

Self-assembled aggregates of a synthetic zinc chlorin in an aqueous suspension with either α-lecithin or Triton X-100 exhibit unique structural and functional properties. Absorption, circular dichroism, fluorescence, and resonance Raman spectra indicate that the supramolecular structure in an aqueous microheterogeneous medium is very similar to that of the bacteriochlorophyll c aggregates in non-polar organic solvents and in chlorosomes, the main light-harvesting antennae of green photosynthetic bacteria. The nature of the aggregates is controlled by structure and/or concentration of the added surfactants. When a small amount of metal-free bacteriochlorin is present it acts as an efficient energy acceptor from the aggregated zinc chlorins. Thus, self-assembly of synthetic zinc chlorins, doped with appropriate energy acceptors and surrounded with surfactants, affords an artificial supramolecular light-harvesting device in aqueous environment.     

The Structure of the Aggregate Form of Bacteriochlorophyll c Showing the Qy Absorption above 740 nm as Determined by the Ring-current Effects on 1H and 13C Nuclei and by 1H–1H Intermolecular NOE Correlations

¹³C-enriched bacteriochlorophyll c (S[I, E] BChl c _F) was suspended in a 1:3 mixture of methylene chloride and carbon tetrachloride to form an aggregate showing the Q_y absorption above 740 nm; changes in the ¹³C chemical shifts were traced when methanol was titrated to dissolve the aggregate, and then, the changes were correlated to the ring-current effects due to the neighboring macrocycles in the aggregate. A pair of aggregate structures has been proposed based on the ring-current effects on both ¹H and ¹³C nuclei; the monomeric units are stacked together to form an inclined column with different sliding directions, in which the y-axis of the molecule is parallel to the long axis of the column. In order to confirm this pair of models, the ring-current effects on the ¹H and ¹³C nuclei were calculated based on both the magnetic-dipole and the loop-current approximations. Further, an application of three-dimensional F1 ¹³C-edited F3 ¹³C-filtered heteronuclear single-quantum correlation-nuclear Overhauser effect spectroscopy to the above aggregate consisting of a 1:1 mixture of ¹³C-labeled and unlabeled BChl c succeeded in detecting selectively the ¹H–¹H intermolecular nuclear Overhauser effect correlations, which established the coexistence of the above pair of stacked structures in the aggregate.     

PHOTOSENSITIZING PROPERTIES OF BACTERIOCHLOROPHYLLIN a and BACTERIOCHLORIN a, TWO DERIVATIVES OF BACTERIOCHLOROPHYLL a

Abstract The photosensitizing properties of two water soluble derivatives of bacteriochlorophyll a , bacteriochlorophyllin a and bacteriochlorin a (lacking the central Mg-ion) were investigated and compared to those of hematoporphyrin derivatives. At physiological pH the oxygen consumption rate of histidine, tryptophan, dithiothreitol and guanosine upon illumination was 3 to 4 times higher when bacteriochlorin a was used as photosensitizer than when hematoporphyrin derivatives were used. Especially bacteriochlorin a proved to be an effective sensitizer for the killing of L929 cells. Because bacteriochlorin a has an absorption maximum at 765 nm in phosphate buffered saline (allowing a light penetration in tissue about ten times larger than at 630 nm) and a high molar absorption coefficient (32 000 M cm⁻¹) it has promising possibilities for the application in photodynamic therapy.     

POTENTIAL PHOTOSENSITIZERS FOR PHOTODYNAMIC TUMOR THERAPY—I. PHOTOPHYSICAL PROPERTIES OF TWO CHLORIN DERIVATIVES

Abstract— Photophysical properties of two chlorin type molecules (CHLI) and (CHLII) were investigated in different solvents. Quantum yields of fluorescence φ_F of S, → T, intersystem crossing φ_T, and of singlet oxygen (¹Δ_g) formation φ_Δ, as well as the Stern-Volmer constants for the quenching of the S, states by oxygen and the bimolecular rate constants of quenching of ¹Δ_g by the chlorins were measured. The values of φ_T and φ_Λ can be given as 0.57 and 0.58 for CHLI and 0.69 and 0.58 for CHLII. The values of the fluorescence quantum yields, the strong absorption of the chlorins in the red (Λ > 630 nm) and the high values of the quantum yields for ¹Δ_g formation recommend the chlorin derivatives as potential markers and photosensitizers for tumor therapy.     

PHOTOREDUCTION OF ZINC PORPHIN BY ASCORBIC ACID*

Abstract— Zinc porphin is photoreduced to zinc chlorin through an intermediate dihydroporphin (PH₂) by ascorbic acid in ethanol containing 1% to 10% (v/v) piperidine. Under the same conditions zinc chlorin is more slowly photoreduced to zinc tetrahydroporphin. The reactions leading to chlorin are photosensitized by the product chlorin and so are autocatalytic in red light. Quantum yields for these reactions range up to 0.05.Other aliphatic amines catalyze these reactions, but at rates peculiar to the amine. The immediate product of reduction of zinc porphin, PH₂, is distinguished by an intense band at 437 nm; it reverts to porphin in the dark in the presence of oxygen or dehydroascorbic acid. Its conversion to chlorin is effected by light absorbed by porphin or chlorin, but not by light absorbed by PH₂ itself. A suggested structure for PH₂, compatible with the observed reactions, has added hydrogens on one bridge carbon and one β-pyrrole carbon. The possibility of an analogy between these reactions and the biochemical conversion of protochlorophyll to chlorophyll is discussed.     

Physicochemical Studies of Demetalation of Light-harvesting Bacteriochlorophyll Isomers Purified from Green Sulfur Photosynthetic Bacteria

Demetalation kinetics of bacteriochlorophylls (BChls) c, d and e from green sulfur photosynthetic bacteria were studied under weakly acidic conditions. Demetalation rate constants of BChl e possessing a formyl group at the 7-position were significantly smaller than those of BChls c and d , which had a methyl group at this position. The activation energy of demetalation of 3¹ R -8,12-diethyl([E,E])-BChl e was 1.5-times larger than that of 3¹ R -[E,E]-BChl c . ¹⁵N-labeled 3¹ R -[E,E]-BChls c and e were purified from cells of green sulfur bacteria grown in a medium containing ¹⁵NH₄Cl, and their ¹⁵N NMR spectra were measured. The chemical shifts of N₂₁, N₂₂ and N₂₃ atoms of 3¹ R -[E,E]-BChl e were lower-field shifted than those of 3¹ R -[E,E]-BChl c , respectively, and especially the difference in chemical shifts of N₂₂ was significantly large. These results suggest that the electron-withdrawing formyl group at the 7-position of BChl e affected an electronic state of the chlorin macrocycle and caused BChl e to be more tolerant for removal of the central magnesium compared with BChls c and d .     

MECHANISM OF PHYSICAL QUENCHING OF SINGLET MOLECULAR OXYGEN BY CHLOROPHYLLS and RELATED COMPOUNDS OF BIOLOGICAL INTEREST

Abstract— The rate constant k₅/ > for physical quenching of singlet oxygen O₂(δ₁;) by the sensitizer in dye-sensitized photooxygenations is determined in the case of chlorophylls a and b (7.3 times 10⁸, 4.2 times 10⁸ M^-1 s^-1 respectively), pheophytins a and b (7.4 times 10⁷, 3.0 times 10⁷ M^-1 s^_1 respectively), tetraphenylporphyrin (4.4 times 10⁷ M^-1 s^_1), magnesium tetraphenylporphyrin (5.0 times 10⁸ M^-1 s^_1), zinc tetraphenylporphyrin (1.5 times 10⁸ M^-1 s^_l) and protoporphyrin IX-dimethylester (9.1 times 10⁷ M ^-1 s^_1) in benzene. These sensitizers show a linear correlation between log k_s^O , and their half-wave oxidation potentials and the value of the slope is similar to that observed for various compounds such as phenols. It is concluded that (i) the interaction between chlorophylls and related compounds with singlet oxygen may involve an exciplex as for phenols, and (ii) physical quenching may be envisaged as a spin-orbit-induced intersystem crossing within the exciplex.     

Synthesis of Zinc Chlorophyll Homo/Hetero‐Dyads and their Folded Conformers with Porphyrin,Chlorin, and Bacteriochlorin π‐Systems

Zinc complex of pyropheophorbide‐b, a derivative of chlorophyll‐b, was covalently dimerized through ethylene glycol diester. The synthetic homo‐dyad was axially ligated with two methanol molecules from the β‐face and both the diastereomerically coordinating methanol species were hydrogen bonded with the keto‐carbonyl groups of the neighboring chlorin in a complex. The resulting folded conformer in a solution was confirmed by visible, ¹H NMR and IR spectra. All the synthetic zinc chlorin homo‐ and hetero‐dyads consisting of pyropheophorbides‐a, b and/or d took the above methanol‐locked and π–π stacked supramolecules in 1% (v/v) methanol and benzene to give redmost (Qy) electronic absorption band(s) at longer wavelengths than those of the corresponding monomeric chlorin composites. The other zinc chlorin and bacteriochlorin homo‐dyads completely formed similar folded conformers in the same solution, while zinc inverse chlorin and porphyrin homo‐dyads partially took such supramolecules. The J‐type aggregation to folded conformers and the redshift values of composite Qy bands were dependent on the electronic and steric factors of porphyrinoid moieties in dyads.     

Synthesis and excited-state photodynamics of a chlorin-bacteriochlorin dyad--through-space versus through-bond energy transfer in tetrapyrrole arrays.

Understanding energy transfer among hydroporphyrins is of fundamental interest and essential for a wide variety of photochemical applications. Toward this goal, a synthetic free base ethynylphenylchlorin has been coupled with a synthetic free base bromobacteriochlorin to give a phenylethyne-linked chlorin-bacteriochlorin dyad (FbC-pe-FbB). The chlorin and bacteriochlorin are each stable toward adventitious oxidation because of the presence of a geminal dimethyl group in each reduced pyrrole ring. A combination of static and transient optical spectroscopic studies indicate that excitation into the Qy band of the chlorin constituent (675 nm) of FbC-pe-FbB in toluene results in rapid energy transfer to the bacteriochlorin constituent with a rate of approximately (5 ps)(-1) and efficiency of >99%. The excited bacteriochlorin resulting from the energy-transfer process in FbC-pe-FbB has essentially the same fluorescence characteristics as an isolated monomeric reference compound, namely a narrow (12 nm fwhm) fluorescence emission band at 760 nm and a long-lived (5.4 ns) Qy excited state that exhibits a significant fluorescence quantum yield (Phif=0.19). F?rster calculations are consistent with energy transfer in FbC-pe-FbB occurring predominantly by a through-space mechanism. The energy-transfer characteristics of FbC-pe-FbB are compared with those previously obtained for analogous phenylethyne-linked dyads consisting of two porphyrins or two oxochlorins. The comparisons among the sets of dyads are facilitated by density functional theory calculations that elucidate the molecular-orbital characteristics of the energy donor and acceptor constituents. The electron-density distributions in the frontier molecular orbitals provide insights into the through-bond electronic interactions that can also contribute to the energy-transfer process in the different types of dyads.     

Chlorin–Bacteriochlorin Energy‐transfer Dyads as Prototypes for Near‐infrared Molecular Imaging Probes: Controlling Charge‐transfer and Fluorescence Properties in Polar Media

The photophysical properties of two energy‐transfer dyads that are potential candidates for near‐infrared (NIR) imaging probes are investigated as a function of solvent polarity. The dyads ( FbC‐FbB and ZnC‐FbB ) contain either a free base (Fb) or zinc (Zn) chlorin (C) as the energy donor and a free base bacteriochlorin (B) as the energy acceptor. The dyads were studied in toluene, chlorobenzene, 1,2‐dichlorobenzene, acetone, acetonitrile and dimethylsulfoxide (DMSO). In both dyads, energy transfer from the chlorin to bacteriochlorin occurs with a rate constant of ～(5–10 ps)^?1 and a yield of >99% in nonpolar and polar media. In toluene, the fluorescence yields (Φ _f = 0.19) and singlet excited‐state lifetimes (τ～5.5 ns) are comparable to those of the benchmark bacteriochlorin. The fluorescence yield and excited‐state lifetime decrease as the solvent polarity increases, with quenching by intramolecular electron (or hole) transfer being greater for FbC‐FbB than for ZnC‐FbB in a given solvent. For example, the Φ _f and τ values for FbC‐FbB in acetone are 0.055 and 1.5 ns and in DMSO are 0.019 and 0.28 ns, whereas those for ZnC‐FbB in acetone are 0.12 and 4.5 ns and in DMSO are 0.072 and 2.4 ns. The difference in fluorescence properties of the two dyads in a given polar solvent is due to the relative energies of the lowest energy charge‐transfer states, as assessed by ground‐state redox potentials and supported by molecular‐orbital energies derived from density functional theory calculations. Controlling the extent of excited‐state quenching in polar media will allow the favorable photophysical properties of the chlorin–bacteriochlorin dyads to be exploited in vivo. These properties include very large Stokes shifts (85 nm for FbC‐FbB , 110 nm for ZnC‐FbB ) between the red‐region absorption of the chlorin and the NIR fluorescence of the bacteriochlorin (λ _f = 760 nm), long bacteriochlorin excited‐state lifetime (～5.5 ns), and narrow (≤20 nm) absorption and fluorescence bands. The latter will facilitate selective excitation/detection and multiprobe applications using both intensity‐ and lifetime‐imaging techniques.     

ACTINOMETRY IN MONOCHROMATIC FLASH PHOTOLYSIS: THE EXTINCTION COEFFICIENT OF TRIPLET BENZOPHENONE AND QUANTUM YIELD OF TRIPLET ZINC TETRAPHENYL PORPHYRIN

Abstract— The extinction coefficient ε_T, of triplet benzophenone in benzene has been directly determined by absolute measurements of absorbed energy and triplet absorbance, Δ D ⁰_T, under demonstrably linear conditions where incident excitation energy, E ₀, and ground state absorbance, A ₀, are both extrapolated to zero. The result, 7220 ± 320 M ^-1 cm^-1 at 530 nm, validates and slightly corrects many measurements relative to benzophenone of triplet extinction coefficients made by the energy transfer technique, and of triplet yields obtained by the comparative method.
As E ₀ and A ₀ both decrease, Δ D ⁰_T becomes proportional to their product. In this situation, the ratio R = (1/ A ₀)(dΔ D ⁰_T/d E ₀) = (ε_T - ε_G)φ_T. Measurements of R , referred to benzophenone, give (ε_T - ε_G)φ_T for any substance, without necessity for absolute energy calibration.
Both absolute and relative laser flash measurements on zinc tetraphenyl porphyrin (ε_T - ε_G at 470 nm = 7.3 × 10⁴ M ^-1 cm^-1) give φ_T= 0.83 ± 0.04.     

WAVELENGTH EFFECTS ON THE PHOTOPROCESSES OF INDOLE AND DERIVATIVES IN SOLUTION

Abstract— The two main primary photoprocesses (electron ejection and H-atom release) for indole, 5-methoxyindole and N-methylindole in various polar and nonpolar solvents were studied as a function of the excitation energy and were correlated with the corresponding fluorescence quantum yields. In hydrocarbon solvents, N–H bond cleavage is the main primary photoprocess from the ¹B_b band of the substrates with the exception of N-methylindole. In alcohols, both processes are of negligible importance. Hydrated electrons (e⁻_aq) are ejected from the relaxed singlet states of all three compounds in aqueous solutions with a similar yield for excitation at 280 and 254 nm (¹L_a and ¹L_b states). The yield increases when the excitation is into the ¹B_b band. The quantum yields of the two primary processes from the higher excited states are generally lower than the fraction of molecules not converting to the fluorescent state. This is explained by an efficient back reaction in competition with a thermally activated radical release from an intermediate state or radical pair formed from the S₂ (¹B_b) state. The non-occurrence of a photoionization energy threshold is discussed.     

Self-assembled zinc chlorin rod antennae powered by peripheral light-harvesting chromophores

The multichromophoric dyads 1, 2 and triad 3 have been synthesized by coupling of the appropriately functionalized chlorin derivative with naphthalene diimide dyes through esterification, and subsequent metalation of the chlorin center with zinc acetate. The self-assembly properties of naphthalene diimide (NDI)-zinc chlorin (ZnChl) dyads 1, 2 and triad 3 have been studied in nonpolar, aprotic solvents by UV-vis, CD, and steady-state emission spectroscopy, revealing formation of rod-like structures by noncovalent interactions of zinc chlorin units, while the appended naphthalene diimide dyes do not aggregate at the periphery of the rod antennae. In all these systems, photoexcitation of the enveloping naphthalene diimides at 540 and 620 nm, respectively, leads to highly efficient energy-transfer processes (FRET; phiET > or = 0.99) to the inner zinc chlorin backbone, as explored by time-resolved fluorescence spectroscopy on the picosecond time scale. The efficiencies of zinc chlorin rod aggregates for the harvesting of solar light are markedly increased from 26% for dyad 2 up to 63% for triad 3, compared to the LH capacity of the monochromophoric aggregates of model system ZnChl 6a. Thus, with the self-assembled zinc chlorin rod antenna based on triad 3, a highly efficient artificial LH system has been achieved.     

Photochemical and electrochemical properties of zinc chlorin-C60 dyad as compared to corresponding free-base chlorin-C60, free-base porphyrin-C60, and zinc porphyrin-C60 dyads.

The photochemical and electrochemical properties of four chlorin-C60 or porphyrin-C60 dyads having the same short spacer between the macrocycle and the fullerene are examined. In contrast with all the previous results on porphyrin-fullerene dyads, the photoexcitation of a zinc chlorin-C60 dyad results in an unusually long-lived radical ion pair which decays via first-order kinetics with a decay rate constant of 9.1 x 10(3) x s(-1). This value is 2-6 orders of magnitude smaller than values reported for all other porphyrin or chlorin donor-acceptor of the molecule dyad systems. The formation of radical cations of the donor part and the radical anion of the acceptor part was also confirmed by ESR measurements under photoirradiation at low temperature. The photoexcitation of other dyads (free-base chlorin-C60, zinc porphyrin-C60, and free-base porphyrin-C60 dyads) results in formation of the ion pairs which decay quickly to the triplet excited states of the chlorin or porphyrin moiety via the higher lying radical ion pair states as is expected from the redox potentials.     

QUENCHING OF TRYPTOPHAN PHOSPHORESCENCE IN ALCOHOL DEHYDROGENASE FROM HORSE LIVER and ITS TEMPERATURE DEPENDENCE

Abstract— The phosphorescence of alcohol dehydrogenase from horse liver (LADH) can be observed at room temperature. The quenching of this long-lived light emission, which comes from a tryptophan residue well buried within the interior of the enzyme structure, was measured. The rate constants for the quenching by the small oxygen molecule and by the I ^-1ion were found to be 1.4 → 10⁸ M ^-1 s^-1 and 10⁸ M ^-1 s^-1, respectively, at room temperature. The temperature dependence of the quenching yields an activation energy of about 14 kcal/mol. This activation energy and the meaning of the accompanying large pre-exponential factor in the Arrhenius equation, A = 10¹⁸ M ^-l s^-1, are discussed in terms of a model in which the quencher threads its way through the protein network.     

LUMINESCENCE STUDIES OF QUINIZARIN AND DAUNORUBICIN

Abstract— The absorption and emission spectra of quinizarin (1,4-dihydroxy-anthraquinone) have been investigated in hydrocarbon and alcoholic solvents. Fluorescence spectra in 3 different Shpolskii matrices were recorded at 14 K. Vibrational analyses of these spectra revealed the presence of 3, 8, and 9 sites in octane, heptane, and hexane matrices, respectively. The fluorescence lifetime was found to be 6.5 ns in hexane and EPA. Fluorescence photoselection measurements in EPA (77 K) showed that the first 4 electronic transitions of quinizarin are polarized parallel, parallel, perpendicular, and parallel to the long molecular axis and can be assigned, in order of increasing energy, to ¹B₂, ¹B₂, ¹A₁ and ¹B₂ (ππ*)→¹A₁(C_2v,) transitions, respectively. The fluorescent transition is assigned as ¹B₁ (ππ*)→¹A¹. The absence of phosphorescence is attributed to the intramolecular hydrogen bonding present which displaces the parent anthraquinone n →* states above the ππ* states, thereby rendering the intersystem crossing (S₁-T₁) radiationless pathway inefficient.
Photoselection measurements on daunorubicin, a substituted quinizarin and known anticancer drug, revealed an absorption band polarization pattern identical to that of quinizarin. These results are in part at variance with assumptions used in previous work on the intercalation specificity of daunorubicin with DNA.     

THE CHROMOPHORES AS ENDOGENOUS SENSITIZERS INVOLVED IN THE PHOTOGENERATION OF SINGLET OXYGEN IN SPINACH THYLAKOIDS

Abstract— The photogeneration of singlet oxygen (¹O₂) from thylakoids and the chromophores involved as endogenous sensitizers were investigated using chloroplasts and thylakoids isolated from spinach. The blue light-induced inhibition kinetics of photosynthetic electron transport and that of CTvCF, ATPase were also studied. The spectral dependence of the generation of ¹O₂ from thylakoid membranes, measured by the imidazole plus RNO method, clearly demonstrated that the Fe-S centers play an important role in ¹O₂ generation, acting as sensitizers in thylakoids. The photoinhibition of the electron transport in isolated chloroplasts was strikingly depressed by a lipid-soluble '0₂ quencher and enhanced by deuterium oxide substitution, indicating that the inhibition processes are mainly mediated by ¹O₂ which is produced via photodynamic activation. The involvement of chloroplast cytochromes in the production of ¹O₂ was deduced from the action spectrum for the photodynamic inhibition of the electron carrier chain. The results obtained from the kinetic studies appear consistent with the involvement of some components such as the Fe-S centers and cytochrome chromophores of the carrier chain in the generation of ¹O₂.