Chlorin Ring Research Articles

Conservation of Lipid Binding Sites in Cytochrome bc Complexes1

Cytochrome bc complexes present in the energy transducing membranes of respiration and photosynthesis share structural and functional homology. By comparing the ten lipid, detergent and pigment positions in the crystal structures of the dimeric photosynthetic cytochrome b6f complex from 2 cyanobacteria2,3 and an alga4, obtained in the presence of synthetic and natural lipids, with the 13 natural lipid and detergent positions in the yeast cytochrome bc1 dimer5, an extensive conservation of lipid sites has been deduced. Seven lipid and detergent sites in the cyanobacterial b6f complex overlap three natural lipid sites in the algal complex, and four sites in the respiratory bc1 complex. The lipids occupying these sites differ between b6f and bc1: the crystal structures of b6f2-4 contain natural sulfolipid and monogalactosyldiacylglycerol, and synthetic phosphatidylcholine whereas the respiratory bc1 complex5 contains cardiolipin, phosphatidic acid and phosphatidyl-ethanolamine. Phosphatidyl-glycerol and digalactosyldiacylglycerol have been identified in the b6f complex by mass spectrometry. The lipid-like chlorophyll-a, β-carotene and eicosane are unique to b6f. Based on structural superposition of b6f and bc1, sequence conservation, analyses of inter-molecular distances, and amino acid and lipid B-factors, the following lipid functions are inferred in b6f: (i) substitution of bc1 trans-membrane helix (TMH) H by a lipid and chlorophyll chlorin ring in b6f; (ii) lipid and β-carotene mediated connection of trans-membrane peripheral and core domains; (iii) stabilization of the iron-sulfur protein TMH, perhaps relevant to catalytic domain motion; (iv) n-side charge and polarity compensation; (v) β-carotene-mediated super-complex with photosystem I reaction center complex. A physiological function for conserved Pet G and N6 subunits is proposed. 1Hasan et al., JMB (in press); 2Yamshita et al., JMB 2007; 3Baniulis et al., JBC 2009; 4Stroebel et al., Nature 2003; 5Solmaz and Hunte, JBC 2008; 6Schneider et al., Res. Microb. 2007; Funding:NIH-GM038323

Computational Studies on Structural and Excited-State Properties of Modified Chlorophyll f with Various Axial Ligands

Time-dependent density functional theory (TDDFT) calculations have been used to understand the excited-state properties of modified chlorophyll f (Chlide f), Chlide a, Chlide b, and axial ligated (with imidazole, H(2)O, CH(3)OH, CH(3)COOH, C(6)H(5)OH) Chlide f molecules. The computed differences among the Q(x), Q(y), B(x), and B(y) band absorbance wavelengths of Chlide a, b, and f molecules are found to be comparable with the experimentally observed shifts for these bands in chlorophyll a (chl a), chl b, and chl f molecules. Our computations provide evidence that the red shift in the Q(y) band of chl f is due to the extended delocalization of macrocycle chlorin ring because of the presence of the -CHO group. The local contribution from the -CHO substituent to the macrocycle chlorin ring stabilizes the corresponding molecular orbitals (lowest unoccupied molecular orbital (LUMO) of the Chlide f and LUMO-1 of the Chlide b). All the absorption bands of Chlide f shift to higher wavelengths on the addition of axial ligands. Computed redox potentials show that, among the axial ligated Chlide f molecules, Chlide f-imidazole acts as a good electron donor and Chlide f-CH(3)COOH acts as a good electron acceptor.

Conservation of Lipid Functions in Cytochrome bc Complexes

Lipid binding sites and properties are compared in two sub-families of hetero-oligomeric membrane protein complexes known to have similar functions in order to gain further understanding of the role of lipid in the function, dynamics, and assembly of these complexes. Using the crystal structure information for both complexes, we compared the lipid binding properties of the cytochrome b 6 f and bc 1 complexes that function in photosynthetic and respiratory membrane energy transduction. Comparison of lipid and detergent binding sites in the b 6 f complex with those in bc 1 shows significant conservation of lipid positions. Seven lipid binding sites in the cyanobacterial b 6 f complex overlap three natural sites in the Chlamydomonas reinhardtii algal complex and four sites in the yeast mitochondrial bc 1 complex. The specific identity of lipids is different in b 6 f and bc 1 complexes: b 6 f contains sulfoquinovosyldiacylglycerol, phosphatidylglycerol, phosphatidylcholine, monogalactosyldiacylglycerol, and digalactosyldiacylglycerol, whereas cardiolipin, phosphatidylethanolamine, and phosphatidic acid are present in the yeast bc 1 complex. The lipidic chlorophyll a and β-carotene (β-car) in cyanobacterial b 6 f, as well as eicosane in C. reinhardtii, are unique to the b 6 f complex. Inferences of lipid binding sites and functions were supported by sequence, interatomic distance, and B-factor information on interacting lipid groups and coordinating amino acid residues. The lipid functions inferred in the b 6 f complex are as follows: (i) substitution of a transmembrane helix by a lipid and chlorin ring, (ii) lipid and β-car connection of peripheral and core domains, (iii) stabilization of the iron–sulfur protein transmembrane helix, (iv) n-side charge and polarity compensation, and (v) β-car-mediated super-complex with the photosystem I complex.

Reaction of methylpheophorbides d and b with amines

A series of formyl analogs of chlorin e 6 13-amides were synthesized in high yields by reaction under mild conditions of primary and secondary amines with methylpheophorides b and d. In contrast with the secondary 13-amides, tertiary 13-amides were found as two isomers differing in the orientation of the amide plane relative to the plane of the chlorin ring. Methylpheophorbides b and d were more reactive toward the amines than methylpheophorbide a.

Molecular Dynamics Simulation Study of Chlorophyll a in Different Organic Solvents.

Herein, we present a new model of chlorophyll a for molecular dynamics simulations based on the optimized potentials for liquid simulations force field. The new model was used to study the structural and dynamic properties of the molecule in three different solvents: water, methanol, and benzene. The results of the simulations show that structural and dynamic properties of the chlorin ring are similar in both methanol and benzene. In methanol and water, the magnesium in the chlorin ring binds the oxygen of the solvent molecules with residence times of 2566 and 1300 ps, respectively. In both methanol and benzene, the phytol tail shows a worm-like chain distribution with a larger persistence length for the molecule in benzene. On the contrary, chlorophyll a in water adopts a more compact structure with the phytol chain folded onto the chlorin ring. This conformation is consistent with the expected conformation of the aggregates of chlorophyll a in aqueous environments. Finally, the rotational time constants obtained with our model from the simulations in methanol (125 ps) and benzene (192 ps) are in good agreement with the value extrapolated from the experimental data.

Direct measurement of singlet oxygen produced by four chlorin-ringed chlorophyll species in acetone solution

We developed an optical system to detect singlet oxygen produced by chlorin-ringed chlorophyll (Chl) species, i.e., Chl a, Chl b, Chl d, and di-vinyl-Chl a, with a high sensitivity and examined the relationship between molecular structures and reaction rates. Chl a in acetone was the lowest producer of singlet oxygen and the most effective quencher; in contrast, Chl b exhibited the opposite properties. These results showed that replacement of side chain(s) from a methyl group to a formyl group on the R7 position on a chlorin ring induced a higher production and lower quenching of singlet oxygen in Chl molecules.

β-Keto ester aminolysis of pheophorbide a methyl ester: a facile route for asymmetric chlorin ring substitution

A chemoselective aminolysis of the β-keto ester of pheophorbide a methyl ester is demonstrated opening a facile access to an asymmetric amide functionalization of a chlorin ring using a range of aromatic and aliphatic, primary and secondary amine nucleophiles. Aminolysis of pheophorbide a methyl ester with trans-1,2-diaminocyclohexane is shown to give a symmetric open face chlorin dimer.

Zinc chlorins for artificial light-harvesting self-assemble into antiparallel stacks forming a microcrystalline solid-state material

We introduce a concept to solve the structure of a microcrystalline material in the solid-state at natural abundance without access to distance constraints, using magic angle spinning (MAS) NMR spectroscopy in conjunction with X-ray powder diffraction and DFT calculations. The method is applied to a novel class of materials that form (semi)conductive 1D wires for supramolecular electronics and artificial light-harvesting. The zinc chlorins 3-devinyl-3(1)-hydroxymethyl-13(2)-demethoxycarbonylpheophorbide a (3',5'-bis-dodecyloxy)benzyl ester zinc complex 1 and 3-devinyl-3(1)-methoxymethyl-13(2)-demethoxycarbonylpheophorbide a (3',5'-bis-dodecyloxy)benzyl ester zinc complex 2, self-assemble into extended excitonically coupled chromophore stacks. (1)H-(13)C heteronuclear dipolar correlation MAS NMR experiments provided the (1)H resonance assignment of the chlorin rings that allowed accurate probing of ring currents related to the stacking of macrocycles. DFT ring-current shift calculations revealed that both chlorins self-assemble in antiparallel pi-stacks in planar layers in the solid-state. Concomitantly, X-ray powder diffraction measurements for chlorin 2 at 80 degrees C revealed a 3D lattice for the mesoscale packing that matches molecular mechanics optimized aggregate models. For chlorin 2 the stacks alternate with a periodicity of 0.68 nm and a 3D unit cell with an approximate volume of 6.28 nm(3) containing 4 molecules, which is consistent with space group P2(1)22(1).

Excited states of photobiological receptors. II. Chlorophylls, phytochrome, and stentorin

In this series on the application of configuration analysis (CA) to the electronic excited states of photobiological receptors, we have extended CA to two important photobiological receptors, phytochrome of photomorphogenic and biological clock systems and stentorin of the photophobic response system of stentors. In addition, the effect of substituents in chlorophylls on the electronic excitation of the chlorin and bacteriochlorin ring systems has been examined. The open-shell calculations of chlorophylls have also been performed. In the singlet excited state of phytochrome, there is a significant charge transfer from ring A to the violin moiety of the phytochrome molecule, with a tendency for the localized excitation in the A ring. The charge transfer contribution to the singlet-singlet excitation is also significant. These results are correlated with the photomorphogenic transformation of phytochrome which involves deprotonation and/or tautomerism of the ring A in the excited state. In the case of stentorin, the excitation resides largely in the aromatic system, with some charge transfer to the carbonyl groups. The photobiological significance of these results are discussed.

Chlorobaculum tepidum regulates chlorosome structure and function in response to temperature and electron donor availability

Green sulfur bacteria (GSB) rely on the chlorosome, a light-harvesting apparatus comprised almost entirely of self-organizing arrays of bacteriochlorophyll (BChl) molecules, to harvest light energy and pass it to the reaction center. In Chlorobaculum tepidum, over 97% of the total BChl is made up of a mixture of four BChl c homologs in the chlorosome that differ in the number and identity of alkyl side chains attached to the chlorin ring. C. tepidum has been reported to vary the distribution of BChl c homologs with growth light intensity, with the highest degree of BChl c alkylation observed under low-light conditions. Here, we provide evidence that this functional response at the level of the chlorosome can be induced not only by light intensity, but also by temperature and a mutation that prevents phototrophic thiosulfate oxidation. Furthermore, we show that in conjunction with these functional adjustments, the fraction of cellular volume occupied by chlorosomes was altered in response to environmental conditions that perturb the balance between energy absorbed by the light-harvesting apparatus and energy utilized by downstream metabolic reactions.

Photophysical properties of a photocytotoxic fluorinated chlorin conjugated to four β-cyclodextrins

A meso-tetrakis(pentafluorophenyl)-chlorin with the reduced pyrrole ring linked to an isoxazolidine ring (FC) has been conjugated to four beta-cyclodextrins (CDFC). The CDFC exhibits excellent water solubility and is a potent photosensitizer towards proliferating NCTC 2544 human keratinocytes. The study by conventional steady state absorption and fluorescence spectroscopies and by time-resolved femto- and nanosecond laser flash spectroscopies suggests that in ethanol and pH 7 buffer the beta-cyclodextrins embed the highly hydrophobic tetrakis(pentafluorophenyl)-chlorin macrocycle and strongly interact with the chlorin rings in the singlet and triplet manifolds. In these solvents, femtosecond spectroscopy suggests that the conjugate undergoes a rapid relaxation in the upper excited singlet states induced by photochemical and/or conformation change(s) at a rate of about 5 ps(-1) to fluorescent states whose lifetime is approximately 8 ns. This interaction is destroyed upon addition of Triton X100 to buffer. Both FC and CDFC strongly fluoresce (Phi(F) approximately 0.5) in micelles. Similar behavior is observed at the triplet level. In ethanol and water, the initial transient triplet state absorbance decays within 1-3 mus yielding a longer lived triplet with spectral properties indistinguishable from that of original difference absorbance spectra. The determination of the molar absorbance in the 440-460 nm region ( approximately 35 000 M(-1) cm(-1)) leads to an estimate of approximately 0.2 for the triplet formation quantum yield of FC in toluene and of FC and CDFC in Triton X100 micelles. Quenching of the CDFC triplets by dioxygen in buffer produces (1)O(2) in a good yield consistent with the effective photocytotoxicity of the chlorin-cyclodextrins conjugate towards cultured NCTC 2544 human keratinocytes. By contrast, FC which aggregates in buffer produces little if any (1)O(2).

Hexanol-Induced Order−Disorder Transitions in Lamellar Self-Assembling Aggregates of Bacteriochlorophyll c in Chlorobium tepidum Chlorosomes

Chlorosomes are light-harvesting complexes of green photosynthetic bacteria. Chlorosomes contain bacteriochlorophyll (BChl) c, d, or e aggregates that exhibit strong excitonic coupling. The short-range order, which is responsible for the coupling, has been proposed to be augmented by pigment arrangement into undulated lamellar structures with spacing between 2 and 3 nm. Treatment of chlorosomes with hexanol reversibly converts the aggregated chlorosome chlorophylls into a form with spectral properties very similar to that of the monomer. Although this transition has been extensively studied, the structural basis remains unclear due to variability in the obtained morphologies. Here we investigated hexanol-induced structural changes in the lamellar organization of BChl c in chlorosomes from Chlorobium tepidum by a combination of X-ray scattering, electron cryomicroscopy, and optical spectroscopy. At a low hexanol/pigment ratio, the lamellae persisted in the presence of hexanol while the short-range order and exciton interactions between chlorin rings were effectively eliminated, producing a monomer-like absorption. The result suggested that hexanol hydroxyls solvated the chlorin rings while the aliphatic tail partitioned into the hydrophobic part of the lamellar structure. This partitioning extended the chlorosome along its long axis. Further increase of the hexanol/pigment ratio produced round pigment-hexanol droplets, which lost all lamellar order. After hexanol removal the spectral properties were restored. In the samples treated under the high hexanol/pigment ratio, lamellae reassembled in small domains after hexanol removal while the shape and long-range order were irreversibly lost. Thus, all the interactions required for establishing the short-range order by self-assembly are provided by BChl c molecules alone. However, the long-range order and overall shape are imposed by an external structure, e.g., the proteinaceous chlorosome baseplate.

Pigment−Pigment and Pigment−Protein Interactions in Recombinant Water-Soluble Chlorophyll Proteins (WSCP) from Cauliflower

Plants contain water-soluble chlorophyll-binding proteins (WSCPs) that function neither as antennas nor as components of light-induced electron transfer of photosynthesis but are likely constituents of regulatory protective pathways in particular under stress conditions. This study presents results on the spectroscopic properties of recombinant WSCP from cauliflower reconstituted with chlorophyll b (Chl b) alone or with mixtures of Chl a and Chl b. Two types of experiments were performed: (a) measurements of stationary absorption spectra at 77 and 298 K and CD spectra at 298 K and (b) monitoring of laser flash-induced transient absorption changes with a resolution of 200 fs in the time domain of up to 100 ps. On the basis of a theoretical analysis outlined by Renger et al. (J. Phys. Chem. B 2007, 111, 10487) the data obtained in part (a) are interpreted within a model where tetrameric WSCP binds predominantly two Chl molecules in the form of an excitonically coupled "open sandwich" dimer with a tilt angle of about 30 degrees between the chlorin planes. The time-resolved measurements on Chl a/Chl b heterodimers are described by two exponential kinetics with time constants of 400 fs and 7 ps. These kinetics are assumed to reflect a heterogeneous population of WSCPs with Chl dimers either in excitonic coupled "open sandwich" or weakly coupled geometric arrays. The 400 fs component is assigned to excited-state relaxations from the upper to the lower excitonic level of the strongly coupled "open sandwich" dimer, while the 7-8 ps component probably indicates excitation energy transfer from 1Chl b* to Chl a in a dimer array with weak coupling due to significantly longer mutual distances between the chlorin rings.

Perturbation of the Structure of P680 and the Charge Distribution on Its Radical Cation in Isolated Reaction Center Complexes of Photosystem II as Revealed by Fourier Transform Infrared Spectroscopy

The structure and the electronic properties of P680 and its radical cation in photosystem II (PSII) were studied by means of Fourier transform infrared spectroscopy (FTIR). Light-induced P680+/P680 FTIR difference spectra in the mid- and near-IR regions were measured using PSII membranes from spinach, core complexes from Thermosynechococcus elongatus, and reaction center (RC) complexes (D1-D2-Cytb559) from spinach. The spectral features of the former two preparations were very similar, indicating that the structures of P680 and its radical cation are virtually identical between membranes and cores and between plants and cyanobacteria. In sharp contrast, the spectrum of the RC complexes exhibited significantly different features. A positive doublet at approximately 1724 and approximately 1710 cm-1 due to the 131-keto C=O stretches of P680+ in the membrane and core preparations were changed to a prominent single peak at 1712 cm-1 in the RC complexes. This observation was interpreted to indicate that a positive charge on P680+ was extensively delocalized over the chlorophyll dimer in RC, whereas it was mostly localized on one chlorophyll molecule (70-80%) in intact P680. The significant change in the electronic structure of P680+ in RC was supported by a dramatic change in the characteristics of a broad intervalence band in the near-IR region and relatively large shifts of chlorin ring bands. It is proposed that the extensive charge delocalization in P680+ mainly causes the decrease in the redox potential of P680+/P680 in isolated RC complexes. This potential decrease explains the well-known phenomenon that YZ is not oxidized by P680+ in RC complexes.

Synthesis of chlorins with a distal vinyl group

A series of chlorins containing a vinyl group on the periphery of the chlorin ring that was attached by linkers of various length, potential monomers for synthesis of polymers containing chlorin via copolymerization, was synthesized from methylpheophorbide a.

Function of two β-carotenes near the D1 and D2 proteins in photosystem II dimers

The antenna proteins in photosystem II (PSII) not only promote energy transfer to the photosynthetic reaction center (RC) but provide also an efficient cation sink to re-reduce chlorophyll a if the electron transfer (ET) from the Mn-cluster is inhibited. Using the newest PSII dimer crystal structure (3.0 Å resolution), in which 11 β-carotene molecules (Car) and 14 lipids are visible in the PSII monomer, we calculated the redox potentials ( E m) of one-electron oxidation for all Car ( E m(Car)) by solving the Poisson–Boltzmann equation. In each PSII monomer, the D1 protein harbors a previously unlocated Car (Car D1) in van der Waals contact with the chlorin ring of Chl Z(D1). Each Car D1 in the PSII dimer complex is located in the interface between the D1 and CP47 subunits, together with another four Car of the other PSII monomer and several lipid molecules. The proximity of Car bridging between Car D1 and plastoquinone/Q A may imply a direct charge recombination of Car +Q A −. The calculated E m(Car D1) and E m(Chl Z(D1)) are, respectively, 83 and 126 mV higher than E m(Car D2) and E m(Chl Z(D2)), which could explain why Car D2 + and Chl Z(D2) + are observed rather than the corresponding Car D1 + and Chl Z(D1) +.

A Second Nitrogenase-like Enzyme for Bacteriochlorophyll Biosynthesis: RECONSTITUTION OF CHLOROPHYLLIDE a REDUCTASE WITH PURIFIED X-PROTEIN (BchX) AND YZ-PROTEIN (BchY-BchZ) FROM RHODOBACTER CAPSULATUS

In most photosynthetic organisms, the chlorin ring structure of chlorophyll a is formed by the reduction of the porphyrin D-ring by the dark-operative nitrogenase-like enzyme, protochlorophyllide reductase (DPOR). Subsequently, the chlorin B-ring is reduced in bacteriochlorophyll biosynthesis to form a bacteriochlorin ring structure. Phenotypic analysis of mutants lacking one of three genes, bchX, bchY, or bchZ, which show significant sequence similarity to the structural genes of nitrogenase, suggests that a second nitrogenase-like enzyme is involved in the chlorin B-ring reduction. However, there is no biochemical evidence for this. Here, we report the reconstitution of chlorophyllide a reductase (COR) with purified proteins. Two Rhodobacter capsulatus strains that overexpressed Strep-tagged BchX and BchY were isolated. Strep-tagged BchX was purified as a single polypeptide, and BchZ was co-purified with Strep-tagged BchY. When BchX and BchY-BchZ components were incubated with chlorophyllide a, ATP, and dithionite under anaerobic conditions, chlorophyllide a was converted to a new pigment with a Qy band of longer wavelength at 734 nm (P734) in 80% acetone. The formation of P734 was dependent on ATP and dithionite. High performance liquid chromatography and mass spectroscopic analysis indicated that P734 is 3-vinyl bacteriochlorophyllide a, which is formed by the B-ring reduction of chlorophyllide a. These results demonstrate that the B-ring of chlorin is reduced by a second nitrogenase-like enzyme and that the sequential actions of two nitrogenase-like enzymes, DPOR and COR, convert porphyrin to bacteriochlorin. The evolutionary implications of nitrogenase-like enzymes to determine the ring structure of (bacterio)chlorophyll pigments are discussed.

In vitro activity of C-20 methyltransferase, BchU, involved in bacteriochlorophyll c biosynthetic pathway in green sulfur bacteria

The activity of a methyltransferase, BchU, which catalyzes methylation at the C-20 position of chlorin ring in the biosynthetic pathway of bacteriochlorophyll c, was investigated in vitro. The bchU gene derived from the photosynthetic green sulfur bacterium, Chlorobium tepidum, was overexpressed in Escherichia coli as a His-tagged protein (His 6-BchU), and the enzyme was purified. In the presence of S-adenosylmethionine, His 6-BchU methylated zinc bacteriopheophorbide d at the C-20 position to give zinc bacteriopheophorbide c. Metal-free bacteriopheophorbide d could not be methylated by the BchU, indicating that the central metal in the chlorin should be required for the recognition by the BchU.

NMR and EPR Studies of the Bis(pyridine) and Bis(tert-butyl isocyanide) Complexes of Iron(III) Octaethylchlorin

The NMR and EPR spectra of a series of pyridine complexes [(OEC)Fe(L)2]+ (L = 4-Me2NPy, Py, and 4-CNPy) have been investigated. The EPR spectra at 4.2 K suggest that, with a decrease of the donor strength of the axial ligands, the complexes change their ground state from (d(xy))2 (d(xz)d(yz))3 to (d(xz)d(yz))4 (d(xy))1. The NMR data from 303 to 183 K show that at any temperature within this range the chemical shifts of pyrrole-8,17-CH2 protons increase with a decrease in the donor strength of the axial ligands. The full peak assignments of the [(OEC)Fe(L)2]+ complexes of this study have been made from COSY and NOE difference experiments. The pyrrole-8,17-CH2 and pyrroline protons show large chemical shifts (hence indicating large pi spin density on the adjacent carbons which are part of the pi system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts, as predicted by the spin densities obtained from molecular orbital calculations, both Hückel and DFT; the DFT calculations additionally show close energy spacing of the highest five filled orbitals (of the Fe(II) complex) and strong mixing of metal and chlorin character in these orbitals that is sensitive to the donor strength of the axial substituents. The pattern of chemical shifts of the pyrrole-CH2 protons of [(OEC)Fe(t-BuNC)2]+ looks somewhat like that of [(OEC)Fe(4-Me2NPy)2]+, while the chemical shifts of the meso-protons are qualitatively similar to those of [(OEP)Fe(t-BuNC)2]+. The temperature dependence of the chemical shifts of [(OEC)Fe(t-BuNC)2]+ shows that it has a mixed (d(xz)d(yz))4 (d(xy))1 and (d(xy))2 (d(xz),d(yz))3 electron configuration that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with a S = 5/2 excited state that lies somewhat more than 2kT at room temperature above the ground state; the observed pattern of chemical shifts is the approximate average of those expected for the two S = 1/2 electronic configurations, which involve the a-symmetry SOMO of a planar chlorin ring with the unpaired electron predominantly in the d(yz) orbital and the b-symmetry SOMO of a ruffled chlorin ring with the unpaired electron predominantly in the d(xy) orbital. A rapid interconversion between the two, with calculated vibrational frequency of 22 cm(-1), explains the observed pattern of chemical shifts, while a favoring of the ruffled conformation explains the negative chemical shift (and thus the negative spin density at the alpha-pyrroline ring carbons), of the pyrroline-H of [TPCFe(t-BuNC)2]CF3SO3 (Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001, 1587-1592). Peak assignments for high-spin (OEC)FeCl have been made by saturation transfer techniques that depend on chemical exchange between this complex and its bis-4-Me2NPy adduct. The contact shifts of the pyrrole-CH2 and meso protons of the high-spin complex depend on both sigma and pi spin delocalization due to contributions from three of the occupied frontier orbitals of the chlorin ring.

Chlorophyll Chemiluminescence Emitted by Chlorin Ring Cleavage with Acetonitrile and Hydrogen Peroxide

Abstract In the presence of acetonitrile and NaOH, when H2O2 was added to the solution containing chlorophyll, it was found that a strong chemiluminescence (CL) was emitted. The blue light was emitted with the decomposition of chlorophyll. It is considered that the emitter contains the decomposition products of chlorophyll based on the following measurement results, such as the reduction of absorbance for chlorophyll and the difference of the spectral form between fluorescence and CL. The CL of the chlorines, such as chlorophyll, and the porphyrin analogue compounds were compared under the same conditions. Consequently, the obtained CL was strong in the chlorin complex which contains Mg or Zn. On the other hand, the CL was not strongly observed with porphyrin compounds. In the chlorophyll CL, when the chlorin ring is oxidized by the generated •O2−, the excited molecule would be produced in the process of ring cleavage, and then CL is emitted to the outside.