Cyclic Tetrapyrroles Research Articles

Corrinoid salvaging and cobamide remodeling in bacteria and archaea.

Cobamides (Cbas) are cobalt-containing cyclic tetrapyrroles used by cells from all domains of life as co-catalyst of diverse reactions. There are several structural features that distinguish Cbas from one another. The most relevant of those features discussed in this review is the lower ligand, which is the nucleobase of a ribotide located in the lower face of the cyclic tetrapyrrole ring. The above-mentioned ribotide is known as the nucleotide loop, which is attached to the ring by a short linker. In Cbas, the nucleobase of the ribotide can be benzimidazole or derivatives of it, purine or derivatives of it, or phenolic compounds. Given the importance of Cbas in prokaryotic metabolism, it is not surprising that prokaryotes have evolved enzymes that cleave part or the entire nucleotide loop. This function is advantageous when Cbas contain nucleobases that somehow interfere with the function of Cba-dependent enzymes in the organism. After cleavage, Cbas are rebuilt via the nucleotide loop assembly (NLA) pathway, which includes enzymes that activate the nucleobase and the ring intermediate, followed by condensation of activated intermediates and a final dephosphorylation reaction. This exchange of nucleobases is known as Cba remodeling. The NLA pathway is used to salvage Cba precursors from the environment.

Matrix Selection Strategies for MALDI-TOF MS/MS Characterization of Cyclic Tetrapyrroles in Blood and Food Samples.

Cyclic tetrapyrrole derivatives such as porphyrins, chlorins, corrins (compounds with a corrin core), and phthalocyanines are a family of molecules containing four pyrrole rings usually coordinating a metal ion (Mg, Cu, Fe, Zn, etc.). Here, we report the characterization of some representative cyclic tetrapyrrole derivatives by MALDI-ToF/ToF MS analyses, including heme b and c, phthalocyanines, and protoporphyrins after proper matrix selection. Both neutral and acidic matrices were evaluated to assess potential demetallation, adduct formation, and fragmentation. While chlorophylls exhibited magnesium demetallation in acidic matrices, cyclic tetrapyrroles with Fe, Zn, Co, Cu, or Ni remained steadfast against demetallation across all conditions. Phthalocyanines and protoporphyrins were also detectable without a matrix using laser desorption ionization (LDI); however, the incorporation of matrices achieved the highest ionization yield, enhanced sensitivity, and negligible fragmentation. Three standard proteins, i.e., myoglobin, hemoglobin, and cytochrome c, were analyzed either intact or enzymatically digested, yielding heme b and heme c ions along with accompanying peptides. Furthermore, we successfully detected and characterized heme b in real samples, including blood, bovine and cod liver, and mussel. As a result, MALDI MS/MS emerged as a powerful tool for straightforward cyclic tetrapyrrole identification, even in highly complex samples. Our work paves the way for a more comprehensive understanding of cyclic tetrapyrroles in biological and industrial settings, including the geochemical field, as these compounds are a source of significant geological and geochemical information in sediments and crude oils.

The Molecular Evolution, Structure, and Function of Coproporphyrinogen Oxidase and Protoporphyrinogen Oxidase in Prokaryotes.

Coproporphyrinogen oxidase (CgoX) and protoporphyrinogen oxidase (PgoX) catalyze the oxidation of the flexible cyclic tetrapyrrole of porphyrinogen compounds into fully conjugated, planar macrocyclic porphyrin compounds during heme biosynthesis. These enzymes are activated via different pathways. CgoX oxidizes coproporphyrinogen III to coproporphyrin III in the coproporphyrin-dependent pathway, whereas PgoX oxidizes protoporphyrinogen IX to protoporphyrin IX in the penultimate step of the protoporphyrin-dependent pathway. The phylogenetic analysis presented herein demonstrates a clear differentiation between the two enzyme classes, as evidenced by the clustering of sequences in distinct clades, and it shows that, at the origin of porphyrinogen-type oxidase evolution, PgoXs from cyanobacteria were found, which were noticeably separated from descendant PgoX representatives of Deltaproteobacteria and all later PgoX variants, leading to many eukaryotic clades. CgoX sequences originating from the monoderm Actinomycetota and Bacillota were well separated from the predecessor clades containing PgoX types and represent a peculiar type of gene speciation. The structural similarities and differences between these two oxidases are discussed based on their protein sequence alignment and a structural comparison.

Synthesis of C3-substituted chlorophyll-a derivatives fused by a π-conjugated E-ring and their physical properties with unique near-infrared absorption

Chlorophylls (Chls) are one of the most important pigments for the light-harvesting systems in photosynthesis. Methyl pheophorbide-a derivative fused by a π-conjugated E-ring with the C131=C132 double bond was found as a synthetic example of the unique Chl-a derivatives, which showed split Soret absorption bands in the purple and ultraviolet-A regions and a broad absorption band at a near-infrared (NIR) region as its monomeric state, but not its self-aggregated species. The 3-vinyl group of the Chl-a derivative was chemically transformed into ethyl, formyl, hydroxymethyl, and dialkoxymethyl groups. All the resulting chlorins exhibited unique wide bands at a NIR region. The substitution effect on the peaks of their wide NIR bands was similar to that on the sharp redmost Qy maxima at a visible region of the corresponding pheophorbides with the C131–C132 single bond. Protons at the C5-, C10-, and C20-positions as well as of the peripheral C2-, C3-, C7-, and C8-substituents of methyl 3-substituted 131,132-didehydro-131-deoxo-pheophorbides-a were upfield-shifted in 1H nuclear magnetic resonance, compared with those of intact pheophorbides. The shifts were ascribed to the less ring current of the cyclic tetrapyrroles by π-conjugation of the E-ring. The non-fluorescent C3-substituted Chl-a derivatives could be useful as new materials absorbing NIR wavelength light.

Dissecting the conformational complexity and mechanism of a bacterial heme transporter

Iron-bound cyclic tetrapyrroles (hemes) are redox-active cofactors in bioenergetic enzymes. However, the mechanisms of heme transport and insertion into respiratory chain complexes remain unclear. Here, we used cellular, biochemical, structural and computational methods to characterize the structure and function of the heterodimeric bacterial ABC transporter CydDC. We provide multi-level evidence that CydDC is a heme transporter required for functional maturation of cytochrome bd, a pharmaceutically relevant drug target. Our systematic single-particle cryogenic-electron microscopy approach combined with atomistic molecular dynamics simulations provides detailed insight into the conformational landscape of CydDC during substrate binding and occlusion. Our simulations reveal that heme binds laterally from the membrane space to the transmembrane region of CydDC, enabled by a highly asymmetrical inward-facing CydDC conformation. During the binding process, heme propionates interact with positively charged residues on the surface and later in the substrate-binding pocket of the transporter, causing the heme orientation to rotate 180°.

Circadian Genes Expression Patterns in Disorders Due to Enzyme Deficiencies in the Heme Biosynthetic Pathway.

Heme is a member of the porphyrins family of cyclic tetrapyrroles and influences various cell processes and signalling pathways. Enzyme deficiencies in the heme biosynthetic pathway provoke rare human inherited metabolic diseases called porphyrias. Protein levels and activity of enzymes involved in the heme biosynthetic pathway and especially 5'-Aminolevulinate Synthase 1 are featured by 24-h rhythmic oscillations driven by the biological clock. Heme biosynthesis and circadian pathways intermingle with mutual modulatory roles. Notably, heme is a ligand of important cogs of the molecular clockwork, which upon heme binding recruit co-repressors and inhibit the transcription of numerous genes enriching metabolic pathways and encoding functional proteins bringing on crucial cell processes. Herein, we assessed mRNA levels of circadian genes in patients suffering from porphyrias and found several modifications of core clock genes and clock-controlled genes expression, associated with metabolic and electrolytic changes. Overall, our results show an altered expression of circadian genes accompanying heme biosynthesis disorders and confirm the need to deepen the knowledge of the mechanisms through which the alteration of the circadian clock circuitry could take part in determining signs and symptoms of porphyria patients and then again could represent a target for innovative therapeutic strategies.

DFT study about capturing of toxic sulfur gases over cyclic tetrapyrrole

The development of sensors that can detect hazardous analytes selectively and accurately, particularly sulfur based irritants, is quite essential. The infinite-conjugation in cyclic conducting polymers make them highly sensitive to toxic analytes. We implemented B3LYP-D3/6–311 + G (d, p) level to explore the sensing mechanism of cyclic tetrapyrrole (CTPy) for reliable detection of carbonyl sulfide, carbon disulfide, hydrogen sulfide sulfur monoxide, sulfur dioxide and sulfur trioxide using the DFT practice. The interaction energies, Eint ranges from –25.6 to –89.2 kJ mol−1 for sulfur gases over CTPy. Charge transfer interactions in complexes are predicted using natural bond orbital (NBO) and charge decomposition (CDA) analysis. The reduced density gradient (RDG) method supports hydrogen bonding and dispersion interactions in the complexes. The decrease in HOMO-LUMO energy gaps, as well as the red shifting of λmax in UV–Visible spectra, demonstrate sensitivity of CTPy towards sulfur gases. The improved conductivity of complexes is owing to production of numerous energy levels inoccupied and virtual orbitals closer to the Fermi level in DOS spectra. Furthermore, PDOS spectra reveal that CTPy is chiefly contribute to energy of HOMO. The recent findings show CTPy has a significant sensitivity to sulfurirritants. We hope that the above findings and their implications will give valuable suggestions for an experimentalist in designing extremely sensitive hazardous analyte sensors employing CTPy.

Collision-induced dissociation as "mass spectrometric filter" for rapid screening of tetrapyrrole derivatives and their chelated metal species in complex biological and environmental samples.

Cyclic tetrapyrroles, such as chlorophylls and their diagenetic derivatives, are structurally diverse and often chelated with certain metal species in the natural environment. A high throughput analytical method enabling quick tetrapyrrole screening in complex samples will promote the study of tetrapyrrole biogeochemistry and probably discoveries of new tetrapyrroles. Total lipid extracts of biological and environmental samples were injected onto a C18 column to separate compounds with a reverse-phase gradient. Collision-induced dissociation (CID) was performed at different energy levels, from 40 to 200 eV, on a quadrupole time-of-flight mass spectrometry (QTOF-MS) to identify cyclic tetrapyrroles in complex matrices. Under 200 eV CID cyclic tetrapyrroles exhibit a unique fragmentation behavior, the production of fragments larger than 300 Da. Utilizing such feature as a filter to extract product ions in the range of 300-500 Da, various cyclic tetrapyrrole derivatives are readily recognized in all tested biological and environmental samples. The 200 eV CID setup also dissociates chelated to porphyrin metals, including Cu, Fe, Mn, Ni, and V, as single-charged ions for direct MS detection. The 200 eV CID setup provides an efficient approach for the identification of cyclic tetrapyrroles, such as chlorophylls and fossil metalloporphyrins, in complex environmental samples. The direct detection of chelated to porphyrin metal ions with QTOF-MS shows the potential for compound-specific metal isotope analysis.

Elevated Levels of an Enzyme Involved in Coenzyme B12 Biosynthesis Kills Escherichia coli.

ABSTRACTCobamides are cobalt-containing cyclic tetrapyrroles involved in the metabolism of organisms from all domains of life but produced de novo only by some bacteria and archaea. The pathway is thought to involve up to 30 enzymes, five of which comprise the so-called “late” steps of cobamide biosynthesis. Two of these reactions activate the corrin ring, one activates the nucleobase, a fourth one condenses activated precursors, and a phosphatase yields the final product of the pathway. The penultimate step is catalyzed by a polytopic integral membrane protein, namely, the cobamide (5′-phosphate) synthase, also known as cobamide synthase. At present, the reason for the association of all putative and bona fide cobamide synthases to cell membranes is unclear and intriguing. Here, we show that, in Escherichia coli, elevated levels of cobamide synthase kill the cell by dissipating the proton motive force and compromising membrane stability. We also show that overproduction of the phosphatase that catalyzes the last step of the pathway or phage shock protein A prevents cell death when the gene encoding cobamide synthase is overexpressed. We propose that in E. coli, and probably all cobamide producers, cobamide synthase anchors a multienzyme complex responsible for the assembly of vitamin B12 and other cobamides.

Physicochemical and biochemical properties of synthetic zinc 131-(un)substituted chlorophyll-a derivatives

Zinc methyl pheophorbides a / a ’, which possess oxo and methoxycarbonyl groups at the C13 1 - and C13 2 -positions, respectively, on the exo -five membered ring fusing the cyclic tetrapyrrole of their chlorin π-skeletons, were prepared via chemical modifications of chlorophyll a produced in phototrophs. The C13 1 -hydroxylated and unsubstituted analogues were synthesized by reduction of the ketone moiety. The C13 1 - and C13 2 -stereochemistry of these semi-synthetic compounds was confirmed by 1 H NMR and circular dichroism spectroscopies. A methoxycarbonyl group at the chiral C13 2 -position in the ketone was stereo-inverted under basic conditions, whereas those in the corresponding alcoholic and dihydro-forms were not. The in vitro BciC enzymatic reaction of the (13 2 R )-ketone was found to stereoselectively hydrolyze the C13 2 -methoxycarbonyl group, followed by spontaneous decarboxylation. Similarly, the (13 2 R )-COOCH 3 group in the (13 1 S )-alcohol was hydrolyzed, whereas the (13 2 R )-COOCH 3 group in the (13 1 R )-alcohol was not. After the enzymatic reaction of the C13 1 -dihydro-compound, its (13 2 S )-demethoxycarbonylated and hydroxylated product was observed. • Zinc 13 1 -hydroxylated and unsubstituted chlorophyll- a derivatives were synthesized. • The 13 1 - and 13 2 -stereochemistry was confirmed by 1 H NMR and CD spectroscopies. • The 13 1 -oxo analog was 13 2 -epimerized, but 13 1 -hydroxy and dihydro forms could not. • A recombinant BciC enzyme stereospecifically hydrolyzed the 13 2 -COOMe in vitro.

Quantum chemical study on sensing of NH3, NF3, NCl3 and NBr3 by using cyclic tetrapyrrole

Toxic gas sensors with ultrahigh sensitivity are highly desirable and those can be achieved with the help of infinite π-conjugation of cyclic conducting polymers. In this DFT study, we demonstrate the adsorption performance of tetracyclic oligopyrrole (CTPy) towards NH3 and nitrogen halides including NF3, NCl3 and NBr3. The CTPy possesses infinite conjugation and highly active cavity which provides an excellent platform for the adsorption of upcoming gas molecules. Thermodynamically, the observed interaction energies of NH3, NF3, NCl3 and NBr3 analytes with CTPy at ωB97XD/6-31+G(d,p) level of theory are −13.14, −3.00, −6.00 and −7.50 kcal/mol, respectively. In addition, the molecular dynamic simulations are performed at GFN2-xTB method to confirm the stability of CTPy and respective complexes. The SAPT0 and NCI analyses reveal that dispersion forces play a significant role to stablize these complexes. Moreover, the electrostatic component also contributes in stabilizing the NH3@CTPy, NCl3@CTPy and NBr3@CTPy complexes. The variation in the electronic properties including HOMO-LUMO gaps of complexes along with the significant NBO charge transfer indicate the increasing conductivity of CTPy upon complexation with reported analytes. The prominent charge transfer on interaction might be due to the increasing π to π* transition, thus the λmax is shifted to longer wavelength in UV–vis spectra.

Improved Synthesis of ortho-Phenylene-bridged Cyclic Tetrapyrroles and Oxidative Fusion Reactions Toward Substituted Tetraaza[8]circulenes.

Hetero[8]circulenes have emerged as novel functionalized heteronanographenes that show various promising functions such as bright fluorescence, charge transporting, and redox reactivities. One of the effective synthetic strategy is the fold-in type oxidative fusion reaction of ortho-phenylene-bridged cyclic tetrapyrroles, whose construction, however, is not well-sophisticated in terms of reproducibility and possibility for versatile derivatization. In this paper, a "reverse" coupling strategy has been developed, which enabled synthesis of opp-type low symmetric analogues of cyclic tetrapyrroles. Oxidative fusion reaction conditions to afford tetraaza[8]circulenes have also been reinvestigated and improved. Substituent effects of cyclic tetrapyrroles and tetrabenzotetraaza[8]circulenes are studied for solid-state structures and packing structures, redox potentials, and optical properties.

Structural effects on epimerization of bacteriochlorophyll a and chlorophyll a revealed using 3-acetyl chlorophyll a

Chlorophyll (Chl) and bacteriochlorophyll (BChl) pigments, which are crucial cyclic tetrapyrroles in photosynthesis, generally have a chiral center in their exo-cyclic five-membered E-ring. Although [Formula: see text]-epimers (primed-type) of (B)Chl pigments are rarely present in photosynthetic organisms, they play key roles in photosynthetic reaction center complexes. The epimerization mechanism of (B)Chl pigments in vivo has not been unraveled. The structural effects on the physicochemical properties of (B)Chl epimerization reactions provide useful information to tackle this question. We analyzed epimerization of three pigments, BChl [Formula: see text], Chl [Formula: see text], and 3-acetyl Chl [Formula: see text], to elucidate the structural factors that are responsible for epimerization reactions. We compared the epimerization kinetics of the three pigments and concluded that the bacteriochlorin skeleton (7,8,17,18-tetrahydroporphyrin) significantly retarded the epimerization kinetics. Thus, BChl [Formula: see text] exhibited slower epimerization kinetics than Chl [Formula: see text] in spite of the presence of the electron-withdrawing 3-acetyl group that accelerates epimerization. In contrast to the large structural effects of (B)Chl molecules on epimerization kinetics, the thermodynamic properties at equilibrium in the epimerization of the three pigments were barely influenced by their molecular structures. This study also demonstrates that a semi-synthetic pigment, 3-acetyl Chl [Formula: see text], is appropriate for comparative analyses of the structural effects of BChl [Formula: see text] and Chl [Formula: see text] on their physicochemical properties.

Reconstitution of 3-Acetyl Chlorophyll a into Light-Harvesting Complex 2 from the Purple Photosynthetic Bacterium Phaeospirillum molischianum.

The manipulation of B800 bacteriochlorophyll (BChl) a in light-harvesting complex 2 (LH2) from the purple photosynthetic bacterium Phaeospirillum molischianum (molischianum-LH2) provides insight for understanding the energy transfer mechanism and the binding of cyclic tetrapyrroles in LH2 proteins since molischianum-LH2 is one of the two LH2 proteins whose atomic-resolution structures have been determined and is a representative of type-2 LH2 proteins. However, there is no report on the substitution of B800 BChl a in molischianum-LH2. We report the reconstitution of 3-acetyl chlorophyll (AcChl) a, which has a 17,18-dihydroporphyrin skeleton, to the B800 site in molischianum-LH2. The 3-acetyl group in AcChl a formed a hydrogen bond with β′-Thr23 in essentially the same manner as native B800 BChl a, but this hydrogen bond was weaker than that of B800 BChl a. This change can be rationalized by invoking a small distortion in the orientation of the 3-acetyl group in the B800 cavity by dehydrogenation in the B-ring from BChl a. The energy transfer from AcChl a in the B800 site to B850 BChl a was about 5-fold slower than that from native B800 BChl a by a decrease of the spectral overlap between energy-donating AcChl a and energy-accepting B850 BChl a.

High selectivity of cyclic tetrapyrrole over tetrafuran and tetrathiophene toward toxic chemicals; A first-principles study

Abstract Although, the sensor applications of straight chain conducting polymers are well explored, but, the cyclic analogues are not much explored in this regard. The density functional theory calculations are performed to study the interaction behaviour of tetracyclic oligomers such as tetrapyrrole (4CP), tetrafuran (4CF) and tetrathiophene (4CT) for the detection of harmful gases like, phosgene, thiophosgene and formaldehyde. The most stable interaction configurations, interaction energies (Eint), interaction distance (Dint), SAPT0 energies, NCI, NBO charge transfer, HOMO-LUMO gaps and maximum absorbance are evaluated to understand the sensitivity and selectivity of these cyclic analogues toward poisonous gases. The results of all these properties illustrate that 4CP exhibits the highest sensitivity towards analytes with the sequence of formaldehyde > phosgene > thiophosgene. The interaction energies are −10.91, −9.03 and −7.58 kcal mol−1 for fd@4CP, ph@4CP and tph@4CP complexes, respectively. SAPT0 results depict that the electrostatic forces have strong influence in these complexes which indicate the presence of strong hydrogen bond between 4CP and analytes. However, this is not the case in other oligomer-analyte complexes especially in 4CT based complexes. The highest sensitivity of 4CP enables it to act as promising sensing material for formaldehyde, phosgene and thiophosgene as compared to 4CF and 4CT.

Metal coordination and peripheral substitution modulate the activity of cyclic tetrapyrroles on αS aggregation: a structural and cell-based study.

The discovery of aggregation inhibitors and the elucidation of their mechanism of action are key in the quest to mitigate the toxic consequences of amyloid formation. We have previously characterized the antiamyloidogenic mechanism of action of sodium phtalocyanine tetrasulfonate ([Na4(H2PcTS)]) on α-Synuclein (αS), demonstrating that specific aromatic interactions are fundamental for the inhibition of amyloid assembly. Here we studied the influence that metal preferential affinity and peripheral substituents may have on the activity of tetrapyrrolic compounds on αS aggregation. For the first time, our laboratory has extended the studies in the field of the bioinorganic chemistry and biophysics to cellular biology, using a well-established cell-based model to study αS aggregation. The interaction scenario described in our work revealed that both N- and C-terminal regions of αS represent binding interfaces for the studied compounds, a behavior that is mainly driven by the presence of negatively or positively charged substituents located at the periphery of the macrocycle. Binding modes of the tetrapyrrole ligands to αS are determined by the planarity and hydrophobicity of the aromatic ring system in the tetrapyrrolic molecule and/or the preferential affinity of the metal ion conjugated at the center of the macrocyclic ring. The different capability of phthalocyanines and meso-tetra (N-methyl-4-pyridyl) porphine tetrachloride ([H2PrTPCl4]) to modulate αS aggregation in vitro was reproduced in cell-based models of αS aggregation, demonstrating unequivocally that the modulation exerted by these compounds on amyloid assembly is a direct consequence of their interaction with the target protein.

Synthesis of Cationic Pyridinium‐(Bacterio)Chlorophyll Conjugates Bearing a Bacteriochlorin, Chlorin, or Porphyrin π‐Skeleton and their Photophysical and Electrochemical Properties

Fine‐tuning the electronic absorption properties of chlorophylls is important for their use as light‐absorbing pigments. By oxidative coupling, a cationic pyridinium group was selectively introduced at the peripheral vinyl group of three types of synthetic chlorophylls with different degrees of π‐conjugation of the cyclic tetrapyrroles (bacteriochlorin, chlorin, and porphyrin). The substitution with the electron‐withdrawing pyridinium group moved redmost electronic absorption and main fluorescence emission maxima to longer wavelengths. This substitution effect depended on the core π‐system and decreased with increasing the degree of dehydrogenation of the cyclic tetrapyrrole (bacteriochlorin > chlorin > porphyrin). Moreover, fluorescence quenching was observed as a consequence of the introduction of the pyridinium group. The electrochemical measurements and theoretical model calculations showed that the quenching is ascribed to an intramolecular electron transfer from the core π‐skeleton to the peripheral cationic pyridinium group.

Coumarins and other components of Daphne oleoides Schreb. subsp. oleoides from Majella National Park

The present study reports about the phytochemical analysis of a sample of Daphne oleoides Schreb. subsp. oleoides (family Thymelaeaceae) collected from the Majella National Park. Twenty components, belonging to different classes of natural products, have been identified by means of spectroscopic and spectrometric techniques: [monomeric (4), bis- (5, 7–10, 20) and trimeric coumarins (11), including aglycones and glycosides, coumarinolignoid (6); flavonoids (16–19); glycosidic furolignans (14, 15), glucosidic phenylpropanoids (12, 13), cyclic tetrapyrrole derivatives (2, 3), unsaturated triglyceride (1)]. Besides the chemosystematic markers of the genus (4–10, 12, 14–18, 20) several other components were identified for the first time in the species (17, 20) and/or in the Daphne genus (1–3, 13, 19). The observed composition was discussed from the chemotaxonomic standpoint and compared with those recognized from a Sardinian accession. It was observed a pronounced difference in the two metabolites patterns, most probably attributable to geographic isolation of the studied populations and, in some extent, also by the different environmental conditions, evidencing a possible tendency of D. oleoides to the infraspecific chemovariability. Considering the wide traditional uses of Daphne spp. in ethnomedicine of several countries, also the pharmacologic potential of the identified secondary metabolites is discussed.

Characterization of 3-Acetyl Chlorophyll a and 3-Acetyl Protochlorophyll a Accommodated in the B800 Binding Sites of Photosynthetic Light-Harvesting Complex 2 in the Purple Photosynthetic Bacterium Rhodoblastus acidophilus.

We present the detailed characterization on the reconstitution of two cyclic tetrapyrrole pigments that have the same substituents but differ in the degree of hydrogenation in the macrocycles from bacteriochlorophyll (BChl) a (7,8,17,18-tetrahydroporphyrin) into the binding sites of B800 BChl a in light-harvesting complex 2 (LH2) of purple photosynthetic bacteria. Both 3-acetyl chlorophyll (Chl) a (17,18-dihydroporphyrin) and 3-acetyl protochlorophyll (PChl) a (porphyrin) were inserted into the B800-binding pockets in LH2, indicating that these pockets allow alteration of the degree of hydrogenation in the cyclic tetrapyrroles. Redshifts of the Qy peak positions of 3-acetyl (P)Chl a by insertion into the B800-binding sites were smaller than that of BChl a. The relative Qy absorbance of 3-acetyl (P)Chl a to B850 BChl a in the reconstituted proteins was significantly smaller than that of B800 BChl a in native LH2 in spite of their high occupancy in the B800-binding sites. These are ascribable to the smaller dipole strengths of 3-acetyl (P)Chl a. We also performed the coreconstitution of both 3-acetyl Chl a and BChl a into the nine B800-binding sites in LH2, indicating that the affinity of 3-acetyl Chl a to the B800-cavity was slightly higher than that of BChl a.

Effects of Cyclic Tetrapyrrole Rings of Aggregate-Forming Chlorophyll Derivatives as Hole-Transporting Materials on Performance of Perovskite Solar Cells

Organic hole-transporting materials (HTMs) are essential components of high-performance perovskite solar cells (PSCs). Three zinc-coordinated chlorophyll derivatives with bacteriochlorin, chlorin, and porphyrin macrocycles, namely, ZnBChl, ZnChl, and ZnPor, respectively, were newly synthesized and employed as HTMs in PSCs. The difference in the π backbones of these HTMs causes differences in their photophysical properties, and thus different hole-extraction abilities, as revealed by steady-state photoluminescence spectra. The power conversion efficiencies (PCEs) of PSCs with a typical mesoporous structure, fluorine-doped tin oxide/compact TiO2/mesoporous TiO2/CH3NH3PbI3/HTM/Ag, are 8.26%, 11.88%, and 0.68% for ZnBChl, ZnChl, and ZnPor, respectively. The small PCE of the ZnPor-based PSC is partially attributed to the small energy gap of the highest occupied molecular orbital (HOMO) levels between ZnPor and CH3NH3PbI3 perovskite. Therefore, we increased this energy gap slightly by shifting the HOMO level of...