Related Porphyrin Research Articles

Metallaantiaromaticity of 10-Platinacorrole Complexes.

The aromaticity of cyclic π-conjugated organometallic compounds is known as metallaaromaticity. π-Conjugated metallacycles, such as metallabenzenes and metallapentalenes, have been investigated in order to understand the involvement of the d electrons from the metal center in the π-conjugated systems of the organic ligands. Here, we report the synthesis of Pd(II) 10-platinacorrole complexes with cyclooctadiene (COD) and norbornadiene (NBD) ligands. While the Pd(II) 10-platinacorrole COD complex adopts a distorted structure without showing appreciable antiaromaticity, the corresponding NBD complex exhibits a distinct antiaromatic character due to its highly planar conformation. Detailed density functional theory (DFT) calculations revealed that two d orbitals are involved in macrocyclic π-conjugation. We furthermore demonstrated that Craig-Möbius antiaromaticity is not present in the studied system. The synthesis of 10-platinacorroles enables a systematic comparison of the antiaromaticity and aromaticity of closely related porphyrin analogues, providing a better understanding of π-conjugation that involves d orbitals.

Active Sites of Cobalt Phthalocyanine in Electrocatalytic CO2 Reduction to Methanol.

Many metal coordination compounds catalyze CO2 electroreduction to CO, but cobalt phthalocyanine hybridized with conductive carbon such as carbon nanotubes is currently the only one that can generate methanol. The underlying structure-reactivity correlation and reaction mechanism desperately demand elucidation. Here we report the first in situ X-ray absorption spectroscopy characterization, combined with ex situ spectroscopic and electrocatalytic measurements, to study CoPc-catalyzed CO2 reduction to methanol. Molecular dispersion of CoPc on CNT surfaces, as evidenced by the observed electronic interaction between the two, is crucial to fast electron transfer to the active sites and multi-electron CO2 reduction. CO, the key intermediate in the CO2 -to-methanol pathway, is found to be labile on the active site, which necessitates a high local concentration in the microenvironment to compete with CO2 for active sites and promote methanol production. A comparison of the electrocatalytic performance of structurally related porphyrins indicates that the bridging aza-N atoms of the Pc macrocycle are critical components of the CoPc active site that produces methanol. In situ X-ray absorption spectroscopy identifies the active site as Co(I) and supports an increasingly non-centrosymmetric Co coordination environment at negative applied potential, likely due to the formation of a Co-CO adduct during the catalysis.

Active Sites of Cobalt Phthalocyanine in Electrocatalytic CO2 Reduction to Methanol

AbstractMany metal coordination compounds catalyze CO2 electroreduction to CO, but cobalt phthalocyanine hybridized with conductive carbon such as carbon nanotubes is currently the only one that can generate methanol. The underlying structure–reactivity correlation and reaction mechanism desperately demand elucidation. Here we report the first in situ X‐ray absorption spectroscopy characterization, combined with ex situ spectroscopic and electrocatalytic measurements, to study CoPc‐catalyzed CO2 reduction to methanol. Molecular dispersion of CoPc on CNT surfaces, as evidenced by the observed electronic interaction between the two, is crucial to fast electron transfer to the active sites and multi‐electron CO2 reduction. CO, the key intermediate in the CO2‐to‐methanol pathway, is found to be labile on the active site, which necessitates a high local concentration in the microenvironment to compete with CO2 for active sites and promote methanol production. A comparison of the electrocatalytic performance of structurally related porphyrins indicates that the bridging aza‐N atoms of the Pc macrocycle are critical components of the CoPc active site that produces methanol. In situ X‐ray absorption spectroscopy identifies the active site as Co(I) and supports an increasingly non‐centrosymmetric Co coordination environment at negative applied potential, likely due to the formation of a Co−CO adduct during the catalysis.

Drastic differences between the release kinetics of two highly related porphyrins in liposomal membranes: mTHPP and pTHPP

HypothesisThe release of hydrophobic compounds from liposomal membranes occurs by partitioning and is thus determined by the physicochemical properties (e.g. logP and water solubility) of the drug. We postulate that even minor structural differences, e.g. the position of the phenolic OH-group of the hydrophobic porphyrins mTHPP and pTHPP (meta vs. para substitution), distinctly affect their partitioning and release behavior from liposomes. ExperimentsThe release and redistribution of mTHPP and pTHPP from lecithin or POPC/POPG liposomes to different acceptor particles (DSPE-mPEG micelles and liposomes) was studied by asymmetrical flow field-flow fractionation to separate donor and acceptor particles. Reversed phase HPLC was applied to detect differences in partitioning. Molecular dynamics (MD) simulations were carried out to obtain molecular insight in the different behavior of the two compounds inside a lipid bilayer. FindingsDespite the minor differences in chemical structure, mTHPP is more hydrophobic and redistributes much slower to both acceptor phases than pTHPP. MD simulations indicate that compared to pTHPP, mTHPP makes stronger hydrogen bonds with the lipid head groups, is oriented more parallel to the lipid tails and is embedded slightly deeper in the membrane.

On-Surface Synthesis of Fused Anthracenyl Porphyrin Derivatives

For the last fifteen years, on-surface synthesis became an extremely popular and powerful method to fabricate nanostructures which otherwise would not have been accessible by conventional wet organic syntheses. The syntheses, realized in the chamber of high vacuum scanning tunneling microscope (STM), permitted the fabrication and the characterization of atomically defined nanostructures such as polycyclic aromatic hydrocarbons (PAH), graphene nanoribbons or 2D covalent networks.1 Among the organic molecules studied by STM, porphyrins are of particular importance because of their electronic and optical properties. Initial studies were mainly devoted to the supramolecular organization of porphyrins and porphyrin derivatives on surface and it is only in 2004 that noble metal surfaces were used as catalytic surface to perform reactions on the porphyrin cores.2 Since then, examples of open-shell porphyrin derivatives3 and conjugated oligomers4 were reported.While the synthesis of fused bis- and tetra-4,5-bis-(2,4,6-trimethylphenoxy)anthracen-9-yl porphyrins containing Ni(II) was reported by Anderson about 10 years ago,5 the formation π-extended porphyrin bearing unsubstituted anthracenyl moieties were never achieved. The formation of such compounds became possible by on-surface synthesis. Herein, we report on the synthesis of fused bis- and tetra-anthracenylporphyrin derivatives via the cyclodehydrogenation the parent meso-substituted molecules on a catalytic Au(111) surface.6 Depending on the annealing temperature, anthracenylporphyrins with the four anthracene fully linked to the porphyrin core or over-oxidized porphyrin in which the anthracene are connected two by two in 2-2’ position were observed by STM (Figure 1). Figure 1. STM images of the fused tetra-anthracenylporphyrins and related porphyrin structures References Clair, S.; de Oteyza, D. G. Chem. Rev. 2019, 119, 4717-4776.Fox, S.; Boyle, R. W. Chem. Commun. 2004, 1322-1323.a) Sun, Q.; Mateo, L. M.; Robles, R.; Ruffieux, P.; Lorente, N.; Bottari, G.; Torres, T.; Fasel, R. J. Am. Chem. Soc. 2020, 142, 18109-18117; b) Zhao, Y.; Jiang, K.; Li, C.; Liu, Y.; Xu, C.; Zheng, W.; Guan, D.; Li, Y.; Zheng, H.; Liu, C.; Luo, W.; Jia, J.; Zhuang, X.; Wang, S. J. Am. Chem. Soc. 2020, 142, 18532-18540.a) Mateo, L. M.; Sun, Q.; Liu, S.-X.; Bergkamp, J. J.; Eimre, K.; Pignedoli, C. A.; Ruffieux, P.; Decurtins, S.; Bottari, G.; Fasel, R.; Torres, T. Angew. Chem., Int. Ed. 2020, 59, 1334-1339; b) Mateo, L. M.; Sun, Q.; Eimre, K.; Pignedoli, C. A.; Torres, T.; Fasel, R.; Bottari, G. Chem. Sci. 2021, 12, 247-252.a) Davis, N. K. S.; Thompson, A. L.; Anderson, H. L. Org. Lett. 2010, 12, 2124-2127; b) Davis, N. K. S.; Thompson, A. L.; Anderson, H. L. J. Am. Chem. Soc. 2011, 133, 30-31.Baljozović, M. et al., in preparation Figure 1

Utility of dipyrromethane in the synthesis of some new A2B2 porphyrin and their related porphyrin like derivatives with their evaluation as antimicrobial and antioxidant agents

In this work, the dipyrromethane derivatives 2a,b were synthesized from an aldehyde and excess of pyrrole at room temperature in solvent-free conditions. The reaction of dipyrromethanes 2a,b with different aromatic aldehydes in the presence of hydrochloric acid or p-toluenesulfonic acid as a catalyst afforded some new porphyrin like and A2B2 porphyrin derivatives 4a-g and 5a-d, respectively. Biological evaluation of the newly synthesized compounds 4a-g and 5a-d showed promising activity as antimicrobial and antioxidant agents. Compounds 4c,4e,4g,5c, and 5d exhibited the highest antimicrobial activity, whereas, compounds 4a,4f,5a,5b, and 5d recorded the highest antioxidant activity.

Phthalocyanine-Cored Fluorophores with Fluorene-Containing Peripheral Two-Photon Antennae as Photosensitizers for Singlet Oxygen Generation.

A series of free base and Zn(II) phthalocyanines featuring fluorenyl antennae linked by methoxy or oxo bridges to the phthalocyanine core (Pc) were synthesized and characterized. Selected linear and nonlinear (two-photon absorption) optical properties of these new compounds were subsequently studied. As previously observed for related porphyrin dendrimers bearing 2-fluorenyl peripheral dendrons, an efficient energy transfer occurs from the peripheral antennae to the central phthalocyanine core following excitation in the fluorenyl-based π–π* absorption band of these chromophores. Once excited, these compounds relax to the ground state, mostly by emitting intense red light or by undergoing intersystem crossing. As a result, the tetrafunctionalized Zn(II) phthalocyanines are fluorescent, but can also efficiently photosensitize molecular oxygen in tetrahydrofurane (THF), forming singlet oxygen with nearly comparable yields to bare Zn(II) phthalocyanine (ZnPc). In comparison with the latter complex, the positive role of the fluorenyl-containing antennae on one- and two-photon brightness (2PA) is presently demonstrated when appended in peripheral (β) position to the phthalocyanine core. Furthermore, when compared to known porphyrin analogues, the interest in replacing the porphyrin by a phthalocyanine as the central core to obtain more fluorescent two-photon oxygen photosensitizers is clearly established. As such, this contribution paves the way for the future development of innovative biphotonic photosensitizers usable in theranostics.

Structural, Photophysical, and Electrochemical Properties of Doubly Fused Porphyrins and Related Fused Chlorins.

The electrochemical and physicochemical properties of tetraphenylporphyrins and tetraphenylchlorins with two fused indanedione (IND) or malononitrile (MN) groups and two antipodal Br, Ph, or H β-substituents are investigated in nonaqueous media. These compounds were synthesized by oxidative fusion of free-base trans-chlorins, followed by metalation. The corresponding free-base di-fused chlorins were also isolated as intermediates and characterized for comparisons. The examined di-fused porphyrins (DFP) and di-fused chlorins (DFC) are represented as MDFP(Y)2(R)2 and H2DFC(Y)2(R)2, where M = 2H, CuII, NiII, ZnII, and CoII, Y is a fused indanedione (IND) or malononitrile group (MN), and R = H, Br, or Ph. The IND- and MN-appended compounds in both series exhibit the expected two one-electron oxidations but quite different redox behavior is observed upon reduction, where the free-base IND-appended chlorins show four reversible one-electron reductions, compared to only two for the related free-base MN-appended chlorins. Although porphyrin trianions and tetraanions have been recently described for derivatives with highly electron-withdrawing and/or π-extending substituents, this seems not to be the case for the doubly fused IND-chlorins, where the first two one-electron additions are proposed to be located at the conjugated macrocycle and the last two at the fused IND groups, each of which is reduced at a different potential, consistent with the behavior expected for two equivalent and interacting redox centers. Unlike the examined chlorins, which are all stable in their electroreduced forms, the electrogenerated anionic forms of the di-fused porphyrins are all highly reactive and characterized by cyclic voltammograms having reduction peaks not only for the synthesized compounds added to solution but also for one or more new redox active species formed at the electrode surface in homogeneous chemical reactions following electron transfer. Comparisons are made between electrochemical behavior of the structurally related porphyrins and chlorins and the sites of electron transfer assigned on the basis of known electrochemical diagnostic criteria. One of the compounds, ZnDFP(MN)2, was also structurally characterized as having a ruffled and twisted macrocyclic conformation.

Electrochemistry of Three-Dimensional Porphyrin Arrays and Related Compounds

Three-dimensional π-conjugated molecules have received extensive attention in many chemical research areas due to their interesting structure and unique characteristics. Among these situations, we found that oxidation of aromatic amines was useful for construction of distorted π-conjugated molecules under laboratory conditions. For example, oxidation of beta-amino substituted porphyrins provided three-dimensional porphyrins in excellent yield. In particular, sequential use of amination and oxidation toward 5,10,15,20-tetramesitylporphyrin effectively afforded highly twisted porphyrin oligomers. These oligomers exhibited high solubility in common organic solvent, which is promising for application to solution-processible molecular wire. In addition, we further investigated the substituent effect on the oxidation of beta-aminoporphyrins. We disclosed that dimerization products were varied dependent on meso-aryl substituents. In this presentation, we will show the electrochemical properties of these porphyrin oligomers in order to investigated the effect of distorted conformation on π-electronic network between porphyrin units. We also found oxidation of N-aryl beta-amino substituted porphyrins produced dehydropyrazine-bridged porphyrin dimers. Recently, we have succeeded in the synthesis of π-extended dihydrophenazine through oxidation of amino-substituted anthracenes. This molecule exhibited conformational change upon electrochemical and chemical redox conditions. Spectral analysis and theoretical calculation revealed that the aromatic and non-aromatic switch of the central dihydropyrazine moiety by two electron oxidation play important role for this electrochemical responsive behaviors. This time, we also prepared π-extended dihydrophenazine based on porphyrins. The structure was clearly elucidated by X-ray diffraction analysis, showing saddle-shape conformation. The electrochemical analysis exhibited irreversible characteristic upon one-electron oxidation and reduction as observed for anthracene derivatives. In this presentation, we will show the synthetic details and electrochemical behaviors of π-extended dihydrophenazines and related porphyrin arrays.

Unravelling the conductance path through single-porphyrin junctions.

Porphyrin derivatives are key components in natural machinery enabling us to store sunlight as chemical energy. In spite of their prominent role in cascades separating electrical charges and their potential as sensitizers in molecular devices, reports concerning their electronic transport characteristics are inconsistent. Here we report a systematic investigation of electronic transport paths through single porphyrin junctions. The transport through seven structurally related porphyrin derivatives was repeatedly measured in an automatized mechanically controlled break-junction set-up and the recorded data were analyzed by an unsupervised clustering algorithm. The correlation between the appearances of similar clusters in particular sub-sets of the porphyrins with a common structural motif allowed us to assign the corresponding current path. The small series of model porphyrins allowed us to identify and distinguish three different electronic paths covering more than four orders of magnitude in conductance.

(Invited) The Reactivity and Mechanistic Studies of Group 14 Corrole Complexes

The discoveries of facile syntheses of meso-aryl substituted corroles in 1999 triggered the revival of the chemistry of corroles. A variety of main group and transition metal elements were inserted into the corrole ring and some of the complexes show similar or superior properties to the related porphyrin complexes. The corrole germanium hydride showed unusual high reactivity with aldehydes, olefins and alkyl halides to form corrole germanium α-hydroxy alkyl and alkyl complexes. More importantly, the efficient photo-activation of N-H bond by TEMPO-Ge corrole complexes was observed and the mechanism was investigated based on EPR analysis. The reactivity of Sn corrole was also studied. Activation of N–H bonds with silicon and its low-valent heavier congeners typically proceeds via 2 electron pathways: i) a direct oxidative addition of the N–H bond to the low-valent metal center or ii) a metal-ligand cooperative process. Light-promoted N–H bond activation by main group element radicals has thus far rarely been encountered, but provides an attractive strategy to activate amines with lower activation energies. We observed the light-driven activation of N–H bonds by an organo-germanium corrole complex [(TPFC)Ge(TEMPO)] (TPFC = tris(pentafluorophenyl)corrole, TEMPO = (2,2,6,6-tetramethylpiperidin-1-yl)oxyl) involving a radical pathway to form germanium amides in high yields at room temperature.

Total synthesis of vitamin B12— a fellowship of the ring

In the synthesis of cobyric acid following an arduous 60-step synthesis of the tetrapyrrole corrigenolide, Woodward demonstrated that ring cyclization yielded unexpectedly the corrin cyclic ether (Fig. 2). Its structure was chosen as the chemistry logo for the 1970 IUPAC meeting. It was a reminder that chemistry can be capricious and beautiful. This article is a tribute to the 50-year career in porphyrins of Professor Kevin M. Smith and describes his early contributions with colleagues in the synthetic quest of a related porphyrin molecule, vitamin B12(Fig. 1a).

Triplet state delocalization in a conjugated porphyrin dimer probed by transient electron paramagnetic resonance techniques.

The delocalization of the photoexcited triplet state in a linear butadiyne-linked porphyrin dimer is investigated by time-resolved and pulse electron paramagnetic resonance (EPR) with laser excitation. The transient EPR spectra of the photoexcited triplet states of the porphyrin monomer and dimer are characterized by significantly different spin polarizations and an increase of the zero-field splitting parameter D from monomer to dimer. The proton and nitrogen hyperfine couplings, determined using electron nuclear double resonance (ENDOR) and X- and Q-band HYSCORE, are reduced to about half in the porphyrin dimer. These data unequivocally prove the delocalization of the triplet state over both porphyrin units, in contrast to the conclusions from previous studies on the triplet states of closely related porphyrin dimers. The results presented here demonstrate that the most accurate estimate of the extent of triplet state delocalization can be obtained from the hyperfine couplings, while interpretation of the zero-field splitting parameter D can lead to underestimation of the delocalization length, unless combined with quantum chemical calculations. Furthermore, orientation-selective ENDOR and HYSCORE results, in combination with the results of density functional theory (DFT) calculations, allowed determination of the orientations of the zero-field splitting tensors with respect to the molecular frame in both porphyrin monomer and dimer. The results provide evidence for a reorientation of the zero-field splitting tensor and a change in the sign of the zero-field splitting D value. The direction of maximum dipolar coupling shifts from the out-of-plane direction in the porphyrin monomer to the vector connecting the two porphyrin units in the dimer. This reorientation, leading to an alignment of the principal optical transition moment and the axis of maximum dipolar coupling, is also confirmed by magnetophotoselection experiments.

Electronic Structure of One-Electron-Oxidized Form of the Methylcobalamin Cofactor: Spin Density Distribution and Pseudo-Jahn–Teller Effect

The electronic and structural properties of the one-electron-oxidized form of methylcobalamin (MeCbl) cofactor have been investigated using density functional theory (DFT) and CASSCF/MC-XQDPT2 calculations. We applied two types of functionals (hybrid and GGA) which produced quite different results in terms of spin density profiles: the B3LYP description was consistent with Co(III) and the π-cation corrin radical while the BP86 result was more in line with the Co(IV) oxidation state. A closer inspection of both outcomes indicates that the oxidized species have a mixed π-cation corrin radical and Co(III)/Co(IV) character. This mixed character was further supported by high-level ab initio CASSCF/MC-XQDPT2 calculations, which reveal the strong mixing of the electronic states due to nondynamical correlation effects. The near degeneracy, which takes place between the ground and first excited state, was consistent with the presence of a pseudo-Jahn-Teller (pJT) effect in the oxidized form of MeCbl. In addition, the DFT-based investigation of the structurally related porphyrin complexes gives a description consistent with corrin-based analogues and reveals that the corrin species have more Co(IV) character. The most important finding of the present study, regardless of the type of functional used, was the significant lowering of dissociation energy (∼35%), which might be due to the partial depopulation of the Co-C σ orbital upon removal of an electron.

The influence of ultrasound on porphyrin-based metallogel formation: efficient control of H- and J-type aggregations

Efficient control of aggregation modes of the porphyrin–Pd2+ complexes by ultrasonication and heating is reported. A porphyrin-based metallogel is formed in the presence of Pd2+, which displays H-type aggregation. Heating the porphyrin derivative in the presence of Pd2+, however, results in J-type aggregation that does not produce a gel. Conclusive evidence was found that the crystal structure of the related porphyrin–Pd2+ complex obtained by heating shows J-type aggregation.

The synthesis, reactivity, and peripheral functionalization of corroles

Corroles are unusual macrocycles that often exhibit chemical reactivity that is distinct from their closely related porphyrin cousins. Standard organic transformations with corroles often result in the formation of unexpected products. A survey of synthetic methods for the preparation of both meso-substituted and β-substituted corroles will give an overview of the different synthetic strategies. This review provides a comprehensive description of the chemical reactivity and functionalization of corroles, focusing especially on reactions at the periphery of the macrocycle. Formylation, nitration, bromination and chlorosulfonation reactions will be explored in detail. In addition, demetalation processes, reactivity of the N-pyrrolic nitrogens and the lability of the macrocycle ring toward expansion and ring-opening reactions will be discussed. Finally, the synthesis of super-structured (picket-fence, capped, and strapped) corroles and isocorroles will be surveyed.

Effects of different storage conditions on quality related porphyrin fluorescence signatures of pork slices

This study evaluated the potential of fluorescence as an indicator of pork quality by determining the effects of various conditions on fluorescence signatures (excitation at 420nm, emission at 550–750nm).Storage of porcine musculus longissimus dorsi in PE bags led to a clear increase in porphyrin fluorescence intensity after approx. 10d post mortem. Modified gas atmosphere (70% O2, 30% CO2) inhibited the fluorescence emission of zinc protoporphyrin and protoporphyrin IX due to quenching by oxygen. Bleaching processes caused similar effects by halogen light exposure during meat storage. However, already formed signals could not be manipulated by oxygen or halogen light. Storage under vacuum reduced the quenching effects and resulted in increased fluorescence intensities. Freezing and thawing of meat samples delayed and reduced the increase in fluorescence intensity. Only minor effects could be detected at long-term frozen storage for two months.Consequently porphyrin fluorescence analysis is a potential means to indicate changes of pork quality and remaining shelf life.

ChemInform Abstract: Metalation of Carbaporphyrinoid Systems

AbstractReview: 79 refs.

New Halogenated Water‐Soluble Chlorin and Bacteriochlorin as Photostable PDT Sensitizers: Synthesis, Spectroscopy, Photophysics, and in vitro Photosensitizing Efficacy

Chlorin and bacteriochlorin derivatives of 5,10,15,20-tetrakis(2-chloro-5-sulfophenyl)porphyrin have intense absorptions in the phototherapeutic window, high water solubility, high photostability, low fluorescence quantum yield, long triplet lifetimes, and high singlet oxygen quantum yields. Biological studies revealed their negligible dark cytotoxicity, yet significant photodynamic effect against A549 (human lung adenocarcinoma), MCF7 (human breast carcinoma) and SK-MEL-188 (human melanoma) cell lines upon red light irradiation (cutoff λ<600 nm) at low light doses. Time-dependent cellular accumulation of the chlorinated sulfonated chlorin reached a plateau at 2 h, as previously observed for the related porphyrin. However, the optimal incubation time for the bacteriochlorin derivative was significantly longer (12 h). The spectroscopic, photophysical, and biological properties of the compounds are discussed in relevance to their PDT activity, leading to the conclusion that the bacteriochlorin derivative is a promising candidate for future in vivo experiments.

Probing the structure and Franck–Condon region of protochlorophyllide a through analysis of the Raman and resonance Raman spectra

AbstractThe structure and Franck–Condon region of protochlorophyllide a, a precursor in the biosynthesis of chlorophyll and substrate of the light‐regulated enzyme protochlorophyllide oxidoreductase (POR), were investigated by Raman and resonance Raman (RR) spectroscopy. The spectroscopic results are compared to the spectra of the structurally closely related porphyrin model compound magnesium octaethylporphyrin (MgOEP), and interpreted on the basis of density functional theory (DFT) calculations. It is shown that the electronic properties of the two porphyrin macrocycles are affected by different vibrational coupling modes, resulting in a higher absorption cross section of protochlorophyllide a in the visible spectral region. Furthermore, a comparison of the Fourier transform (FT)‐Raman and RR spectra of protochlorophyllide a indicates the modes that are resonantly enhanced upon excitation. Based on vibrational normal mode calculations, these modes include CC ring‐breathing and CC stretching vibrations of the porphyrin macrocycle. In particular, the strong band at 1703 cm−1 can be attributed to the CO carbonyl vibration of the cyclopentanone ring, which is attached in conjugation to the π‐electron path of the porphyrin ring system. The enhancement of that mode upon electronically resonant excitation is discussed in the light of the reaction model suggested for the photoreduction of protochlorophyllide a in the POR. Copyright © 2009 John Wiley &amp; Sons, Ltd.