Chlorophyll and pheophytin protonated and deprotonated ions: Observation and theory

Pheophytin a and chlorophyll a have been investigated by electrospray mass spectrometry in the positive and negative modes, in view of the importance of the knowledge of their properties in photosynthesis. Pheophytin and chlorophyll are both observed intensely in the protonated mode, and their main fragmentation route is the loss of their phytyl chain. Pheophytin is observed intact in the negative mode, while under collisions, it is primarily cleaved beyond the phytyl chain and loses the attaching propionate group. Chlorophyll is not detected in normal conditions in the negative mode, but addition of methanol solvent molecule is detected. Fragmentation of this adduct primarily forms a product (-30 amu) that dissociates into dephytyllated deprotonated chlorophyll. Semi-empirical molecular dynamics calculations show that the phytyl chain is unfolded from the chlorin cycle in pheophytin a and folded in chlorophyll a. Density functional theory calculations have been conducted to locate the charges on protonated and deprotonated pheophytin a and chlorophyll a and have found the major location sites that are notably more stable in energy by more than 0.5 eV than the others. The deprotonation site is found identical for pheophytin a and the chlorophyll a-methanol adduct. This is in line with experiment and calculation locating the addition of methanol on a double bond of deprotonated chlorophyll a.

Effects of peripheral substituents on epimerization kinetics of formylated chlorophylls

C132-[Formula: see text]-epimers of chlorophyll (Chl) molecules are important cofactors in the photosystem I reaction centers in oxygenic photosynthetic organisms; however, their production mechanism is still unclear. The reaction properties of Chl epimerization are helpful for a better understanding of the molecular mechanism of the in vivo formation of Chl C132-[Formula: see text]-epimers. We report herein the kinetic properties of the epimerization of formylated Chl molecules, Chl [Formula: see text] and Chl [Formula: see text], by use of triethylamine. Both Chl [Formula: see text] and Chl [Formula: see text] performed faster epimerization kinetics than Chl [Formula: see text], indicating that the electron-withdrawing ability of the formyl groups directly linked to the chlorin macrocycle is responsible for acceleration of the epimerization. Comparing the rate constants of the two mono-formylated Chl molecules indicated that the epimerization of Chl [Formula: see text] was faster than that of Chl [Formula: see text]. This difference is rationalized by invoking a combination of the inductive effects of the C3- and C7-substituents in Chls; the sums of Hammett [Formula: see text] parameters of the C3- and C7-substituents exhibited high correlations with the epimerization rate constants of Chls [Formula: see text], [Formula: see text], and [Formula: see text].

Ni/Pd-catalyzed Suzuki-Miyaura cross-coupling of alcohols and aldehydes and C-N cross-coupling of nitro and amines via domino redox reactions: base-free, hydride acceptor-free

Domino oxidation-Suzuki-Miyaura cross-coupling of benzyl alcohols with phenylboronic acid and domino reduction-C-N cross-coupling of the nitro compounds with aryl halides were carried out using a strong Ni/Pd bimetallic redox catalyst. The catalyst bearing a copolymer with two Ni/Pd coordinated metals in porphyrin (derived from demetalated chlorophyll b) and salen-type ligands, and pyridine moiety as a base functionality all immobilized on magnetite NPs was synthesised and characterized. The domino oxidation cross-coupling reaction was accomplished under molecular O2 in the absence of any hydride acceptor or/and base. Also, the domino reduction C-N cross-coupling reaction was performed in the presence of NaBH4 without the need for any base and co-reductant. This multifunctional catalyst gave moderate to good yields for both coupling reactions with high chemoselectivity. A wide investigation was conducted to determine its mechanism and chemoselectivity.

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.

Microdroplet fusion mass spectrometry: accelerated kinetics of acid-induced chlorophyll demetallation

Kinetics of acid-induced chlorophyll demetallation was recorded in microdroplets by fusing a stream of microdroplets containing 40 μM chlorophyll a or b dissolved in methanol with a stream of aqueous microdroplets containing 35 mM hydrochloric acid (pH = 1·46). The kinetics of the demetallation of chlorophyll in the fused microdroplets (14 ± 6 μm diameter; 84 ± 18 m s⁻¹ velocity) was recorded by controlling the traveling distance of the fused microdroplets between the fusion region and the inlet of a mass spectrometer. The rate of acid-induced chlorophyll demetallation was about 960 ± 120 times faster in the charged microdroplets compared with that reported in bulk solution. If no voltage was applied to the sprayed microdroplets, then the acceleration factor was about 580 ± 90, suggesting that the applied voltage is not a major factor determining the acceleration. Chlorophyll a was more rapidly demetallated than chlorophyll b by a factor of ~26 in bulk solution and ~5 in charged microdroplets. The demetallation kinetics was second order in the H⁺ concentration, but the acceleration factor of microdroplets compared with bulk solution appeared to be unchanged in going from pH = 1·3 to 7·0. The water:methanol ratio of the fused microdroplets was varied from 7:3 to 3:7 causing an increase in the reaction rate of chlorophyll a demetallation by 20%. This observation demonstrates that the solvent composition, which has different evaporation rates, does not significantly affect the acceleration. We believe that a major portion of the acceleration can be attributed to confinement effects involving surface reactions rather than either to evaporation of solvents or to the introduction of charges to the microdroplets.

In Vitro Enzymatic Activities of Bacteriochlorophyll a Synthase Derived from the Green Sulfur Photosynthetic Bacterium Chlorobaculum tepidum

The activity of an enzyme encoded by the CT1610 gene in the green sulfur photosynthetic bacterium Chlorobaculum tepidum, which was annotated as bacteriochlorophyll (BChl) a synthase, BchG (denoted as tepBchG), was examined in vitro using the lysates of Escherichia coli containing the heterologously expressed enzyme. BChl a possessing a geranylgeranyl group at the 17-propionate residue (BChl aGG) was produced from bacteriochlorophyllide (BChlide) a and geranylgeranyl pyrophosphate in the presence of tepBchG. Surprisingly, tepBchG catalyzed the formation of BChl a bearing a farnesyl group (BChl aF) as in the enzymatic production of BChl aGG, indicating loose recognition of isoprenoid pyrophosphates in tepBchG. In contrast to such loose recognition of isoprenoid substrates, BChlide c and chlorophyllide a gave no esterifying product upon being incubated with geranylgeranyl or farnesyl pyrophosphate in the presence of tepBchG. These results confirm that tepBchG undoubtedly acts as the BChl a synthase in Cba. tepidum. The enzymatic activity of tepBchG was higher than that of BchG of Rhodobacter sphaeroides at 45 °C, although the former activity was lower than the latter below 35 °C.

Demetalation kinetics of the zinc chlorophyll derivative possessing two formyl groups: effects of formyl groups conjugated to the chlorin macrocycle on physicochemical properties of photosynthetic pigments

Demetalation kinetics of zinc chlorophyll derivative 1 possessing two formyl groups directly linked to the A- and B-rings of the chlorin macrocycle, which was synthesized from chlorophyll b, was examined under acidic conditions and compared with those of Zn chlorins 2 and 3 possessing a single formyl group in the A- and B-ring, respectively, as well as Zn chlorin 4 lacking any formyl group to unravel the substitution effects on demetalation properties of chlorophyllous pigments. Demetalation kinetics of diformylated Zn chlorin 1 was slower than those of monoformylated Zn chlorins 2 and 3, indicating that the effect of the electron-withdrawing formyl group on demetalation kinetics was amplified by introduction of two formyl groups to the chlorin macrocycle. High correlations were observed between demetalation rate constants of Zn chlorins 1–4 and the sum of Hammett σ parameters of the 3- and 7-substituents on the chlorin macrocycle, indicating that the combination of electron-withdrawing abilities of the substituents linked directly to the cyclic tetrapyrrole was responsible for demetalation properties of zinc chlorophyll derivatives.

Conversion of chlorophyll b to chlorophyll a precedes magnesium dechelation for protection against necrosis in Arabidopsis

Chlorophyll is a deleterious molecule that generates reactive oxygen species and must be converted to nontoxic molecules during plant senescence. The degradation pathway of chlorophyll a has been determined; however, that of chlorophyll b is poorly understood, and multiple pathways of chlorophyll b degradation have been proposed. In this study, we found that chlorophyll b is degraded by a single pathway, and elucidated the importance of this pathway in avoiding cell death. In order to determine the chlorophyll degradation pathway, we first examined the substrate specificity of 7-hydroxymethyl chlorophyll a reductase. 7-hydroxymethyl chlorophyll a reductase reduces 7-hydroxymethyl chlorophyll a but not 7-hydroxymethyl pheophytin a or 7-hydroxymethyl pheophorbide a. These results indicate that the first step of chlorophyll b degradation is its conversion to 7-hydroxymethyl chlorophyll a by chlorophyll b reductase, although chlorophyll b reductase has broad substrate specificity. In vitro experiments showed that chlorophyll b reductase converted all of the chlorophyll b in the light-harvesting chlorophyll a/b protein complex to 7-hydroxymethyl chlorophyll a, but did not completely convert chlorophyll b in the core antenna complexes. When plants whose core antennae contained chlorophyll b were incubated in the dark, chlorophyll b was not properly degraded, and the accumulation of 7-hydroxymethyl pheophorbide a and pheophorbide b resulted in cell death. This result indicates that chlorophyll b is not properly degraded when it exists in core antenna complexes. Based on these results, we discuss the importance of the proper degradation of chlorophyll b.

Synthesis of ¹⁸O-labeled photosynthetically active chlorophylls at the 3- or 7-carbonyl group with high regioselectivity

The 3- and 7-formyl groups of chlorophyll-d (Chl-d) and bacteriochlorophyll-e (BChl-e), respectively, were regioselectively labeled with an isotopically stable oxygen-18 (¹⁸O) atom to give 3¹-¹⁸O-labeled Chl-d and 7¹-¹⁸O-labeled BChl-e (ca. 90% ¹⁸O) by exchanging the carbonyl oxygen atoms in the presence of acidic H₂ ¹⁸O (ca. 95% ¹⁸O). Another photosynthetically active chlorophyll, BChl-a possessing the 3-acetyl group was treated under similar acidic conditions to afford a trace amount of 3¹-¹⁸O-labeled BChl-a and further demetallated compound, the corresponding 3¹-¹⁸O-labeled bacteriopheophytin-a as the major product with 55% ¹⁸O-degree. The FT-IR spectra of ¹⁸O-(un)labeled chlorophylls in the solution and the solid states showed that the 3- and 7-carbonyl stretching vibration modes moved to about a 30-cm⁻¹ lower wavenumber by ¹⁸O-labeling at the 3¹- and 7¹-oxo moieties. In artificial chlorosome-like self-aggregates of BChl-e, the ¹⁸O-labeled 7-carbonyl stretching mode was completely resolved from the specially hydrogen-bonded 13-C=O stretching mode, evidently indicating no interaction of the 7-CHO with other functional groups in the supramolecules.