Comparison of Physicochemical Properties of Metallobacteriochlorophylls and Metallochlorophylls

Factors controlling the reactivity of divalent metal ions towards pheophytin a

In this study, we evaluate the factors which determine the reactivity of divalent metal ions in the spontaneous formation of metallochlorophylls, using experimental and computational approaches. Kinetic studies were carried out using pheophytin a in reactions with various divalent metal ions combined with non- or weakly-coordinative counter ions in a series of organic solvents. To obtain detailed insights into the solvent effect, the metalations with the whole set of cations were investigated in three solvents and with Zn²⁺ in seven solvents. The reactions were monitored using electronic absorption spectroscopy and the stopped-flow technique. DFT calculations were employed to shed light on the role of solvent in activating the metal ions towards porphyrinoids. This experimental and computational analysis gives detailed information regarding how the solvent and the counter ion assist/hinder the metalation reaction as activators/inhibitors. The metalation course is dictated to a large extent by the reaction medium, via either the activation or deactivation of the incoming metal ion. The solvent may affect the metalation in several ways, mainly via H-bonding with pyrrolenine nitrogens and the activation/deactivation of the incoming cation. It also seems to affect the activation enthalpy by causing slight conformational changes in the macrocyclic ligand. These new mechanistic insights contribute to a better understanding of the “metal–counterion–solvent” interplay in the metalation of porphyrinoids. In addition, they are highly relevant to the mechanisms of metalation reactions catalyzed by chelatases and explain the differences between the insertion of Mg²⁺ and other divalent cations.

Redox potential of chlorophyll d in vitro

Chlorophyll (Chl) d is a major chlorophyll in a novel oxygenic prokaryote Acaryochloris marina. Here we first report the redox potential of Chl d in vitro. The oxidation potential of Chl d was +0.88 V vs. SHE in acetonitrile; the value was higher than that of Chl a (+0.81 V) and lower than that of Chl b (+0.94 V). The oxidation potential order, Chl b>Chl d>Chl a, can be explained by inductive effect of substituent groups on the conjugated pi-electron system on the macrocycle. Corresponding pheophytins showed the same order; Phe b (+1.25 V)>Phe d (+1.21 V)>Phe a (+1.14 V), but the values were significantly higher than those of Chls, which are rationalized in terms of an electron density decrease in the pi-system by the replacement of magnesium with more electronegative hydrogen. Consequently, oxidation potential of Chl a was found to be the lowest among Chls and Phes. The results will help us to broaden our views on photosystems in A. marina.

Theoretical study on the thermodynamic properties of chlorophyll d-peptides coordinating ligand

The chlorophyll d containing cyanobacterium, Acaryochloris marina has provided a model system for the study of chlorophyll replacement in the function of oxygenic photosynthesis. Chlorophyll d replaces most functions of chlorophyll a in Acaryochloris marina. It not only functions as the major light-harvesting pigment, but also acts as an electron transfer cofactor in the primary charge separation reaction in the two photosystems. The Mg-chlorophyll d-peptide coordinating interaction between the amino acid residues and chlorophylls using the latest semi-empirical PM5 method were examined. It is suggested that chlorophyll d possesses similar coordination ligand properties to chlorophyll a, but chlorophyll b possesses different ligand properties. Compared with other studies involving theoretical correlation and our prior experiments, this study suggests that the chlorophyll a-bound proteins will bind chlorophyll d without difficulty when chlorophyll d is available.

Energy and electron transfer in the photosynthetic reaction center complex of Acidiphilium rubrum containing Zn-bacteriochlorophyll a studied by femtosecond up-conversion spectroscopy

A photosynthetic reaction center (RC) complex was isolated from a purple bacterium, Acidiphilium rubrum. The RC contains bacteriochlorophyll a containing Zn as a central metal (Zn-BChl a) and bacteriopheophytin a (BPhe a) but no Mg-BChl a. The absorption peaks of the Zn-BChl a dimer (P(Zn)), the accessory Zn-BChl a (B(Zn)), and BPhe a (H) at 4 K in the RC showed peaks at 875, 792, and 753 nm, respectively. These peaks were shorter than the corresponding peaks in Rhodobacter sphaeroides RC that has Mg-BChl a. The kinetics of fluorescence from P(Zn)(*), measured by fluorescence up-conversion, showed the rise and the major decay with time constants of 0.16 and 3.3 ps, respectively. The former represents the energy transfer from B(Zn)(*) to P(Zn), and the latter, the electron transfer from P(Zn) to H. The angle between the transition dipoles of B(Zn) and P(Zn) was estimated to be 36 degrees based on the fluorescence anisotropy. The time constants and the angle are almost equal to those in the Rb. sphaeroides RC. The high efficiency of A. rubrum RC seems to be enabled by the chemical property of Zn-BChl a and by the L168HE modification of the RC protein that modifies P(Zn).

Effects of heavy central metal on the ground and excited states of chlorophyll

Chlorophylls, owing to their adjustable pi-electron system and intense, well-separated electronic transitions, can serve as convenient intrinsic spectroscopic probes of ligand-metal center interactions. They are also interesting for their photosensitizing properties. In order to examine the heavy-atom effects on the chlorophyll triplet state, a key intermediate in chlorophyll-photosensitized reactions, the synthesis of a novel Pt(II)-substituted chlorophyll a was carried out, and the effects of the substitution on steady-state and transient photophysical properties of chlorophyll were studied by absorption and fluorescence spectroscopies, and by laser flash photolysis. The presence of highly electronegative platinum as the central ion increases the energies of the chlorophyll main absorption transitions. As laser flash photolysis experiments show, in air-equilibrated solutions, chlorophyll triplets are efficiently quenched by molecular oxygen. Interestingly, this quenching by oxygen is more effective with metal-containing pigments, in spite of the increased spin-orbit coupling, introduced with the central metals. This points to occurrence of nonspecific interactions of molecular oxygen with metallochlorophylls. The differences in the effects exerted on the pigment triplet by the central metal become distinct after the removal of oxygen. The lifetime of a Pt-chlorophyll triplet remains very short, in the range of only a few microseconds, unlike in the free-base and Mg- and Zn-substituted chlorophylls. Such drastic shortening of the triplet lifetime can be attributed to a large heavy-atom effect, implying that strong interactions must occur between the central Pt(II) ion and the chlorophyll macrocycle, which lead to a more efficient spin-orbit coupling in Pt-chlorophyll than in Pt-porphyrins.

The Formation of Zn-Chl a in Chlorella Heterotrophically Grown in the Dark with an Excessive Amount of Zn2+

Chlorella, when heterotrophically cultivated in the dark, is able to grow with Zn2+ at 10-40 mM, which is 10 times the concentration lethal to autotrophically grown cells. However, the lag phase is prolonged with increasing concentrations of Zn2+; for example, in this study, 1 d of the control lag phase was prolonged to about 16 d with Zn2+ at 16.7 mM (x2,000 of the control). Once the cells started to grow, the log phase was finished within 4-6 d regardless of Zn concentration, which was almost the same as that of the control. The photosysystem I reaction center chlorophyll, P700, and the far-red fluorescence were detected only after the late log phase of the growth curve, suggesting that chlorophyll-protein complexes can be organized after cell division has ceased. Interestingly, at more than 16.7 mM of Zn2+, Zn-chlorophyll a was accumulated and finally accounted for about 25% of the total chlorophyll a in the late stationary phase. We found that the Zn-chlorophyll a was present in the thylakoid membranes and not in the soluble fractions of the cells. The rather low fluorescence yield at around 680 nm in the stationary phase suggests that Zn-chlorophyll a can transfer its excitation energy to other chlorophylls. Before accumulation of Zn-chlorophyll a, a marked amount of pheophytin a was temporally accumulated, suggesting that Zn-chlorophyll a could be chemically synthesized via pheophytin a.

Aerobic anoxygenic photosynthetic bacteria with zinc-bacteriochlorophyll

Naturally occurring chlorophyllous pigments, which function as the cofactor in the early photochemical reaction of photosynthesis, have been proven beyond question to be magnesium-complexed porphyrin derivatives. Phototrophic organisms that use (bacterio)chlorophylls ([B]Chls) containing metals other than Mg were unknown for a long time. This common knowledge of natural photosynthesis has recently been modified by the striking finding that a novel purple pigment, zinc-chelated-BChl (Zn-BChl) a, is present as the major and functional pigment in species of the genus Acidiphilium. Acidiphilium species are obligately acidophilic chemoorganotrophic bacteria that grow and produce photopigments only under aerobic conditions. Although the mechanism of photosynthesis with Zn-BChl a in Acidiphilium species is similar to that seen in common purple bacteria, some characteristic photosynthetic features of the acidophilic bacteria are also found. The discovery of natural photosynthesis with Zn-BChl has not only provided a new insight into our understanding of bacterial photosynthesis but also raised some interesting questions to be clarified. The major questions are why the acidophilic bacteria have selected Zn-BChl for their photosynthesis and how they synthesize Zn-BChl and express photosynthetic activity with it in their natural habitats. In this article we review the current knowledge of the biology of Acidiphilium as aerobic photosynthetic bacteria with Zn-BChl a and discuss the interesting topics noted above.