Steric Control of Bacteriochlorophyll Ligation

Dominant role of excitons in photosynthetic color-tuning and light-harvesting

Photosynthesis is a vital process that converts sunlight into energy for the Earth's ecosystems. Color adaptation is crucial for different photosynthetic organisms to thrive in their ecological niches. Although the presence of collective excitons in light-harvesting complexes is well known, the role of delocalized excited states in color tuning and excitation energy transfer remains unclear. This study evaluates the characteristics of photosynthetic excitons in sulfur and non-sulfur purple bacteria using advanced optical spectroscopic techniques at reduced temperatures. The exciton effects in these bacteriochlorophyll a-containing species are generally much stronger than in plant systems that rely on chlorophylls. Their exciton bandwidth varies based on multiple factors such as chromoprotein structure, surroundings of the pigments, carotenoid content, hydrogen bonding, and metal ion inclusion. The study nevertheless establishes a linear relationship between the exciton bandwidth and Qy singlet exciton absorption peak, which in case of LH1 core complexes from different species covers almost 130 nm. These findings provide important insights into bacterial color tuning and light-harvesting, which can inspire sustainable energy strategies and devices.

Nanoformulation of Tetrapyrroles Derivatives in Photodynamic Therapy: A Focus on Bacteriochlorin

Photodynamic therapy (PDT) is a well-known remedial treatment for cancer, infections, and various other diseases. PDT uses nontoxic dyes called photosensitizers (PS) that are activated in visible light at the proper wavelength to generate ROS (reactive oxygen species) that aid in killing tumor cells and destroying pathogenic microbes. Deciding a suitable photosensitizer is essential for enhancing the effectiveness of photodynamic therapy. It is challenging to choose the photosensitizer that is appropriate for specific pathological circumstances, such as different cancer species. Porphyrin, chlorin, and bacteriochlorin are tetrapyrroles used with proper functionalization in PDT, among which some compound has been clinically approved. Most photosensitizers are hydrophobic, have minimum solubility, and exhibit cytotoxicity due to the dispersion in biological fluid. This paper reviewed some nanotechnology-based strategies to overcome these drawbacks. In PDT, metal nanoparticles are widely used due to their enhanced surface plasmon resonance. The self-assembled nano-drug carriers like polymeric micelles, liposomes, and metal-based nanoparticles play a significant role in solubilizing the photosensitizer to make them biocompatible.

Fast Photochemical Oxidation of Proteins Maps the Topology of Intrinsic Membrane Proteins: Light-Harvesting Complex 2 in a Nanodisc

Although membrane proteins are crucial participants in photosynthesis and other biological processes, many lack high-resolution structures. Prior to achieving a high-resolution structure, we are investigating whether MS-based footprinting can provide coarse-grained protein structure by following structural changes that occur upon ligand binding, pH change, and membrane binding. Our platform probes topology and conformation of membrane proteins by combining MS-based footprinting, specifically fast photochemical oxidation of proteins (FPOP), and lipid Nanodiscs, which are more similar to the native membrane environment than are the widely used detergent micelles. We describe here results that show a protein's outer membrane regions are more heavily footprinted by OH radicals whereas the regions spanning the lipid bilayer remain inert to the labeling. Nanodiscs generally exhibit more protection of membrane proteins compared to detergent micelles and less shielding to those protein residues that exist outside the membrane. The combination of immobilizing the protein in Nanodiscs and footprinting with FPOP is a feasible approach to map extra-membrane protein surfaces, even at the amino-acid level, and to illuminate intrinsic membrane protein topology.

Lessons from chlorophylls: modifications of porphyrinoids towards optimized solar energy conversion

Practical applications of photosynthesis-inspired processes depend on a thorough understanding of the structures and physiochemical features of pigment molecules such as chlorophylls and bacteriochlorophylls. Consequently, the major structural features of these pigments have been systematically examined as to how they influence the S₁ state energy, lifetimes, quantum yields, and pigment photostability. In particular, the effects of the macrocyclic π-electron system, central metal ion (CMI), peripheral substituents, and pigment aggregation, on these critical parameters are discussed. The results obtained confirm that the π-electron system of the chromophore has the greatest influence on the light energy conversion capacity of porphyrinoids. Its modifications lead to changes in molecular symmetry, which determine the energy levels of frontier orbitals and hence affect the S₁ state properties. In the case of bacteriochlorophylls aggregation can also strongly decrease the S₁ energy. The CMI may be considered as another influential structural feature which only moderately influences the ground-state properties of bacteriochlorophylls but strongly affects the singlet excited-state. An introduction of CMIs heavier than Mg²⁺ significantly improves pigments' photostabilities, however, at the expense of S₁ state lifetime. Modifications of the peripheral substituents may also influence the S₁ energy, and pigments’ redox potentials, which in turn influence their photostability.

Theoretical study on excited states of bacteriochlorophyll a in solutions with density functional assessment

The excited-state properties of bacteriochlorophyll (BChl) a in triethylamine, 1-propanol, and methanol are investigated with the time-dependent density functional theory by using the quantum mechanical and molecular mechanical reweighting free energy self-consistant field method. It is found that no prevalent density functionals can reproduce the experimental excited-state properties, i.e., the absorption and reorganization energies, of BChl a in the solutions. The parameter μ in the range-separated hybrid functional is therefore optimized to reproduce the differences of the absorption energies in the solutions. We examine the origin of the differences of the absorption energies in the solutions and find that sensitive balance between contributions of structural changes and solute-solvent interactions determines the differences. The accurate description of the excitation with the density functional with the adjusted parameter is therefore essential to the understanding of the excited-state properties of BChl a in proteins and also the mechanism of the photosynthetic systems.

High-pressure and theoretical studies reveal significant differences in the electronic structure and bonding of magnesium, zinc, and nickel ions in metalloporphyrinoids

High pressure in combination with optical spectroscopy was used to gain insights into the interactions between Mg(2+), Zn(2+), and Ni(2+) ions and macrocyclic ligands of porphyrinoid type. In parallel, the central metal ion-macrocycle bonding was investigated using theoretical approaches. The symmetry properties of the orbitals participating in this bonding were analyzed, and pigment geometries and pressure/ligation effects were computed within DFT. Bacteriopheophytin a was applied as both a model chelator and a highly specific spectroscopic probe. The analysis of solvent and pressure effects on the spectral properties of the model Mg(2+), Zn(2+), and Ni(2+) complexes with bacteriopheophytin a shows that various chemical bonds are formed in the central pocket, depending on the valence configuration of the central metal ion. In addition, the character of this bonding depends on symmetry of the macrocyclic system. Since in most cases it is not coordinative bonding, these results challenge the conventional view of metal ion bonding in such complexes. In (labile) complexes with the main group metals, the metal ion-macrocycle interaction is mostly electrostatic. Significantly, water molecules are not preferred as a second axial ligand in such complexes, mainly due to the entropic constraints. The metal ions with a closed d shell may form (stable) complexes with the macrocycle via classical coordination bonds, engaging their p and s orbitals. Transition metals, due to the unfilled d shell, do form much more stable complexes, because of strong bonding via both coordination and covalent interactions. These conclusions are confirmed by DFT computations and theoretical considerations, which altogether provide the basis to propose a consistent and general mechanism of how the central metal ion and its interactions with the core nitrogens govern the physicochemical properties of metalloporphyrinoids.

Spectral shift mechanisms of chlorophylls in liquids and proteins

Origins of non-excitonic spectral shifts of chlorophylls that can reach -1,000 cm(-1) in pigment-protein complexes are actively debated in literature. We investigate possible shift mechanisms, basing on absorption and fluorescence measurements in large number of liquids. Transition wavelength in solvent-free state was estimated (±2 nm) for chlorophyll a (Chl a, 647 nm), Chl b (624 nm), bacteriochlorophyll a (BChl a, 752 nm), and pheophytines. The dispersive-repulsive shift is a predominating mechanism. It depends on polarizability difference between the ground and the excited state Δα and the Lorenz-Lorentz function of refractive index of solvent (n). The approximate (± 2Å(3)) increase of polarizability Δα is close to 15Å(3) for S(1) bands of Chl a, BChl a, and BPheo a, slightly larger for Chl b (18Å(3)), and less for Pheo a (11Å(3)). The effect of solvent polarity, expressed in terms of static dielectric permittivity (ε) is relatively minor, but characteristic for different pigments and transitions. Remarkably, maximum influence of ε on S(1) band of BChl a is less (-20 ± 10 cm(-1)) than that for Chl a (-50 ± 10 cm(-1)), and not correlated with dipole moment changes on excitation Δμ (∼2D and 0.1 ± 0.1D, respectively). Hydrogen bonding in protic solvents produces red shifts in Chl a (-60 cm(-1)) and BChl a (-100 cm(-1)), but not in Chl b. Second axial ligand of BChl a has no influence on the S(1) band, whereas the S(2) transition suffers a -400 to -600 cm(-1) down shift. Aromatic character of solvent is responsible for a ∼-100 cm(-1) red shift of both Q transitions in BChl a. The S(1) bands in chlorophylls are relatively insensitive with respect to dielectric properties and specific solvation. Therefore, nontrivial mechanisms, yielding large site-energy shifts are expected in photosynthetic chlorophyll-proteins.

Ligation of water to magnesium chelates of biological importance

Water binding to several Mg(2+) chelates, ethylenediamine, ethylenediamine-N,N'-diacetate, porphyrin, chlorophyll a and bacteriochlorophyll a, to form five- and six-coordinate complexes is studied by means of density functional theory. The results obtained for magnesium chelates are compared with the properties of the respective aqua complexes and the influence of the permittivity of environment on the ligand binding energies is discussed. Although the most common coordination number of Mg(2+) is six, in the tetrapyrrolic chelates it is reduced to five because the accommodation of the sixth water ligand results in no gain in energy. This is in line with the experimental observations made for coordination of chlorophylls in vivo. The binding between Mg(2+) and water is mostly of electrostatic nature, which is supported by the finding that its energy is correlated both with the electron density of the chelator and with electrostatic potential determined at the ligand binding site.

Ligation of water to magnesium chelates of biological importance

Water binding to several Mg(2+) chelates, ethylenediamine, ethylenediamine-N,N'-diacetate, porphyrin, chlorophyll a and bacteriochlorophyll a, to form five- and six-coordinate complexes is studied by means of density functional theory. The results obtained for magnesium chelates are compared with the properties of the respective aqua complexes and the influence of the permittivity of environment on the ligand binding energies is discussed. Although the most common coordination number of Mg(2+) is six, in the tetrapyrrolic chelates it is reduced to five because the accommodation of the sixth water ligand results in no gain in energy. This is in line with the experimental observations made for coordination of chlorophylls in vivo. The binding between Mg(2+) and water is mostly of electrostatic nature, which is supported by the finding that its energy is correlated both with the electron density of the chelator and with electrostatic potential determined at the ligand binding site.

Design principles for chlorophyll-binding sites in helical proteins

The cyclic tetrapyrroles, viz. chlorophylls (Chl), their bacterial analogs bacteriochlorophylls, and hemes are ubiquitous cofactors of biological catalysis that are involved in a multitude of reactions. One systematic approach for understanding how Nature achieves functional diversity with only this handful of cofactors is by designing de novo simple and robust protein scaffolds with heme and/or (bacterio)chlorophyll [(B)Chls]-binding sites. This strategy is currently mostly implemented for heme-binding proteins. To gain more insight into the factors that determine heme-/(B)Chl-binding selectivity, we explored the geometric parameters of (B)Chl-binding sites in a nonredundant subset of natural (B)Chl protein structures. Comparing our analysis to the study of a nonredundant database of heme-binding helical histidines by Negron et al. (Proteins 2009;74:400-416), we found a preference for the m-rotamer in (B)Chl-binding helical histidines, in contrast to the preferred t-rotamer in heme-binding helical histidines. This may be used for the design of specific heme- or (B)Chl-binding sites in water-soluble helical bundles, because the rotamer type defines the positioning of the bound cofactor with respect to the helix interface and thus the protein-binding site. Consensus sequences for (B)Chl binding were identified by combining a computational and database-derived approach and shown to be significantly different from the consensus sequences recommended by Negron et al. (Proteins 2009;74:400-416) for heme-binding helical proteins. The insights gained in this work on helix- (B)Chls-binding pockets provide useful guidelines for the construction of reasonable (B)Chl-binding protein templates that can be optimized by computational tools.

Structural and electronic effects in the metalation of porphyrinoids. Theory and experiment

The structure-reactivity relationships in metalation reactions of porphyrinoids have been studied using experimental and theoretical methods. A series of eight porphyrinoic ligands, derivatives of chlorophylls, was prepared in which both the peripheral groups and the degrees of saturation of the macrocycle were systematically varied. To reveal the solvent and structural factors which control the interactions of these macroligands with metal centers, their interactions with reactive Zn(2+) and inert Pt(2+) ions were investigated using absorption spectroscopy. In parallel, quantum chemical calculations (density functional theory, DFT) were performed for the same set of molecules to examine the influence of structural and electronic factors on the energy of the frontier orbitals, the nucleophilicity/electronegativity of the macrocycle, its hardness, and conformation. These static descriptors of chemical reactivity, relevant to metalation reactions, were verified against the results obtained in the experimental model. The experimentally obtained kinetic data clearly show that the solvent has a crucial role in the activation of the incoming metal center. In terms of chelator structure, the largest effects concern the size of the delocalized pi-electron system and the presence of side groups. Both the DFT calculations and experimental results show the strong influence of the macrocycle rigidity and of the peripheral groups on the chelating ability of porphyrinoids. In particular, the peripheral functionalization of the macrocyclic system seems to drastically reduce its reactivity toward metal ions. The effect of peripheral groups is two-fold: (i) a lower electron density on the core nitrogens, and (ii) increased rigidity of the macrocycle. The outcomes of the theoretical and experimental analyses are discussed also in terms of their relevance to the mechanism of biological metal insertion in the biosynthesis of heme and chlorophyll.

Characterization of the FMO protein from the aerobic chlorophototroph, Candidatus Chloracidobacterium thermophilum

Candidatus Chloracidobacterium (Cab.) thermophilum is a recently discovered aerobic chlorophototroph belonging to the phylum Acidobacteria. From analyses of genomic sequence data, this organism was inferred to have type-1 homodimeric reaction centers, chlorosomes, and the bacteriochlorophyll (BChl) a-binding Fenna-Matthews-Olson protein (FMO). Here, we report the purification and characterization of Cab. thermophilum FMO. Absorption, fluorescence emission, and CD spectra of the FMO protein were measured at room temperature and at 77 K. The spectroscopic features of this FMO protein were different from those of the FMO protein of green sulfur bacteria (GSB) and suggested that exciton coupling of the BChls in the FMO protein is weaker than in FMO of GSB especially at room temperature. HPLC analysis of the pigments extracted from the FMO protein only revealed the presence of BChl a esterified with phytol. Despite the distinctive spectroscopic properties, the residues known to bind BChl a molecules in the FMO of GSB are well conserved in the primary structure of the Cab. thermophilum FMO protein. This suggests that the FMO of Cab. thermophilum probably also binds seven or possibly eight BChl a(P) molecules. The results imply that, without changing pigment composition or structure dramatically, the FMO protein has acquired properties that allow it to perform light harvesting efficiently under aerobic conditions.

An overview on chlorophylls and quinones in the photosystem I-type reaction centers

Minor but key chlorophylls (Chls) and quinones in photosystem (PS) I-type reaction centers (RCs) are overviewed in regard to their molecular structures. In the PS I-type RCs, the prime-type chlorophylls, namely, bacteriochlorophyll (BChl) a' in green sulfur bacteria, BChl g' in heliobacteria, Chl a' in Chl a-type PS I, and Chl d' in Chl d-type PS I, function as the special pairs, either as homodimers, (BChl a')(2) and (BChl g')(2) in anoxygenic organisms, or heterodimers, Chl a/a' and Chl d/d' in oxygenic photosynthesis. Conversions of BChl g to Chl a and Chl a to Chl d take place spontaneously under mild condition in vitro. The primary electron acceptors, A (0), are Chl a-derivatives even in anoxygenic PS I-type RCs. The secondary electron acceptors are naphthoquinones, whereas the side chains may have been modified after the birth of cyanobacteria, leading to succession from menaquinone to phylloquinone in oxygenic PS I.

Interplay between acetate ions, peripheral groups, and reactivity of the core nitrogens in transmetalation of tetrapyrroles

The mechanism of acetate-assisted transmetalation of tetrapyrroles was investigated in a model system consisting of chlorophyll a and copper(II) acetate in organic solvents by using a spectroscopic and kinetic approach. Surprisingly, acetate ions bind to the central Mg in chlorophyll much more strongly than do acetonitrile, methanol and even pyridine, one of the best ligands in chlorophyllic systems. This exceptionally strong non-symmetrical axial ligation of the central Mg by acetate causes its out-of-plane displacement and deformation of the tetrapyrrole ring, thus facilitating the interaction with an incoming CuII complex. This mechanism is controlled by a keto-enol tautomerism of the chlorophyll isocyclic ring. Additionally, depending on solvent, acetate activates the incoming metal ions. These new insights allow to suggest a mechanism for the acetate method of metal exchange in tetrapyrrolic macrocycles, which resembles biological insertion of metal ions into porphyrins. It also provides a guideline for the design of more efficient methods for the metalation of porphyrins and related macrocycles.

Understanding chlorophylls: central magnesium ion and phytyl as structural determinants

Phytol, a C20 alcohol esterifying the C-17(3) propionate, and Mg2+ ion chelated in the central cavity, are conservative structural constituents of chlorophylls. To evaluate their intramolecular structural effects we prepared a series of metal- and phytyl-free derivatives of bacteriochlorophyll a and applied them as model chlorophylls. A detailed spectroscopic study on the model pigments reveals meaningful differences in the spectral characteristics of the phytylated and non-phytylated pigments. Their analysis in terms of solvatochromism and axial coordination shows how the central Mg and phytyl residue shape the properties of the pigment. Surprisingly, the presence/absence of the central Mg has no effect on the solvatochromism of (bacterio)chlorophyll pi-electron system and the hydrophobicity of phytyl does not interfere with the first solvation shell of the chromophore. However, both residues significantly influence the conformation of the pigment macrocycle and the removal of either residue increases the macrocycle flexibility. The chelation of Mg has a flattening effect on the macrocycle whereas bulky phytyl residue seems to control the conformation of the chromophore via steric interactions with ring V and its substituents. The analysis of spectroscopic properties of bacteriochlorophyllide (free acid) shows that esterification of the C-17(3) propionate is necessary in chlorophylls because the carboxyl group may act as a strong chelator of the central Mg. These observations imply that the truncated chlorophylls used in theoretical studies are not adequate as models of native chromophores, especially when fine effects are to be modeled.

Photostability of Bacteriochlorophyll a and Derivatives: Potential Sensitizers for Photodynamic Tumor Therapy

The photostabilities of bacteriochlorophyll a and several of its derivatives, which are of interest as potential sensitizers in photodynamic tumor therapy, were investigated. The pigments were irradiated with light >630 nm in organic solvents (acetone, tetrahydrofuran, pyridine, methanol, ethanol, n-propanol, 2-propanol and toluene) and in aqueous detergent solutions (cetyl-trimethyl-ammonium bromide [CTAB], lauryldimethyl-aminoxide [LDAO] or sodium dodecyl-sulfate [SDS] and Triton X-100 [TX100]). Their stabilities in these different solvents were determined in the presence and absence of an external sensitizer (pyromethyl-pheophorbide a), oxygen, sodium ascorbate and inert gas (Ar) or vacuum. The photodegradation products of bacteriochlorophyll a in acetone solution were isolated, purified by HPLC and analyzed by their absorption spectra and mass spectroscopy. Besides the well-known dehydrogenation products, such as [3-acetyl]-chlorophyll a, which were obtained as by-products, the major products had low absorption in the visible-near infrared spectral range. The spectral signature of the major component of these products was characteristic of linear open-chain tetrapyrroles, but they lacked the characteristic protonation-deprotonation behavior and reactivity of bilins with Zn(++).