Mutual control of axial and equatorial ligands: model studies with [Ni]-bacteriochlorophyll-a

Origin of the Enhanced Binding Capability toward Axial Nitrogen Bases of Ni(II) Porphyrins Bearing Electron-Withdrawing Substituents: An Electronic Structure and Bond Energy Analysis

Axial coordination to metalloporphyrins is important in many biological and catalytic processes. Experiments found the axial coordination of nitrogenous bases to nickel(II) porphyrins to be strongly favored by electron-withdrawing substituents such as perfluorophenyls at the meso carbon positions. Careful analysis of the electronic structure reveals that the natural explanation in terms of density change of the nickel(II) porphyrin system (in particular the metal), does not apply. Electron density changes, by the assumed inductive or polarizing effects on the metal or on the porphyrin ring system, are slight. The effect is caused by a remarkable through-space electric field effect on the metalloporphyrin system, originating from the charge distribution inside the perfluorphenyl groups (mostly the C-F dipoles).

Surface directed reversible imidazole ligation to nickel(ii) octaethylporphyrin at the solution/solid interface: a single molecule level study

STM and DFT calculations are used to study the reversible binding of imidazole to NiOEP supported on HOPG in solution.

Could structural similarity of specific domains between animal globins and plant antenna proteins provide hints important for the photoprotection mechanism?

Non photochemical quenching is a fundamental mechanism in photosynthesis, which protects plants against excess excitation energy and is of crucial importance for their survival and fitness. In the last decades hundreds of papers have appeared that describe the role of antenna regulation in protection or the so called qE response. However, the exact quenching site is still obscure. Previously overlooked features of the antenna may provide hints towards the elucidation of its functionality and of the quenching mechanism. Recently it was demonstrated that the catalytic domain of human myoglobin that binds the pigment (i.e. heme) is similar in structure to the domain of the light harvesting complex II of pea that binds Chl a 614 (former known as b3). In addition, it is well accepted that conformational changes of the chlorophyll macrocycle result in reversible changes of fluorescence (the lowest fluorescence corresponds to non planar macrocycle). Here we put forward a hypothesis regarding the molecular mechanism that leads to the formation of a quenching center inside the antenna proteins. Our main suggestion is that a conformational change of helix H5 (known also as helix D) forces conformational changes in the macrocycle of Chl a 614 is implicated in the ΔA535 absorbance change and quenching during photoprotective qE. The specific features (some of them similar to those of heme domain of globins) of the b3 domain account for these traits. The model predicts that antenna proteins having b3 pigments (i.e. LHCII, CP29, CP26) can act as potential quenchers.

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.

Comparison of chemical properties of iron, cobalt, and nickel porphyrins, corrins, and hydrocorphins

Density functional calculations have been used to compare the geometric, electronic, and functional properties of the three important tetrapyrrole systems in biology, heme, coenzyme B 12, and coenzyme F430, formed from iron porphyrin ( Por ), cobalt corrin ( Cor ), and nickel hydrocorphin ( Hcor ). The results show that the flexibility of the ring systems follows the trend Hcor > Cor > Por and that the size of the central cavity follows the trend Cor < Por < Hcor . Therefore, low-spin Co I, Co II, and Co III fit well into the Cor ring, whereas Por seems to be more ideal for the higher spin states of iron, and the cavity in Hcor is tailored for the larger Ni ion, especially in the high-spin Ni II state. This is confirmed by the thermodynamic stabilities of the various combinations of metals and ring systems. Reduction potentials indicate that the +I and +III states are less stable for Ni than for the other metal ions. Moreover, Ni – C bonds are appreciably less stable than Co - C bonds. However, it is still possible that a Ni – CH 3 bond is formed in F 430 by a heterolytic methyl transfer reaction, provided that the donor is appropriate, e.g. if coenzyme M is protonated. This can be facilitated by the adjacent SO 3− group in this coenzyme and by the axial glutamine ligand, which stabilizes the Ni III state. Our results also show that a Ni III– CH 3 complex is readily hydrolysed to form a methane molecule and that the Ni III hydrolysis product can oxidize coenzyme B and M to a heterodisulphide in the reaction mechanism of methyl coenzyme M reductase.

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.

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.

Modulation of Fragmental Charge Transfer via Hydrogen Bonds. Direct Measurement of Electronic Contributions†

Hydrogen bonds play an important role in an overwhelming variety of fields from biology to surface and supramolecular chemistry. The term "hydrogen bond" refers to a wide range of interactions with various covalent and polar contributions. In particular, hydrogen bonds have an important role in the folding and packing of peptides and nucleic acids. Recent studies also point to the importance of hydrogen bonding in the context of second-shell interactions, in metal binding and selectivity in metalloproteins, and in controlling the dynamics of membrane proteins. In this study, we demonstrate and quantify the modulation of fragmental charge transfer from hydrogen-bonded ligands to a metal center, by employing our recently introduced molecular potentiometer. The molecular details that affect this type of fragmental charge transfer are presented and a path for transferring chemical information is demonstrated. We found that H-bond interactions in the extended positions of axial ligands provide an effective means of modulating the amount of fragmental charge transfer to a metal center, thereby dramatically influencing the electronic properties of the ligand, the binding affinity, and the binding of additional ligands. The magnitude of fragmental charge-transfer modulation induced by a single ligand-solvent H-bond interaction is comparable to those induced by covalent substitution, although H-bond enthalpy is only on the order of several kilojoules per mole. Importantly, we find a significant change in the ligand electronic properties, even for weak C-H...O=C H-bond formation, where the bond enthalpy is substantially lower than for conventional H-bond interactions. The excess fragmental charge transferred to the metal center, deduced from the spectroscopic measurements, correlates well with the computationally determined values. Our findings underscore the importance of second-shell interactions in the active sites of enzymes, beyond the structural and electrostatic importance that is widely recognized today.

Steric Control of Bacteriochlorophyll Ligation

The axial coordination of central Mg(2+) ion in chlorophylls is of great structural and functional importance for virtually all photosynthetic chlorophyll proteins; however, little thermodynamic data are available on the ligand binding to these pigments. In the present study, spectral deconvolution of the bacteriochlorophyll Q(X) band serves to determine the ligand binding equilibria and relationships between thermodynamic parameters of ligand binding, ligand properties, and steric interactions occurring within the pigment. On the basis of the temperature effects on coordination, DeltaH degrees, DeltaS degrees, and DeltaG degrees of binding various types of ligands (acetone, dimethylformamide, imidazole, and pyridine) to diastereoisomeric bacteriochlorophylls were derived from respective van't Hoff's plots. At ambient temperatures, only ligation by imidazole and pyridine occurs spontaneously while DeltaG degrees becomes positive for ligation by acetone and dimethylformamide, due to a relatively large entropic effect, which is dominating when the energetic effects of ligation are small. It reflects, in quantitative terms, the control of the equatorial coordination of the Mg(2+) ion via the axial coordination: a "hard" free Mg(2+) ion is made into a softer center through the coordination of tetrapyrrole. Pigment structural features have comparable effects on the energetic and entropic contributions to the difference of ligation free energy between the diastereoisomers of bacteriochlorophyll. DeltaS degrees and DeltaH degrees values are consistently lower for the S epimer, most likely due to the steric crowding between bulky substituents. The two epimers show a 5 J.mol(-1).K(-1) difference in DeltaS degrees values, regardless of the ligand type, while the difference in DeltaH degrees amounts to 1.7 kJ.mol(-1), depending on the ligand. Such steric control of ligation would relate to the partial diastereoselectivity of chlorophyll self-assembly and, in particular, the very high diastereoselectivity of the ligation of chlorophylls in photosynthetic proteins.

Design of amphiphilic protein maquettes: controlling assembly, membrane insertion, and cofactor interactions

We have designed polypeptides combining selected lipophilic (LP) and hydrophilic (HP) sequences that assemble into amphiphilic (AP) alpha-helical bundles to reproduce key structure characteristics and functional elements of natural membrane proteins. The principal AP maquette (AP1) developed here joins 14 residues of a heme binding sequence from a structured diheme-four-alpha-helical bundle (HP1), with 24 residues of a membrane-spanning LP domain from the natural four-alpha-helical M2 channel of the influenza virus, through a flexible linking sequence (GGNG) to make a 42 amino acid peptide. The individual AP1 helices (without connecting loops) assemble in detergent into four-alpha-helical bundles as observed by analytical ultracentrifugation. The helices are oriented parallel as indicated by interactions typical of adjacent hemes. AP1 orients vectorially at nonpolar-polar interfaces and readily incorporates into phospholipid vesicles with >97% efficiency, although most probably without vectorial bias. Mono- and diheme-AP1 in membranes enhance functional elements well established in related HP analogues. These include strong redox charge coupling of heme with interior glutamates and internal electric field effects eliciting a remarkable 160 mV splitting of the redox potentials of adjacent hemes that leads to differential heme binding affinities. The AP maquette variants, AP2 and AP3, removed heme-ligating histidines from the HP domain and included heme-ligating histidines in LP domains by selecting the b(H) heme binding sequence from the membrane-spanning d-helix of respiratory cytochrome bc(1). These represent the first examples of AP maquettes with heme and bacteriochlorophyll binding sites located within the LP domains.

Design of amphiphilic protein maquettes: enhancing maquette functionality through binding of extremely hydrophobic cofactors to lipophilic domains

We demonstrate coordination of the extremely hydrophobic 13(2)-OH-Ni-bacteriochlorophyll (Ni-BChl) to the lipophilic domain of a novel, designed amphiphilic protein maquette (AP3) dispersed in detergent micelles [Discher et al. (2005) Biochemistry 44, 12329-12343]. Sedimentation velocity and equilibrium experiments and steady-state absorption spectra indicate that Ni-BChl-AP3 is a four-helix bundle containing one Ni-BChl axially ligated by one or two histidines. The nature of the ligation was pursued with ultrafast visible spectroscopy. While it is well established that light excitation of axially ligated mono- and bisimidazole Ni-BChl in solution leads to rapid imidazole dissociation and nanosecond recombination, there is no evidence of axial ligand dissociation in the light-excited Ni-BChl-AP3. This indicates that Ni-BChl is confined within the AP3 protein, ligated to histidines with severely restricted mobility. Dissociation constants show that Ni-BChl binding to AP3 is considerably weaker than the nanomolar range usual for heme and hydrophilic (HP) maquettes; moreover, there is a tendency for the Ni-BChl-AP3 four-helix bundles to dimerize into eight-helix bundles. Nevertheless, the preparation of the Ni-BChl-AP3 four-alpha-helix maquettes, supported by time-resolved spectroscopic analysis of the nature of the ligation, provides a viable new approach to AP maquette designs that address the challenges involved in binding extremely hydrophobic cofactors.

Uniform Approach to Bacteriochlorophyll-Based Monolayers on Conducting, Semiconducting, and Insulating Substrates

A general approach is demonstrated for the formation of monolayers comprised of free-base and metalated Bacteriochlorophyll-based derivatives providing a new vehicle for studying photosynthetic motifs and chromophore thin-film interactions. Accessibility to covalent and self-assembled systems on conducting, semiconducting, and insulating substrates is realized utilizing identical molecular building blocks. The monolayers retain the optical features typical for the new systems in solution. Molecular organization of chromophore interaction motifs can be sequentially designed using preassembled building blocks in solution and expressed in the thin film optical properties. For instance, intramolecular pi-pi stacking is conserved for the dimeric Ni-based chromophores as deduced from the spectroscopic measurements of the monolayers and in solution.

Direct Experimental Evaluation of Charge Scheme Performance by a Molecular Charge-Meter

The remarkable advances accomplished in the past two decades in theoretical and computational capabilities have made the in silico study of complex chemical systems feasible. However, this progress is in strong contrast to the lag in experimental capabilities relating to the measurement of fundamental chemical quantities within convoluted environments such as solvents or protein milieu. As a result, many works rely extensively on predictions provided by ab initio methodologies without having independent experimental support. Such a proliferation of theory and computational approaches without being substantiated by appropriate experimental data is undesirable. The feasibility of using nickel-bacteriochlorophyll as a molecular potentiometer was recently demonstrated for the systematic evaluation of fragmental charge density transfer for metal complexes in solution, thus providing an experimental assay with high accuracy and sensitivity (better than +/-0.005 e(-); Yerushalmi, R.; Baldridge, K. K.; Scherz, A. J. Am. Chem. Soc. 2003, 125, 12706-12707). Here the experimentally determined fragmental charge density transfer values measured by the molecular potentiometer for metal complexes in solvent are used to provide, for the first time, an independent and critical experimental evaluation of theoretical approaches commonly used in determining atomic charges and fragmental charge density transfer among interacting molecular systems. Importantly, these findings indicate that the natural population analysis (NPA) charge analysis is highly robust and well-suited for determining charge transfer processes involving donor-acceptor coordination interactions. The majority of computational charge schemes fail to provide an accurate chemical picture for the whole range of systems considered here. In cases where the role of electronic correlation varies significantly among chemically related structures, as with mono- and biligated complexes, the widely used electrostatic potential fit-based methods for evaluating atomic charges may prove to be problematic for predictive studies. In such cases, alternative methods that do not rely on the net dipole moment or other higher multipoles of the system for determining charges should be employed.

Control of redox transitions and oxygen species binding in Mn centers by biologically significant ligands; model studies with [Mn]-bacteriochlorophyll a

Mn-superoxide dismutase (Mn-SOD), which protects the cell from the toxic potential of superoxide radicals (O(2)(-*)), is the only type of SOD which resides in eukaryotic mitochondria. Up-to-date, the exact catalytic mechanism of the enzyme and the relationship between substrate moieties and the ligands within the active site microenvironment are still not resolved. Here, we set out to explore the possible involvement of hydroperoxyl radicals ((*)OOH) in the catalytic dismutaion by following the interplay of Mn(III)/Mn(II) redox transitions, ligands binding, and evolution or consumption of superoxide radical, using a new model system. The model system encompassed an Mn atom chelated by a bacteriochlorophyll allomer macrocycle (BChl) in aerated aprotic media that contain residual water. The redox states of the Mn ion were monitored by the Q(y) electronic transitions at 774 and 825 nm for [Mn(II)]- and [Mn(III)]-BChl, respectively (Geskes, C.; Hartwich, G.; Scheer, H.; Mantele, W.; Heinze, J. J. Am. Chem. Soc. 1995, 117, 7776) and confirmed by electron spin resonance spectroscopy. Evolution of (*)OOH radicals was monitored by the ESR spin-trap technique using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The experimental data suggest that the [Mn]-BChl forms a (HO(-))[Mn(III)]-BChl(OOH) complex upon solvation. Spectrophotometeric titrations with tetrabutylamonnium acetate (TBAA) and 1-methylimidazole (1-MeIm) together with ESI-MS measurements indicated the formation of a 1:1 complex with [Mn]-BChl for both ligands. The coordination of ligands at low concentrations to [Mn(III)]-BChl induced a release of a (*)OOH radical and a [Mn(III)]-BChl --> [Mn(II)]-BChl transition at higher concentrations. The estimated equilibrium constants for the total redox reaction ( )()are 1.9 x 10(4) +/- 1 x 10(3) M(-)(1) and 12.3 +/- 0.6 M(-)(1) for TBAA and 1-MeIm, respectively. The profound difference between the equilibrium constants agrees with the suggested key role of the ligand's basicity in the process. A direct interaction of superoxide radicals with [Mn(III)]-BChl in a KO(2) acetonitrile (AN) solution also resulted in [Mn(III)]-BChl --> [Mn(II)]-BChl transition. Cumulatively, our data show that the Mn(III) center encourages the protonation of the O(2)(-)(*) radical in an aprotic environment containing residual water molecules, while promoting its oxidation in the presence of basic ligands. Similar coordination and stabilization of the (*)OOH radical by the Mn center may be key steps in the enzymatic dismutation of superoxide radicals by Mn-SOD.