High-valent transition metal centers versus noninnocent ligands in metallocorroles: insights from electrochemistry and implications for high-valent heme protein intermediates

For relatively electron-rich corrole ligands, the halfwave potentials for oxidation of Cu(III), Sn(IV)Ph, Fe(IV)Ph, and Fe(IV)-O-Fe(IV) complexes are significantly lower than those of Sn(IV)Cl, Fe(IV)Cl, Mn(IV)Cl, and Cr(V)(O) complexes, suggesting that the corrole ligand is relatively electron-rich or 'innocent' in the former group of complexes and that it is relatively electron-deficient or 'noninnocent' in the latter group. Both the formal charge of the central metal ion and the nature of the axial ligand, if any, appear to be key determinants of the electronic character of the corrole ligand in metallocorrole complexes, a theme that has interesting resonances with recent findings on high-valent heme protein intermediates. However, for very strongly electron-deficient ligands such as meso-tris(pentafluorophenyl)corrole (TPFPC) and beta-octabromo-meso-tris(pentafluorophenyl)corrole (Br(8)TPFPC), which cannot sustain significant radical character, the various metal complexes all exhibit comparable halfwave potentials for oxidation and the ligand may be considered to be relatively innocent.     

High-valent corrole metal complexes

This commentary concentrates on corrole complexes with the three metal ions that are most relevant to oxidation catalysis: chromium, manganese, and iron. Particular emphasis is devoted to the only recently introduced meso-triarylcorroles and a comparison with the traditionally investigated beta-pyrrole-substituted corroles. Based on a combination of spectroscopic methods, electrochemistry, and X-ray crystallography, it is concluded that in most high-valent metallocorroles the corrole is not oxidized. Both experimental (for (oxo)chromium(V) corrole) and computational (for (oxo)manganese(V) corrole) evidence indicate that the stabilization of high-valent metal ions by corroles originates from a combination of short metal-nitrogen bonds and large metal out-of-plane displacements in the corrole, which lead to quite unexpected interactions of the oxo-metal pi* orbitals with the in-plane orbitals of the corrole.     

Cobalt(IV) corroles as catalysts for the electroreduction of O2: reactions of heterobimetallic dyads containing a face-to-face linked Fe(III) or Mn(III) porphyrin

A series of heterobinuclear cofacial porphyrin-corrole dyads containing a Co(IV) corrole linked by one of four different spacers in a face-to-face arrangement with an Fe(III) or Mn(III) porphyrin have been examined as catalysts for the electroreduction of O(2) to H(2)O and/or H(2)O(2) when adsorbed on the surface of a graphite electrode in air-saturated aqueous solutions containing 1M HClO(4). The examined compounds are represented as (PCY)M(III)ClCo(IV)Cl where P is a porphyrin dianion, C is a corrole trianion and Y is a biphenylene (B), 9,9-dimethylxanthene (X), dibenzofuran (O) or anthracene (A) spacer. The catalytic behavior of the seven investigated dyads in the two heterobimetallic (PCY)MClCoCl series of catalysts is compared on one hand to what was previously reported for related dyads with a single Co(III) corrole macrocycle linked to a free-base porphyrin with the same set of linking bridges, (PCY)H(2)Co, and on the other hand to dicobalt porphyrin-corrole dyads of the form (PCY)Co(2) which were shown to efficiently electrocatalyze the four electron reduction of O(2) at a graphite electrode in acid media. Comparisons between the four series of porphyrin-corrole dyads, (PCY)Co(2), (PCY)H(2)Co, (PCY)FeClCoCl and (PCY)MnClCoCl, show that in all cases the biscobalt dyads catalyze O(2) electroreduction at potentials more positive by an average 110mV as compared to the related series of compounds containing a Co(III) or Co(IV) corrole macrocycle linked to a free-base metalloporphyrin or a metalloporphyrin with an Fe(III) or Mn(III) central metal ion. The data indicates that the E(1/2) values where electrocatalysis is initiated is related to the initial site of electron transfer, which is the Co(III)/Co(II) porphyrin reduction process in the case of (PCY)Co(2) and the Co(IV)/Co(III) corrole reduction in the case of (PCY)MnClCoCl, (PCY)FeClCoCl and (PCY)H(2)Co. The overall data also suggests that the catalytically active form of the biscobalt dyad in (PCY)Co(2) contains a Co(II) porphyrin and a Co(IV) corrole.     

Aluminum speciation and morphological differentiation in murine neuroblastoma cells

Aluminum is known as a neurotoxic metal ion in experimental animals as well as in humans. The present study was carried out to determine whether and how the physicochemical properties of the metal coordination sphere (metal speciation) can influence the differentiation of murine neuroblastoma cells as has been observed previously in this laboratory (1). Results herein reported indicate that while the aluminum lipophilic species—particularly aluminum acetylacetonate (Alacac₃) and aluminum maltolate (Almalt₃), both hydrolitically stable and differently lipophilic—are both rather cytotoxic, metal hydrophilic species show different neuritogenic properties indicating the ability of Al(III), when diversely coordinated, to produce different biological effects.     

Migration of electron excitation energy in mixed associates of chlorophyll and its derivatives]

The intracomplex migration is investigated of electron excitation energy between molecules which are a part of a mixed associate. Weakly fluorescent and nonfluorescent associates of chlorophyll and its analogues as well as of bacteriochlorophyll and neoxanthine have been used as donors and acceptors. The migration of excitation energy with variation of quantum yield of donor emission from 5-10(-3) to less than or equal to 10(-4) and also with very weak overlapping of luminescence and absorption spectra for the pair D leads to A is experimentally discovered by luminescence excitation spectra of mixed pigment aggregates. A specific disturbing action of Pd and Cu ions on chlorophyll a luminescence with formation of mixed complexes of chlorophyll with Pd-pheophytin a and Cu-pheophytin b is observed. The presence of Pd and Cu ions in the structure of the mixed complexes gives rise to a considerable reduction of chlorophyll apparently due to increasing intersystem crossing probability in the chlorophyll molecule. The probability of intracomplex energy migration in systems under investigation amounts to approximately 10(12) sec-1 according to the obtained estimations. The probable mechanism of energy migration in mixed associates is discussed. The obtained results may be used for elucidation of a possible role of the mixed aggregates and effective intracomplex migration of excitation energy in photosynthetic apparatus of green plants.     

Thermodynamics of ATP hydrolysis from membrane electrode measurements of metal-ion ATP and ADP complexation

Free energies for the ATP-ADP hydrolysis reaction have been recalculated on the basis of new thermodynamic formation constants for metal-ion ATP and ADP complexes as determined by potentiometric measurements with ion-selective membrane electrodes. It is shown that free-energy maps are sharply altered at metal-ion levels greater than 10^?3m because of the effect of previously unrecognized 2:1 complexes of divalent metal ions with ATP and ADP. New formation constants for ADP complexes with sodium, potassium, calcium, and magnesium are also reported.     

Phosphorescence of octaethylchlorine, isobacteriooctaethylchlorine and their metal complexes]

The phosphorescence of dihydrooctaethylporphin (octaethylchlorin or OEC), of its complexes with magnesium, zinc, copper and palladium, and of zinc and palladium complexes of isobacteriooctaethylchlorin (5,6,7,8-tetrahydrooctaethylporphin with adjacent hydrogenated pyrrole rings or THOEP-ADJ) has been investigated. The phosphorescence spectra and phosphorescence excitation spectra as well as the ratio of fluorescence and phosphorescence yields and the triplet state lifetume have been measured. It has been shown that the singlet-triplet interval is about 4100 cm-1 for OEC complexes and about 4300 cm-1 for THOEP-ADJ complexes, and depends wealky on the nature of the metal atom forming the complex. The triplet level position of chlorophyll alpha is discussed. It is concluded that the maximum of chlorophyll alpha phosphorescence spectrum must be located at 895 nm.     

Low-lying electronic states of carotenoids.

Four all-trans carotenoids, spheroidene, 3,4-dihydrospheroidene, 3,4,5,6-tetrahydrospheroidene, and 3,4,7,8-tetrahydrospheroidene, have been purified using HPLC techniques and analyzed using absorption, fluorescence and fluorescence excitation spectroscopy of room temperature solutions. This series of molecules, for which the extent of pi-electron conjugation decreases from 10 to seven carbon-carbon double bonds, exhibits a systematic crossover from S2----S0 (1(1)Bu----1(1)Ag) to S1----S0 (2(1)Ag----1(1)Ag) emission with decreasing chain length. Extrapolation of the S1----S0 transition energies indicates that the 2(1)Ag states of longer carotenoids have considerably lower energies than previously thought. The energies of the S1 states of spheroidenes and other long carotenoids are correlated with the S1 energies of their chlorophyll partners in antenna complexes of photosynthetic systems. Implications for energy transfer in photosynthetic antenna are discussed.     

Nonequivalence of the metal binding sites of conalbumin. Calorimetric and spectrophotometric studies of aluminum binding.

Differential scanning calorimetric experiments show that addition of Al(III) to conalbumin increases its denaturation temperature by 5 degrees, from 60 to 68 degrees. Only one Al(III) bound per conalbumin molecule produces this change in heat stability; additional bound Al(III) does not affect the heat stability. Since Al(III) displaces both Cu(II) bound at the metal binding sites of conalbumin, binding of aluminum takes place at the same metal binding sites. The binding constant for the second Al(III) is at least 100-fold less than that for the binding of the first Al(III), and both are displaced by added iron. The order of increasing heat stability of the metal ion complexes of conalbumin, Cu(II), Al(III), Fe(III), is the order of increasing binding constant for these metal ions.     

Comaprative binding study of aluminum and chromium to human transferrin

The characteristics of aluminum and chromium binding to apotransferrin (apo-tf) have been investigated and compared. Both metal ions were taken up by human transferrin forming complexes with the maximum absorbances at 405 nm for chromium-transferrin (cr-tf) and 240 nm for aluminum-transferrin (Al-tf). In the presence of citric acid, chromium binding to transferrin is five times more than aluminum. The binding of aluminum or chromium to apo-transferrin was reduced by 18 and 22% in the presence of 200 ng/mL of iron. The binding of both metals to apo-tf appears to be pH dependent. In acidic pHs, less chromium and more aluminum binding occurred.     

Exchange of pigment-binding amino acids in light-harvesting chlorophyll a/b protein

Four amino acids in the major light-harvesting chlorophyll (Chl) a/b complex (LHCII) that are thought to coordinate Chl molecules have been exchanged with amino acids that presumably cannot bind Chl. Amino acids H68, Q131, Q197, and H212 are positioned in helixes B, C, A, and D, respectively, and, according to the LHCII crystal structure [Kühlbrandt, W., et al. (1994) Nature 367, 614-621], coordinate the Chl molecules named a(5), b(6), a(3), and b(3). Moreover, a double mutant was analyzed carrying exchanges at positions E65 and H68, presumably affecting Chls a(4) and a(5). All mutant proteins could be reconstituted in vitro with pigments, although the thermal stability of the resulting mutant versions of recombinant LHCII varied significantly. All complexes reconstituted with the mutant proteins contained fewer chlorophyll molecules per two lutein molecules than complexes reconstituted with the wild-type protein. However, the chlorophyll-binding amino acids could not be unambiguously assigned to binding either chlorophyll a or b, as in most cases more than one chlorophyll molecule was lost due to the mutation. The changes in Chl stoichiometries suggest that in LHCII some chlorophyll positions can be filled with either Chl a or b. Only some of the point mutations in LHCII affected the ability of the apoprotein to assemble into trimeric LHCII upon insertion into isolated thylakoid membranes. Among these were exchanges of H68 with either F or L, suggesting that the stability of the LHCII trimer significantly depends on this amino acid or the Chl molecule named a(5) that is attached to it and is located close to the center of the trimeric complex. The ion pair bridge between E65 and R185 in LHCII does not appear to be essential for the proper folding of the protein.     

A GREEN DINOFLAGELLATE WITH CHLOROPHYLLS a and b: MORPHOLOGY,FINE STRUCTURE OF THE CHLOROPLAST AND CHLOROPHYLL COMPOSITION1

A green-colored marine unicell has been grown in unialgal culture and its morphology, chloroplast fine structure, and chlorophyll composition investigated. The organism is typical of dinoflagellates in its shape, flagellation, nucleus, mitochondria, and trichocysts. It is similar to Gymnodinium but possesses fine body scales. Chloroplasts and two kinds of vesicles bounded by double membranes, but no organelles obviously identifiable as nuclei or mitochondria, are associated in ribosome-dense cytoplasm separated by a double membrane from the dinophycean cytoplasm. The chloroplasts are unlike any previously reported for dinoflagellates. Each is enclosed by an envelope consisting of a double membrane. Chloroplast lamellae consist of three appressed thylakoids. Interlamellar pyrenoids are present. Pigment analysis reveals chlorophylls a and b but not chlorophyll c. It seems likely that the organism is an undescribed dinoflagellate containing an endosymbiont with chlorophylls a and b and that the reduction of the endosymbiont nucleus and mitochondria has permitted a more initmate symbiosis.     

Very strong metal ligand aromatic cation-π interactions in transition metal complexes: intermolecular interaction in tetraphenylborate salts

Strong intermolecular interactions between ligands in cationic metal complexes and aromatic rings of tetraphenylborate anion, so-called metal ligand aromatic cation-π (MLACπ) interactions, were found by screening Cambridge Structural Database. Distances between phenyl ring and ligand are shorter in these structures than in previously reported MLACπ interactions by 0.2 Å.     

Methane and methyl halide activation by Sc atoms: Infrared spectra and DFT calculations for the CH3-ScX and CH2-ScHX complexes

Reactions of laser-ablated scandium atoms with methane and methyl halides have been done during condensation in excess argon, and matrix infrared spectra were recorded for the products. Scandium is as effective as titanium in producing insertion products (CH₃-ScX, X = H, F, Cl, Br) and higher oxidation-state methylidene complexes (CH₂-ScHX) following α-hydrogen migration. However, unlike the Group 4-6 metal complexes, the Group 3 methylidene complexes do not show agostic distortion, and the computed C-Sc bond lengths of the methylidene complexes are slightly shorter than those for the insertion products. This shows that carbon-transition metal bond lengths in the double bond range are needed to support the agostic interaction. The computed spin densities are consistent with weak C(p)-Sc(d) π-bonding in the methylidene complexes.     

Photosynthesis in trees: organization of chlorophyll and photosynthetic unit size in isolated gymnosperm chloroplasts

Chloroplasts have been isolated in high yield from several gymnosperms and from two deciduous trees. The organization of chlorophyll in the chloroplasts of these woody species is basically similar to that in angiosperm crop plants and green algae. The tree chloroplasts contain two chlorophyll proteins, the P700-chlorophyll a-protein and the major light-harvesting chlorophyll a/b-protein, the size, spectral characteristics, and function of which are the same as the equivalent complexes previously isolated from other classes of green plants. All the gymnosperms have chlorophyll/P700 ratios (photosynthetic unit sizes) 1.6 to 3.8 times larger than that typically found in crop plants; the deciduous trees have units of intermediary size. The presence of fewer but larger photosynthetic units in the woody species can partially account for their lower photosynthetic rate and explains why their photosynthetic processes saturate at lower light intensities. Chloroplasts of shade needles have large units containing a greater proportion of the light-harvesting chlorophyll a/b-protein than those of sun needles.     

Crystal structures of dialkylglycine decarboxylase inhibitor complexes.

The crystal structures of four inhibitor complexes of dialkylglycine decarboxylase are reported. The enzyme does not undergo a domain closure, as does aspartate aminotransferase, upon inhibitor binding. Two active-site conformations have been observed in previous structures that differ in alkali metal ion content, and two active-site conformations have been shown to coexist in solution when a single type of metal ion is present. There is no indication of coexisting conformers in the structures reported here or in the previously reported structures, and the observed conformation is that expected based on the presence of potassium in the enzyme. Thus, although two active-site conformations coexist in solution, a single conformation, corresponding to the more active enzyme, predominates in the crystal. The structure of 1-aminocyclopropane-1-carboxylate bound in the active site shows the aldimine double bond to the pyridoxal phosphate cofactor to be fully out of the plane of the coenzyme ring, whereas the Calpha-CO2(-) bond lies close to it. This provides an explanation for the observed lack of decarboxylation reactivity with this amino acid. The carboxylate groups of both 1-aminocyclopropane-1-carboxylate and 5'-phosphopyridoxyl-2-methylalanine interact with Ser215 and Arg406 as previously proposed. This demonstrates structurally that alternative binding modes, which constitute substrate inhibition, occur in the decarboxylation half-reaction. The structures of d and l-cycloserine bound to the active-site show that the l-isomer is deprotonated at C(alpha), presumably by Lys272, while the d-isomer is not. This difference explains the approximately 3000-fold greater potency of the l versus the d-isomer as a competitive inhibitor of dialkylglycine decarboxylase.     

Impact of the invasive polychaete Marenzelleria viridis on the biogeochemistry of sandy marine sediments

Strong correlations of soil total organic carbon (OC) with iron and aluminum phases reported frequently make it important to quantify these organic matter (OM) associations, but selective extractants sometimes contain OC. Soil nitrogen is often predominantly organic and might serve as a proxy for OM. We therefore investigated nitrogen associations with Fe and Al using several selective extractants that use reductive, complexation, and alkaline approaches. Total dissolved nitrogen (TDN) correlated strongly with extracted Fe and Al across seventeen samples, including highly- and weakly-weathered soils, iron-rich ultrabasic soils, podzolic, and volcanic soils. Typically a quarter to a third of total soil nitrogen was dissolved by the various extractions, though higher fractions (up to 60%) were found in spodic-horizon and volcanic surface-horizon samples. Similar proportions were found for OC, using three OC-free extractants, indicating that TDN provides a useful surrogate for assessing OM partitioning via extractants that contain OC. Use of TDN:metal ratios in extractant solutions allows estimation of extracted OM that could have been sorptively associated with metal oxide/hydroxides and poorly-crystalline aluminosilicates. These ratios were often high in extractions targeted at these adsorbents, and imply that usually most of the extracted TDN consists instead of organo–metal complexes. The dynamics of these complexes may have stronger control on accumulation/remobilization of soil OM than those of metal oxyhydroxides and poorly-crystalline aluminosilicates.     

Formation of stable and metastable porphyrin- and corrole-iron(IV) complexes and isomerizations to iron(III) macrocycle radical cations

Oxidations of three porphyrin-iron(III) complexes (1) with ferric perchlorate, Fe(ClO₄)₃, in acetonitrile solutions at −40 °C gave metastable porphyrin-iron(IV) diperchlorate complexes (2) that isomerized to known iron(III) diperchlorate porphyrin radical cations (3) when the solutions were warmed to room temperature. The 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetramesitylporphyrin (TMP), and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) systems were studied by UV-visible spectroscopy. Low temperature NMR spectroscopy and effective magnetic moment measurements were possible with the TPP and TMP iron(IV) complexes. Reactions of two corrole systems, 5,10,15-tris(pentafluorophenyl)corrole (TPFC) and 5,15-bis(pentafluorophenyl)-10-p-methoxyphenylcorrole (BPFMC), also were studied. The corrole-iron(IV) chlorides reacted with silver salts to give corrole-iron(IV) complexes. The corrole-iron(IV) nitrate complexes were stable at room temperature. (TPFC)-iron(IV) toslyate, (TPFC)-iron(IV) chlorate, and (BPFMC)-iron(IV) chlorate were metastable and rearranged to their electronic isomers iron(III) corrole radical cations at room temperature. (TPFC)-iron(III) perchlorate corrole radical cation was the only product observed from reaction of the corrole-iron(IV) chloride with silver perchlorate. For the metastable iron(IV) species, the rates of isomerizations to the iron(III) macrocycle radical cation electronic isomers in dilute acetonitrile solutions were relatively insensitive to electron demands of the macrocyclic ligand but reflected the binding strength of the ligand to iron. Kinetic studies at varying temperatures and concentrations indicated that the mechanisms of the isomerization reactions are complex, involving mixed order reactivity.     

Magnetic circular dichroism spectroscopy of cobalt tetraphenyltetraacenaphthoporphyrin

The first MCD spectral data for an open shell first row transition metal complex of tetraphenyltetraacenaphthoporphyrin (TPTANP) are reported. The B (or Soret) band of cobalt tetraphenyltetraacenaphthoporphyrin (Co(II)TPTANP(-2)) exhibits an anomalous negative Faraday A(1) term as was reported previously in the case of ZnTPTANP, while a positive A(1) term is observed for the Q band. INDO/1 geometry optimizations predict that the TPTANP ligand is saddled due to steric hindrance at the ligand periphery to a slightly lesser extent than is the case with ZnTPTANP. The Q and B bands of CoTPTANP arising from the pi-system are blue shifted relative to those of ZnTPTANP, based on the "hypso" effect reported previously for planar porphyrin complexes of d(6-9) transition metals.     

Decreasing the chlorophyll a/b ratio in reconstituted LHCII: structural and functional consequences.

Trimeric (bT) and monomeric (bM) light-harvesting complex II (LHCII) with a chlorophyll a/b ratio of 0.03 were reconstituted from the apoprotein overexpressed in Escherichia coli. Chlorophyll/xanthophyll and chlorophyll/protein ratios of bT complexes and 'native' LHCII are rather similar, namely, 0.28 vs 0. 27 and 10.5 +/- 1.5 vs 12, respectively, indicating the replacement of most chlorophyll a molecules with chlorophyll b, leaving one chlorophyll a per trimeric complex. The LD spectrum of the bT complexes strongly suggests that the chlorophyll b molecules adopt orientations similar to those of the chlorophylls a that they replace. The circular dichroism (CD) spectra of bM and bT complexes indicate structural arrangements resembling those of 'native' LHCII. Thermolysin digestion patterns demonstrate that bT complexes are folded and organized like 'native' trimeric LHCII. Surprisingly, in the bT complexes at 77 K, half of the excitations that are created on either chlorophyll b or xanthophyll are transferred to chlorophyll a. No or very limited triplet transfer from chlorophyll b to xanthophyll appears to take place. However, the efficiency of triplet transfer from chlorophyll a to xanthophyll is close to 100%, even higher than in 'native' LHCII at 77 K. It is concluded from the triplet-minus-singlet and CD results that the single chlorophyll a molecule that on the average is present in each bT complex binds preferably next to a xanthophyll molecule at the interface between the monomers.