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PubMed2001

Synthesis and cleavage reactions of metal-metal-bonded [Mo(2)(S(2)CNR(2))(6)](OTf)(2), a source of the Tris(dithiocarbamato)molybdenum(IV) fragment.

Halide abstraction from the chlorotris(dialkyldithiocarbamato)molybdenum(IV) complexes MoCl(S(2)CNR(2))(3) (R = Et, Me) with silver triflate produces the diamagnetic dimeric complexes [Mo(2)(S(2)CNR(2))(6)](OTf)(2) in good yield. The crystallographically determined structure of the diethyldithiocarbamato complex indicates that the dimer consists of two pentagonal bipyramids sharing an axial edge, with a Mo-Mo separation (2.8462(8) A) indicative of a metal-metal bond. A qualitative analysis of the bonding indicates that this bond is of order 2 and consists of one normal sigma bond and one relatively weak "skewed pi" interaction. The dimers [Mo(2)(S(2)CNR(2))(6)](OTf)(2) react with a variety of reagents to give monomeric seven-coordinate complexes, including the new cationic molybdenum(IV) complex [Mo(PMe(2)Ph)(S(2)CNEt(2))(3)](OTf), which has been structurally characterized. Kinetic studies of the reaction of [Mo(2)(S(2)CNEt(2))(6)](OTf)(2) with halides indicate the presence of competing dissociative and associative substitution pathways, although neutral donors may react by different mechanisms.

Journal Article↗

PubMed1992

The cerebellum and the adaptive coordination of movement.

Based on a review of cerebellar anatomy, neural discharge in relation to behavior, and focal ablation syndromes, we propose a model of cerebellar function that we believe is both comprehensive as to the available information (at these levels) and unique in several respects. The unique features are the inclusion of new information on (a) cerebellar output--its replicative representation of body maps in each of the deep nuclei, each coding a different type and context of movement, and each appearing to control movement of multiple body parts more than of single body parts; and (b) the newly assessed long length of the parallel fiber. The parallel fiber, by virtue of its connection through Purkinje cells to the deep nuclei, appears optimally designed to combine the actions at several joints and to link the modes of adjacent nuclei into more complex coordinated acts. We review the old question of whether the cerebellum is responsible for the coordination of body parts as opposed to the tuning of downstream executive centers, and conclude that it is both, through mechanisms that have been described in the cerebellar cortex. We argue that such a mechanism would require an adaptive capacity, and support the evidence and interpretation that it has one. We point out that many parts of the motor system may be involved in different types of motor learning for different purposes, and that the presence of the many does not exclude an existence of the one in the cerebellar cortex. The adaptive role of the cerebellar cortex would appear to be specialized for combining simpler elements of movement into more complex synergies, and also in enabling simple, stereotyped reflex apparatus to respond differently, specifically, and appropriately under different task conditions. Speed of learning and magnitude of memory for both novel synergies and task-specific performance modifications are other attributes of the cerebellar cortex.

Adaptation, Physiological↗

PubMed2004

Mechanism of dioxygen activation in 2-oxoglutarate-dependent enzymes: a hybrid DFT study.

The reaction mechanism for dioxygen activation in 2-oxoglutarate-dependent enzymes has been studied by means of hybrid density functional theory. The results reported here support a mechanism in which all chemical transformations take place on a quintet potential-energy surface. More specifically, the activated dioxygen species attacks the carbonyl group of the co-substrate producing the Fe(II)-persuccinate-CO(2) complex, which readily releases the carbon dioxide molecule. The step in which the Fe(II)-peracid-CO(2) complex is formed is found to be rate-limiting and irreversible. Subsequent heterolysis of the Obond;O bond in the Fe(II)-persuccinate complex proceeds in two one-electron steps and produces the high-valent iron-oxo species Fe(IV)dbond;O, which is most likely to be responsible for oxidative reactions catalyzed by 2-oxoglutarate-dependent enzymes. The concerted pathway for simultaneous Obond;O and Cbond;C bond cleavage on the septet potential-energy surface is found to be less favorable. The relative stability of different forms of the active iron-oxo species is assessed, and the quintet five-coordinate complex is found to be most stable.

Binding Sites↗

PubMed1988

Use of hemopexin domains and monoclonal antibodies to hemopexin to probe the molecular determinants of hemopexin-mediated heme transport.

Plasmin cleaves rabbit serum apohemopexin (Mr = 60,000) at a single site producing a heme-binding domain (I, Mr = 35,000) and a second domain (II, Mr = 25,000) (W. T. Morgan and A. Smith (1984) J. Biol. Chem. 259, 12001-12005). The absorbance spectra of heme-domain I are indicative of a bis-histidyl coordination complex with the central heme iron atom. Chemical modification of the 5 histidine residues of apo-domain I with diethylpyrocarbonate abolished heme binding, supporting this assignment. Upon binding heme, domain I migrates more rapidly in sucrose gradients, and, in sedimentation velocity experiments, the s value of domain I increases from 3.17 +/- 0.04 to 3.71 +/- 0.09, a notably large increase which indicates that the domain becomes much more compact. This conformational change which plays a pivotal role in hemopexin function requires the bis-histidyl coordination with heme iron and leads to a tighter association between domain I and domain II shown by the co-migration of heme-domain I and domain II in sucrose gradients. In turn, the association of heme-domain I with domain II increases the thermal stability of the heme-domain I chromophore. Results of binding studies using mouse hepatoma cells and isolated domains indicate that domain I not only binds heme but also plays a vital part in the hemopexin-receptor interaction. The change in conformation of domain I upon heme binding and the association between domains I and II induced by heme are both notable determinants of the strength of the hemopexin-receptor interaction, but an intact "hinge region" between the domains is not necessary for receptor binding. The importance of both domains in bringing about the transport function of hemopexin is confirmed by the ability of three (two specific for domain I and one for domain II) of seven monoclonal antibodies raised against hemopexin to inhibit the hemopexin-receptor interaction.

Animals↗

PubMed1992

1,10-Phenanthroline-copper, a footprinting reagent for single-stranded regions of RNAs.

The 1,10-phenanthroline-cuprous complex (OP-Cu) with hydrogen peroxide as a coreactant nicks the single-stranded loops and bulges of RNA stem-loop structures more rapidly than the double-stranded stems. This chemical nuclease is therefore a useful footprinting reagent for these regions and can be used to monitor both intramolecular and intermolecular hybridization of single-stranded domains. The formation of A-form structures characteristic of either RNA-RNA or RNA-DNA duplexes inhibits scission because it blocks the binding site of the coordination complex in single-stranded loops and not because the oxidatively sensitive hydrogens of the ribose moiety are blocked. The C-4' and C-1' hydrogens are accessible to solvent in A-structures.

Base Sequence↗

PubMed1993

Homodinuclear (Pt,Pt) and heterodinuclear (Ru,Pt) metal compounds as DNA-protein cross-linking agents: potential suicide DNA lesions.

Homodinuclear (Pt,Pt) and heterodinuclear (Ru,Pt) metal compounds having the generalized formula M(a)NH2(CH)4NH2M(b) are shown to form specific DNA lesions which can efficiently cross-link proteins to DNA. In this study, the homodinuclear case is represented by M(a) = M(b) = [cis-Pt(Cl2)-(NH3)] and the heterodinuclear case is represented by M(a) = [cis-RuCl2(DMSO)3] and M(b) = [cis-PtCl2(NH3)]. Native and denaturing polyacrylamide gel electrophoresis was used to show the formation of ternary coordination complexes between the metal-treated 49-bp DNA fragment and the Escherichia coli UvrA and UvrB DNA repair proteins. Treatment with proteinase K results in loss of the DNA-protein cross-links. DNA-protein cross-links formed between UvrA and DNA previously modified with the dinuclear metal compounds are reversible with the reducing agent beta-mercaptoethanol. The DNA lesion responsible for efficient DNA-protein cross-linking is most probably a DNA-DNA interstrand cross-link in which each metal atom is coordinated with one strand of the DNA helix. The formation of DNA repair protein associated DNA cross-links, potential "suicide adducts", suggests a novel action mechanism for these anticancer compounds. In addition, these dinuclear metal compounds should be very useful agents for the investigation of a wide range of protein-DNA interactions.

Adenosine Triphosphatases↗

PubMed2001

Ligand-field analysis of an Er(III) complex with a heptadentate tripodal N4O3 ligand.

Polarized absorption and emission spectra of trigonal single crystals of an Er(III) complex coordinated to a heptadentate tripodal ligand are reported at temperatures between 8 and 298 K. The assigned energy levels below the onset of ligand absorption (< 25 000 cm(-1)) are fitted to a parametrized electronic Hamiltonian. The C(3) site symmetry of the Er(III) ion requires eight parameters for a full description of the ligand field within a one-electron operator description. This compound shows unusually large splittings of the multiplets, and the fitted parameters imply that this heptadentate ligand imparts the largest ligand field reported for an Er(III) complex. The ligand field was also interpreted within the angular overlap model (AOM). We derive the AOM matrix to include both sigma and anisotropic pi bonding and show that a useful description of the C(3) ligand field can be made using only five parameters. The success of the AOM description is encouraging for applications on isomorphous complexes within the lanthanide series and in describing the ligand field of low-symmetry complexes with less parameters than in the usual spherical harmonic expansion.

Journal Article↗

PubMed2005

Ligand orientation control in low-spin six-coordinate (porphinato)iron(II) species.

The synthesis of a low-spin six-coordinate iron(II) porphyrinate in which the two axial ligands are forced to have a relative perpendicular orientation has been successfully accomplished for the first time. The reaction of four-coordinate (tetramesitylporphinato)iron(II) with 2-methylimidazole leads to the preparation of [Fe(TMP)(2-MeHIm)(2)] which cocrystallizes with five-coordinate [Fe(TMP)(2-MeHIm)]. The six-coordinate complex accommodates the sterically crowded pair of imidazoles with a strongly ruffled core and relative perpendicular orientation. This leads to shortened equatorial bonds of 1.963(6) A and slightly elongated axial Fe-N bond lengths of 2.034(9) A that are about 0.04 A shorter and 0.03 A longer, respectively, in comparison to those of the bis-imidazole-ligated iron(II) species with parallel oriented axial ligands. The Mossbauer spectrum shows a pair of quadrupole doublets that can be assigned to the components of the cocrystallized crystalline solid. High-spin five-coordinate [Fe(TMP)(2-MeHIm)] has DeltaE(Q) = 2.25 mm/s and delta = 0.90 mm/s at 15 K. The quadrupole splitting, DeltaE(Q), for [Fe(TMP)(2-MeHIm)(2)] is 1.71 mm/s, and the isomer shift is 0.43 mm/s at 15 K. The quadrupole splitting value is significantly larger than that found for low-spin iron(II) derivatives with relative parallel orientations for the two axial ligands. Mossbauer spectra thus provide a probe for ligand orientation when structural data are otherwise not available.

Crystallography, X-Ray↗

PubMed1998

The roles of individual gamma-carboxyglutamate residues in the solution structure and cation-dependent properties of conantokin-T.

The solution structure of the Ca2+-loaded conantokin-T (con-T), a gamma-carboxyglutamate (Gla)-containing 21-residue peptide (NH2-G1EgammagammaY5QKMLgamma10NLRgammaA15EVKKN20A-CONH2,gam ma = Gla), has been elucidated by use of distance geometry calculations with experimental distances derived from two-dimensional 1H NMR spectroscopy. An end-to-end alpha-helix was the dominant conformation in solution, similar to that of apo-con-T, except that reorientation of several side chains occurred in the Ca2+-coordinated complex. The most notable examples of this were those of Gla10 and Gla14, which were more optimally positioned for complexation with Ca2+. In addition to the stabilization offered to the alpha-helix by Ca2+ binding, hydrophobic clustering of the side chains of Tyr5, Met8, Leu9, and Leu12, and ionic interactions between Lys7 and Gla3/Gla10 and between Arg13 and Gla14, along with hydrogen bonding between Gln6 and Gla10, were among the side chain interactions likely playing a significant role in maintenance of the alpha-helical conformation. Docking of Ca2+ in the con-T structure was accomplished using genetic algorithm-molecular dynamics simulation approaches. The results showed that one Ca2+ ion is most likely coordinated by four side chain oxygen atoms, two each from Gla10 and Gla14. Another bound Ca2+ ion has as its donor sites three oxygen atoms, two from Gla3 and one from Gln6. To examine the functional roles of the individual Gla residues, a series of variant peptides have been synthesized with Ala substituted for each Gla residue, and several properties of the resulting variants have been examined. The data obtained demonstrated the importance of Gla10 and Gla14 in stabilizing binding of the highest affinity Ca2+ site and in governing the conformational change induced by Ca2+. The critical nature of Gla3 and Gla4 in inhibition of the spermine-induced potentiation of the binding of MK-801 to open ion channels of the N-methyl-D-aspartate receptor was established, as well as the role of Gla4 in stabilizing the apo-con-T alpha-helical conformation.

1-Carboxyglutamic Acid↗

PubMed2001

The encapsulation of ferrocyanide by copper(II) complexes of tripodal tetradentate ligands. Novel H-bonding networks incorporating heptanuclear and pentanuclear heterometallic assemblies.

Substitution of the weakly binding aqua ligand in [Cu(tren)OH2](2+) and [Cu(tpa)OH2](2+) (tren = tris(2-aminoethyl)amine; tpa = tris(2-pyridylmethyl)amine) by a cyano ligand on ferricyanide results in the assembly of heteropolynuclear cations around the cyanometalate core. In water, the reduction of the Fe(III) core to Fe(II) generates complexes that feature heteropolycations in which ferrocyanide is encapsulated by the Cu(II) moieties: [(Cu(tpa)CN)6Fe][ClO4]8-3H2O 1, [(Cu(tren)CN)6Fe][ClO4]8-10H2O 2, [(Cu(tren)CN)6Fe][Fe(CN)6]2[ClO4]2-15.8H2O 3, and [(Cu(tren)CN)6Fe][(Cu(tren)CN)4Fe(CN)2][Fe(CN)6)]4-6DMSO-21H2O 4. The formation of discrete molecules, in preference to extended networks or polymeric structures, has been encouraged through the use of branched tetradentate ligands in conjunction with copper(II), a metal center with the propensity to form five-coordinate complexes. Complex 3 crystallizes in the monoclinic space group P2(1)/c (#14) with a = 14.8674(10), b = 25.9587(10), c = 27.5617(10) A, beta = 100.8300(10) degrees, and Z = 4, and it is comprised of almost spherical heptanuclear cations, [(Cu(tren)CN)6Fe](8+), whose charge is balanced by two ferricyanide and two perchlorate counteranions. Complex 4 crystallizes in the triclinic space group P1 (# 1) with a = 14.8094(8), b = 17.3901(7), c = 21.1565(11) A, alpha = 110.750(3), beta = 90.206(2), gamma = 112.754(3) degrees, and Z = 1, and it is comprised of the heptanuclear [(Cu(tren)CN)6Fe](8+) cation and pentanuclear [(Cu(tren)CN)4Fe(CN)2](4+) cation, whose terminal cyano ligands are oriented trans to each other. The charge is balanced exclusively by ferricyanide counteranions. In both complexes, H-bonding interactions between hydrogens on primary amines of the tren ligand, terminal cyano groups of the ferricyanide counterions, and the solvent of crystallization generate intricate 3D H-bonding networks.

Journal Article↗

PubMed1998

Electronic State Structure and Optical Properties of Tb(oda)(3)(3)(-) Complexes in Trigonal Na(3)[Tb(oda)(3)].2NaClO(4).6H(2)O Crystals.

Polarized optical absorption and emission measurements are used to locate and assign 95 crystal-field energy levels split out of the 4f( )(8) electronic configuration of Tb(3+) in single crystals of Na(3)[Tb(oda)(3)].2NaClO(4).6H(2)O (where oda denotes an oxydiacetate ligand). The absorption measurements span the 235-490 nm wavelength range, and the emission measurements span the 485-685 nm wavelength range. The combined absorption and emission spectra measurements provide access to the energy-level structures of 46 different 4f( )(8)[SL]J multiplet manifolds of Tb(3+) (all multiplet manifolds with baricenter energies <42 400 cm(-)(1) above ground). The site symmetry of the Tb(3+) ions in Na(3)[Tb(oda)(3)].2NaClO(4).6H(2)O is D(3), and the point-group symmetry of the tris-terdentate Tb(oda)(3)(3)(-) coordination complexes is also D(3). The Tb(oda)(3)(3)(-) complexes are chiral, and they exist in just one, fully resolved enantiomeric form in single crystals of Na(3)[Tb(oda)(3)].2NaClO(4).6H(2)O. The crystals exhibit strong chiroptical activity in their absorption and emission spectra, and results obtained from both circularly polarized and linearly polarized optical spectra measurements are used in making transition line assignments. The energy-level data acquired from the spectroscopic measurements are analyzed in terms of a model Hamiltonian that includes consideration of both isotropic and nonisotropic 4f electron/crystal-field interactions, and the interaction parameters derived from this analysis are discussed and then compared with those obtained for other Na(3)[Ln(oda)(3)].2NaClO(4).6H(2)O systems and for Tb(3+) in other crystalline hosts.

Journal Article↗

PubMed1996

Photoperturbation of the heme a3-CuB binuclear center of cytochrome c oxidase CO complex observed by Fourier transform infrared spectroscopy.

Purified cytochrome c oxidase CO complex from beef heart has been studied by Fourier transform infrared absorbance difference spectroscopy. Photolysis at 10-20 Kelvin results in dissociation of a3FeCO, formation of CuBCO, and perturbation of the a3-heme and CuB complex. The vibrational perturbation spectrum between 900 and 1700 cm-1 contains a wealth of information about the binuclear center. Appearance in infrared photoperturbation difference spectra of virtually all bands previously reported from resonance Raman spectra indicate the importance of polarization along the 4-vinyl:8-formyl axis, which results in the reduction of heme symmetry to C2v. Frequency-shifted bands due to the 8-formyl and 4-vinyl groups of the a3-heme have been identified and quantitated. The frequency shifts have been interpreted as being due to a change in porphyrin polarization with change in spin state of the iron by photodissociation of CO or perturbation of the CuB coordination complex.

Animals↗

PubMed1977

Resonance Raman spectroscopy of arsanilazocarboxypeptidase A: determination of the nature of the azotyrosyl-248-zinc complex.

Resonance Raman spectra of arsanilazotryosyl-248 carboxypeptidase A (peptidyl-L-amino-acid hydrolase, EC 3.4.12.2) exhibit only the vibrational bands of its chromophoric azotyrosyl-248 residue uncomplicated by background interference from either water or other components of the protein. The resonance Raman spectra contain multiple, discrete bands which change as a function of pH, thereby demonstrating the existence of interconvertible species of the azotyrosine probe in solution. Spectra of model azophenols and of the apoazoenzyme establish the identity of these species. All conclusions about the azoenzyme based on the resonance Raman spectra, including the apparent pK values for the interconversion of these species, are in complete agreement with those drawn earlier from studies by absorption spectroscopy. In addition, the properties of resonance Raman bands that have been identified with the motions of specific atoms of azotyrosyl-248 provide details of the interactions of specific atoms of this chromophore with the catalytic zinc atom at the active site. In particular, this has allowed elucidation of the structure of the azotyrosyl-248-zinc coordination complex. Such experiments are also providing information on the effects of crystallization on the enzyme and on its interaction with inhibitors. The important potential of resonance Raman spectroscopy for the study of the structure of chromophoric components of active enzymatic sites and of metal complex ions is discussed.

Arsenicals↗

PubMed2026

Methylome profiling of SetDB1-deficient ESCs reveals coordinated epigenetic cross-talk during pluripotency.

SetDB1 is best known for catalyzing H3K9me3, but it also influences H3K27me3 deposition, CTCF-binding, and DNA methylation (DNAme). Given the interplay between DNAme and the other epigenetic features, we profiled DNAme following Setdb1 knockout (KO) in ground-state and serum-grown mouse embryonic stem cells (ESCs) to illuminate DNAme-dependent and -independent functions of SetDB1. Time-course whole-genome bisulfite sequencing of serum-grown ESCs shows that nearly half of SetDB1 binding sites are enriched with DNAme and H3K9me3, primarily at retrotransposons. Upon Setdb1 KO, both H3K9me3 and DNAme are reduced, with DNAme rapidly removed at many sites by TET enzymes. Some retrotransposons, primarily IAPs, are TET-resistant and lose DNAme slowly via passive dilution. Notably, SetDB1-mediated regulation of H3K27me3, CTCF-binding, and SMAD3 are uncoupled from the DNAme-H3K9me3 axis, and from each other. AlphaFold modeling and co-immunoprecipitation mass spectrometry suggest this uncoupling involves competitive binding to distinct SetDB1 protein domains, highlighting the complex coordination underlying SetDB1 functions.

AlphaFold modeling↗

PubMed2001

Ring-opening polymerisation of silver-diphosphine [M2L3] coordination cages to give [M2L3]infinity coordination polymers.

[M2L3] coordination cages and linear [M2L3]infinity polymers of the rigid, bridging diphosphines bis(diphenylphosphino)acetylene (dppa) and trans-1,2-bis(diphenylphosphino)ethylene (dppet) with silver(I) salts have been investigated in the solution and solid states. Unlike flexible diphosphines, 1:1 dppa/AgX mixtures do not selectively form discrete [Ag2(diphos)2(X)2] macrocycles; instead dynamic mixtures of one-, two- and three-coordinate complexes are formed. However, 3:2 dppa/AgX ratios (X = SbF6. BF4, O3SCF3 or NO3) do lead selectively to new [M2L3] triply bridged cage complexes [Ag2(dppa)3(X)2] 1a-d (X = SbF6 a, BF4 b, O3SCF3 c, NO3 d), which do not exhibit Ag-P bond dissociation at room temperature on the NMR time scale (121 MHz). Complexes la-d were characterised by X-ray crystallography and were found to have small internal cavities, helical conformations and multiple intramolecular aromatic interactions. The nucleophilicity of the anion subtly influences the cage shape: Increasing nucleophilicity from SbF6 (1a) through BF4 (1b) and O3SCF3 (1c) to NO3 (1d) increases the pyramidal distortion at the AgP3 centres, stretching the cage framework (with Ag...Ag distances increasing from 5.48 in 1a to 6.21 A in 1d) and giving thinner internal cavities. Crystal packing strongly affected the size of the helical twist angle, and no correlation between this parameter and the Ag-Ag distance was observed. When crystalline 1c was stored in its supernatant for 16 weeks, conversion occured to the isostoichiometric [M2L3]infinity coordination polymer [Ag(dppa)2Ag(dppa)(O3SCF3)2]infinity (1c'). X-ray crystallography revealed a structure with ten-membered Ag2(dppa)2 rings linked into infinite one-dimensional chains by a third dppa unit. The clear structural relationship between this polymer and the precursor cage 1c suggests a novel example of ring-opening polymerisation. With dppet, evidence for discrete [M2L3] cages was also found in solution, although 31P NMR spectroscopy suggested some Ag-P bond dissociation. On crystallisation, only the corresponding ring-opened polymeric structures [M2L3]infinity could be obtained. This may be because the greater steric bulk of dppet versus dppa destabilises the cage and favours the ring-opening polymerisation.

Journal Article↗

PubMed2026

Molecular Drivers of Mutualistic Association Between Anemone and Anemonefish.

The anemone-anemonefish mutualism is one of the most iconic in the marine environment. While the evolution of this mutualistic relationship has contributed to the ecological success of both partners, the underlying molecular processes that establish and maintain it remain poorly understood, particularly how anemonefish tolerate anemone venom. Here, we characterize the transcriptional dynamics in both the anemonefish Amphiprion clarkii and its host anemone Entacmaea quadricolor 48 h after association, providing a rare insight into the coordinated molecular processes in both partners that underlie symbiosis establishment. Upon acclimation with an anemone, anemonefish showed differential regulation of sensory perception and memory genes in key brain regions, indicating activation of neural pathways that may facilitate host recognition and mutualism establishment. In the fish's skin, altered expression of genes involved in neurotransmitter release, cytoskeleton organization, and venom receptor proteins points to mechanisms of resistance to anemone venom. This resistance is particularly remarkable since anemone hosting fish exhibited increased expression of genes encoding mechanoreceptors, putative venom-associated proteins, and ion channels involved in nematocyst discharge, indicating the anemone does indeed mount an active response to their mutualistic partner. By simultaneously capturing the molecular responses of both symbiotic partners, our results reveal the complex, coordinated interplay of molecular events in both species that play a pivotal role in establishing this mutualistic relationship.

Symbiosis↗

PubMed2004

High-frequency and -field EPR of a pseudo-octahedral complex of high-spin Fe(II): bis(2,2'-bi-2-thiazoline)bis(isothiocyanato)iron(II).

A pseudo-octahedral complex of high-spin Fe(II), bis(2,2'-bi-2-thiazoline)bis(isothiocyanato)iron(II), which has a cis-FeN'2N4 chromophore, has been investigated by high-frequency, high-field electron paramagnetic resonance (HFEPR). Complementary Mössbauer and DC magnetic susceptibility studies were also performed. HFEPR spectra of powder samples were recorded at frequencies up to 700 GHz and over a magnetic field range of 0-25 T. Analysis of the field-frequency data set yields the following set of spin Hamiltonian parameters for S = 2: D = +12.427(12) cm-1, E = +0.243(3) cm-1; gx = 2.147(3), gy = 2.166(3), gz = 2.01(1). The parameters are analyzed by use of a simple crystal-field model. This study represents the first precise determination by HFEPR of spin Hamiltonian parameters in six-coordinate high-spin Fe(II) and indicates the applicability of HFEPR to the study of high-spin Fe(II) in coordination complexes and biological model compounds.

Computer Simulation↗

PubMed2000

Orientation of iron bleomycin and porphyrin complexes on DNA fibers.

Bleomycin (Blm) is an antitumor agent that requires iron and oxygen for strand cleavage of DNA. In this study, ferric bleomycin, Fe(III)Blm, or the nitric oxide adduct of ferrous bleomycin, ON-Fe(II)Blm, were bound to one-dimensionally oriented DNA fibers. Reductive nitrosylation of Fe(III) complexes took place in situ on B-form DNA fibers. Electron paramagnetic resonance (EPR) spectra were obtained as a function of the angle phi between the magnetic field B and the fiber axis Zf. For comparison, EPR spectra were acquired for ON-Fe(II)TMpyP and ON-Fe(II)TMpyP-Im on oriented DNA fibers, where TMpyP is 5,10,15,20-tetrakis(1-methyl-4-pyridino)porphyrin and Im is imidazole. EPR spectra showed both low-spin Fe(III)Blm and ON-Fe(II)Blm bound to B-form DNA in two slightly different binding orientations in the ratio of 1:0.2. With A-form DNA, a fraction of bound Fe(III)Blm was high spin. Specifically, the angle beta between the fiber axis Zf and the g axis, gz, perpendicular to or nearly perpendicular to the equatorial plane of the iron complex was estimated as 20 degrees and 25 degrees for ON-Fe(II)Blm and 30 degrees and 25 degrees for Fe(III)Blm, respectively. The angle gamma that determines the orientation of gx and gy axes was estimated as 90 degrees for the two ON-Fe(II)Blm species and 10 degrees for the two Fe(III)Blm species, respectively. The NO was held rigidly in place as the temperature increased from 123 K to room temperature for ON-Fe(II)Blm but not for ON-Fe(II)TMpyP or ON-Fe(II)TMpyP-Im. It is hypothesized that the NO is structurally oriented by hydrogen bonding like the peroxide is held in HO2(-)-Co(III)Blm (Wu et al. J. Am. Chem. Soc. 1996, 118, 1281-1294). The EPR parameters are consistent with a six-coordinate complex for ON-Fe(II)Blm, although the superhyperfine structure from the trans nitrogen was not detected. The increase in g value anisotropy upon binding ON-Fe(II)Blm to DNA fiber may be caused by an increase in the overlap of d pi and 2p pi* orbitals induced by an interaction of NO with DNA and/or by a perturbation of d orbitals due to the pyrimidine-guanine interaction. It is concluded that the EPR parameters of ON-Fe(II)Blm and Fe(III)Blm bound to oriented DNA support the hypothesis that FeBlm species bind to DNA with adduct structures similar to those formed by related CoBlm species and DNA.

Animals↗