Spin-state Equilibrium Research Articles

Spectral properties of myeloperoxidase and its ligand complexes

The effects of ligands with various field strengths on the optical absorption spectrum of myeloperoxidase have been investigated. As is the case with other hemoproteins, the Soret peak in the optical absorption spectra at 77 K moves to longer wavelengths when strong-field ligands are present, whereas binding of such ligands as chloride and fluoride, which stabilize the high-spin state, shows the opposite effect. With a ligand of intermediate field strength, such as azide, the optical spectrum is not affected at room temperature, but lowering of the temperature results in the formation of the low-spin form of the enzyme. Similarly, in native myeloperoxidase a spin state equilibrium is found in which the low-spin state is favoured at high ionic strength and displays corresponding changes in the optical spectra. From the ligand- and the temperature-induced changes in the optical spectra of the ferric enzyme it is concluded that the band at 620–630 nm is an α band of the low-spin heme iron species, whereas the bands at 500 and 690 nm are probably ‘charge-transfer’ bands ∗ ∗ ‘Charge transfer’ does not refer to the pure charge-transfer transitions, which occur in the near infrared, but to mixed transitions of ligand and charge transfer. of the heme with the iron in the high-spin state.

Temperature-dependent spin-state equilibrium in aquo and hydroxo ferric heme octapeptide complexes. Model systems for the spin equilibrium of ferric hemoproteins

Temperature-dependent spin-state equilibrium in aquo and hydroxo ferric heme octapeptide complexes. Model systems for the spin equilibrium of ferric hemoproteins

59Co NMR studies of the spin-state equilibria in rare-earth cobaltites

Hyperfine interaction parameters reveal differences in the nature of spin-state equilibria in the lighter and heavier rare-earth cobaltites; the crystal-field parameter is lower in the lighter cobaltites. Temperature variation of the quadrupolar coupling constant is also more marked in the lighter rare-earth cobaltites, with NdCoO 3 showing evidence for a structural phase transition.

Spectral characterization of cytochrome P-450 of a strain of Candida tropicalis grown on tetradecane.

Several properties of the cytochrome P-450 induced in the yeast Candida tropicalis by growth on tetradecane have been studied by differential visible spectroscopy on microsomes. The spectral changes typical of this cytochrome have been obtained by subtraction of an unspecific spectral change, possibly due to the presence of other hemoproteins in microsomes, from the experimental difference spectra. Like the previously described cytochromes P-450 from yeast and mammalian liver, C. tropicalis cytochrome P-450 is in spin-state equilibrium at ambient temperature: about 30% of the originally low-spin cytochrome is converted to the high-spin state upon increasing the ionic strength of the medium, whereas 30% of the originally high-spin cytochrome is converted to the low-spin state upon addition of hydrophobic alcohols. C. tropicalis cytochrome P-450 readily binds nitrogenous ligands, isocyanides and phosphines in the ferric and ferrous state with spectral characteristics similar to those reported for other yeast or mammalian cytochromes P-450. It also reacts sucessively with cumylhydroperoxide and 1,3-benzodioxole to form a high-valent iron-oxo species and an iron-carbene metabolite complex. However it fails to produce any spectral or spin-state change upon addition of hydrophobic non-coordinating compounds such as n-tetradecane, its substrate in vivo.

The structural flexibility of ferric cytochrome c: Regulation of the spin-state equilibrium by an anti-arthritic gold(I) compound at neutral pH

The structural flexibility of ferric cytochrome c: Regulation of the spin-state equilibrium by an anti-arthritic gold(I) compound at neutral pH

Some properties of macrocyclic complexes in solution which are related to the ligand ring size

The relation between the structural features of ligands and the properties of their metal complexes, which has been generally observed in coordination chemistry, is demonstrated dramatically in the case of tetra-aza macrocycles. In particular solution properties of macrocyclic complexes (thermodynamic and kinetic stabilities, redox behaviour, spin-state equilibria) are strongly dependent on the size of the ligand cavity. For instance, in the formation of Ni(II) and Cu(II) complexes with a series of 12- to 16-membered cyclic tetramines, the most exothermic reactions occur with the 14-membered ring; expansion or contraction of the ligand cavity results in a substantial decrease of the enthalphy of formation, up to 60%. Likewise, the electrode potential values for the Ni(II)/Ni(III) couple, for the same macrocyclic systems, show a completely analogous ring size dependence: the attainment of the +3 state is easiest for the symmetric 14-membered ligand and becomes progressively more difficult by either an increase or decrease in size of the ligand cavity (up to 0.4 V), This behaviour seems to be related to the strength of the in-plane metal–macrocycle interactions, which in turn derive from the relative compatibility between the geometrical features of the ligand and the electronic requirements of the transition metal ion, in the appropriate oxidation state.

Temperature-dependent spin-state equilibrium in an azide-ferric heme octapeptide complex. A model system for the spin equilibriums of ferric heme proteins

Temperature-dependent spin-state equilibrium in an azide-ferric heme octapeptide complex. A model system for the spin equilibriums of ferric heme proteins

A KINETIC STUDY OF THE REACTION OF THE 5,5,7R(S),12,12,14R(S)-HEXAMETHYL-1S(R),4S(R),8S(R),11S(R)-TETRAAZACYCLOTETRADECANENICKEL(II) ION AND DIMETHYLSULFOXIDE INVOLVING A SINGLET-TRIPLET SPIN-STATE EQUILIBRIUM

Abstract A kinetic study on the spin-state equilibrium in dimethylsulfoxide-water media, [NiL]2+ (singlet, square planar) + 2 DMSO \ightleftharpoons [NiL(dmso)2]2+ (triplet, pseudo Oh), is carried out with a temperature-jump method, where L denotes the title ligand (here-after abbreviated as α-rac-Me6[14]aneN4). The results are consistent with a two-step mechanism involving a rate-determining step [NiL(dmso)]2+ \ightleftharpoons [NiL]2+ + DMSO via formation of a 5-coordinated intermediate [NiL(dmso)]2+.

Spin state equilibria in soybean ferric leghemoglobin and its complexes with formate and acetate

The interactions of fluoride, acetate and formate with soybean ferric leghemoglobin a have been investigated by 1H NMR spectroscopy. In the presence of fluoride or acetate leghemoglobin is locked into a high spin ferric conformation whilst the formate complex exists as an equilibrium mixture of high and low spin states. Both formate and acetate ligate directly to the iron and the different magnetic properties of the complexes are attributed to steric constraints within the heme pocket.

Axial ligand effects on myoglobin stability.

Reversible guanidine hydrochloride denaturation has been applied to obtain the first quantitative estimate of ligand-induced changes in hemoprotein conformational free energy. It is found that strong field (low spin) complexes, e.g. cyanometmyoglobin (MbCN) and azido metmyoglobin (MbN3), are 1.0 +/- 0.1 kcal/mol more stable than the high spin analogs aquometmyoglobin (MbH2O) and fluorometmyoglobin (MbF). This observed stability increment is essentially independent of the model chosen for data analysis. These results demonstrate the value of denaturation titration in measuring the stabilization of hemoprotein conformation by ligand binding. The denaturation of MbN3 appears complex. This complexity may be quantitatively accounted for by considering spin state equilibria. Applying this correction, MbCN and MbN3 have essentially the same stability in spite of steric differences in the two proteins. This result implies metal spin state is more important than ligand stereochemistry in determining the conformational free energy of myoglobin.

High spin–low spin crossover and antiferromagnetic interactions in tris(1-pyrrolidinecarbodithioato)iron (III) and the 4-morpholine (FeM) and dibutyl analogs, effect of recrystallization solvent, and crystal structure of FeM⋅nitrobenzene

High sensitivity magnetic susceptibility determinations, especially in the range 1.2–4.2 K on pure and dilute tris(pyrrolidinecarbodithioato)iron(III) (FeP) in its high spin form, show that a maximum at about 2 K is caused by antiferromagnetic interactions. The analogous chromium(III) complex does not exhibit significant antiferromagnetism compared to that of the iron complex, and it is likely that the upper e electrons possessed by the iron and not by the chromium are responsible for the bulk of the antiferromagnetism. As the iron atoms are about 9 Å apart in discrete molecules, the antiferromagnetic interactions presumably occur between unpaired spins delocalized on to the ligands of adjacent molecules. This is in keeping with NMR evidence that spin delocalization is greater in the iron(III) than in the chromium(III) complex. When diluted with large amounts of the cobalt(III) analog (CoP), FeP exhibits a spin state equilibrium. Thus, the structure of the FeP molecule is modified slightly (presumably with shortening of the Fe–S bond) to approach that of the CoP host lattice, which has a shorter metal–sulfur bond. The previous history of the samples of ferric dithiocarbamate complexes is shown to be far more important than had previously been suspected: When crystallized from benzene, FeP exhibits a high spin–low spin equilibrium, in constrast with the pure high spin behavior of the complex when not crystallized from benzene. The effect of adding 7% of benzene to the lattice is much greater than that of adding 50% of CoP. The dibutyl analog shows similar effects. The tris(4-morpholinecarbodithioato-S,S’)iron(III) complex FeM is shown, by single crystal x-ray data, to contain short Fe–S bond lengths (average 2.353 Å) when recrystallized from nitrobenzene. This indicates that the complex is principally low spin, in keeping with the observed magnetism and with the general strong solvent effect on the spin state. It is now proposed that the difference in Fe–S bond lengths between FeP crystallized from chloroform and FeP from benzene (the reverse of the expected differences) is due to experimental error. Crystal data for FeM⋅nitrobenzene: space group P21/c, Z=4, a=9.713(3) Å, b=31.419(8) Å, c =9.718(2) Å, β=105.04(2) °, V=2864 Å3, R=3.3%, 2712 reflections.

Synthesis and characterization of square-planar iron(II) complexes with dianionic tetraaza macrocyclic ligands and their novel derivatives

A series of square-planar iron (II) complexes has been prepared with synthetic dianionic unsaturated tetraaza macrocyclic ligands that are devoid of functional substituents and vary in ring size from 14 to 16 members. The temperature dependence of the magnetic susceptibility confirms the unusual intermediate spin (S = 1) ground state for these square-planar derivatives. In addition, dicationic iron (II) complexes of these ligands have been prepared by protonation of the neutral square-planar complexes. Two types of protonation products have been obtained, including the complexes of tetradentate bis-..beta..-diimine macrocyclic ligands and two novel derivatives in which acetonitrile molecules have added to the apical ..gamma..-carbon of the charged six-membered chelate rings. Ring size effects appear most dramatically in the reactions of the planar complexes with acetonitrile, in which case ring size determines the nature of the product. However, it is also evident both in electronic and Moessbauer spectral data that ring size exerts a great influence on ligand field strengths. The six-coordinate complexes Fe(macrocycle)-(pyridine)/sub 2//sup 2 +/ containing 14- and 15-membered ring macrocyclic ligands have low-spin ground states, while a 16-membered ring derivative exhibits a spin-state equilibrium.

Relation between magnetism and structure of iron(III) complexes exhibiting an S = 3/2 .dblharw. S = 5/2 spin state equilibrium. Structures of chloroform and water solvated tris(4-morpholinecarbodithioato-S,S')iron(III) and -manganese(III)

Relation between magnetism and structure of iron(III) complexes exhibiting an S = 3/2 .dblharw. S = 5/2 spin state equilibrium. Structures of chloroform and water solvated tris(4-morpholinecarbodithioato-S,S')iron(III) and -manganese(III)

Electronic and magnetic properties of LaNi 1− xCo xO 3, LaCo 1− xFe xO 3 and LaNi 1− xFe xO 3

LaNi 1− x Co x O 3 shows itinerant d-electron behavior similar to LaNiO 3 up to x = 0.5. In the range 0.5 < x < 1.0, the cobalt spin state equilibrium is markedly affected; the localized-itinerant electron transition of LaCoO 3 is not seen when x < 0.95. In LaCo 1− x Fe x O 3, itinerancy of d-electrons decreases with increase in x and the compositions with x > 0.5 are similar to LaFeO 3. If x > 0.1, the localized-itinerant electron transition is not seen and the cobalt spin state equilibrium is considerably altered. In LaNi 1− x Fe x O 3, itinerancy decreases with increase in x. These observations can be satisfactorily explained in terms of Goodenough's energy band schemes.

Circular dichroism and raman studies of the allosteric transition in methemoglobin

Summary Circular Dichroism and resonant Raman spectra have been measured for methemoglobin with and without inositol hexaphosphate (IHP) between pH 5 and pH 9. The transition induced by IHP is accompanied by large changes in the rotational strength of the Soret band. Without IHP, the rotational strength of methemoglobin euqals that of oxyhemoglobin; with IHP it equals that of deoxyhemoglobin. The rotational strength of the 260 nm band shows no change with IHP; the Raman bands change in a manner indicative of a shift in spin state equilibrium. The most direct explanation of these results is that IHP changes quaternary structure with few tertiary changes. It is proposed that the Soret rotational strength is a quantitative marker for quaternary structure.

ChemInform Abstract: MAGNETIC AND MOESSBAUER PROPERTIES OF BIS(N,N‐DISUBSTITUTED DITHIOCARBAMATO)DITHIOLENEIRON COMPLEXES, EVIDENCE FOR A SINGLET‐TRIPLET SPIN‐STATE EQUILIBRIUM

Chemischer InformationsdienstVolume 5, Issue 33 Organoelement Compounds ChemInform Abstract: MAGNETIC AND MOESSBAUER PROPERTIES OF BIS(N,N-DISUBSTITUTED DITHIOCARBAMATO)DITHIOLENEIRON COMPLEXES, EVIDENCE FOR A SINGLET-TRIPLET SPIN-STATE EQUILIBRIUM L. H. PIGNOLET, L. H. PIGNOLETSearch for more papers by this authorG. S. PATTERSON, G. S. PATTERSONSearch for more papers by this authorJ. F. WEIHER, J. F. WEIHERSearch for more papers by this authorR. H. HOLM, R. H. HOLMSearch for more papers by this author L. H. PIGNOLET, L. H. PIGNOLETSearch for more papers by this authorG. S. PATTERSON, G. S. PATTERSONSearch for more papers by this authorJ. F. WEIHER, J. F. WEIHERSearch for more papers by this authorR. H. HOLM, R. H. HOLMSearch for more papers by this author First published: August 20, 1974 https://doi.org/10.1002/chin.197433384AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume5, Issue33August 20, 1974 RelatedInformation

Pressure-Temperature Relationships for Fe(II) Complexes of 1,10-Phenanthroline and 2,2′-Bipyridine

The complexes of the type Fe(phen)2X2 and Fe(bipy)2X2 (phen = 1, 10-phenanthroline, bipy = 2,2'-bipyridine) exist in high spin states (X = Cl−, Br−, N3−, NCO−, OAc−, HCOO−) or low spin states (X = CN−, CNO−, NO2−) or can exhibit the phenomenon of magnetic spin crossover. A recent study of the effect of pressure on the Mossbauer spectra for this type of complex indicates that high pressure is capable of partial conversion of the high spin state to the low spin state. However, there is still some disagreement concerning the interpretation of the Mossbauer data in systems which are in spin state equilibrium, as evidenced by some recent work on Fe(III) complexes with N, N-dialkyldithiocarbamates. High pressure studies have also been made on these Fe (III) dithio chelates. The results of these investigations on the solution electronic spectra indicate that the spin state is again pressure-sensitive.

Spin state equilibria and localized versus collective d-electron behaviour in neodymium and gadolinium trioxocobaltate(III)

Magnetic susceptibility measurements and Mossbauer spectra show that NdCoO3 and GdCoO3 contain predominantly low-spin CoIII ions at low temperatures which transform partially to high-spin Co3+ until a certain temperature. Beyond this temperature, there is a transfer of eg electrons from Co3+ to CoIII giving rise to di- and tetra-valent Co species followed by short-range ordering. Co3+ ions completely disappear at about 1000 K where there is a gradual transition from localized electron behaviour to collective behaviour. The cobaltates are semi-metals beyond this transition. The spin state equilibria and the electronic transition described here find support from electron transport properties, Mossbauer parameters as well as differential thermal analysis and X-ray data.

Pressure-temperature relationships of the spin-state equilibrium in a five-coordinate complex of cobalt(II) thiocyanate with an nnp tridentate ligand

Pressure-temperature relationships of the spin-state equilibrium in a five-coordinate complex of cobalt(II) thiocyanate with an nnp tridentate ligand

Spin-State Equilibria in Holmium Cobaltate

M\"ossbauer and magnetic susceptibility studies of HoCo${\mathrm{O}}_{3}$ have shown that there is coexistence of low-spin Co(III) ions and high-spin ${\mathrm{Co}}^{3+}$ ions; Co(III) being more predominant at low temperatures. The population of Co(III) and ${\mathrm{Co}}^{3+}$ equalizes above a particular temperature with these ions occupying alternate oxygen octahedra, leading to an ordered phase. The ordering transition is evidenced by the temperature variation of Lamb-M\"ossbauer factor, x-ray Debye-Waller factor, and inverse susceptibility. Electrical-conductivity data reflect these changes in the spin-state equilibria and show that at around 1080 K, HoCo${\mathrm{O}}_{3}$ becomes metallic. At this temperature, a first-order localized electron-collective electron transition seems to occur. Co(II) and ${\mathrm{Co}}^{4+}$ are not formed by electron transfer from ${\mathrm{Co}}^{3+}$ to Co(III) as in LaCo${\mathrm{O}}_{3}$. This behavior is correlated with the variation of covalency in the cobaltates.