Characterization of a modified nitrogenase Fe protein from Klebsiella pneumoniae in which the 4Fe4S cluster has been replaced by a 4Fe4Se cluster

The Azotobacter vinelandii nifS gene product has been used with selenocysteine to reconstitute Klebsiella pneumoniae nitrogenase Fe protein. Chemical analysis and extended X-ray absorption fine structure (EXAFS) spectroscopy show that the 4Fe4S cluster present in the native protein is replaced by a 4Fe4Se cluster. As well, EXAFS spectroscopy shows that the bond lengths to the cysteine thiolate ligands shrink by 0.05 Å (from 2.28 to 2.23 Å) upon reduction, whereas the Fe–Fe distance is essentially unchanged. Thus, the core of the 4Fe4Se cluster remains essentially static on reduction, whilst the external cysteine thiolate ligands are pulled in towards the cluster. Compared with native (S)–Fe protein, the (Se)–Fe protein has a 20-fold increased rate of MgATP-induced Fe chelation, a sixfold decreased specific activity for acetylene reduction, a fivefold decreased rate of MgATP-dependent electron transfer from (Se)–Fe protein to MoFe protein, and a fourfold increase in the ATP to 2e ^? ratio. The high ATP to 2e ^? ratio and decreased specific activity are consistent with a lower rate of dissociation of oxidized (Se)–Fe protein from reduced MoFe protein. Thus, the relatively small adjustments in the Fe protein structure necessary to accommodate the 4Fe4Se cluster are transmitted both to adjacent residues that dock at the surface of the MoFe protein and to the ATP hydrolysis sites located approximately 19 Å away.     

A binuclear copper(II) complex of the schiff base derived from 1,1′-(2,6-pyridyl)-bis-1,3-butanedione and 3-amino-1-propanol

The reaction of Cu(ClO₄)₂·6H₂O with the Schiff base derived from 1,1′-(2,6-pyridyl)-bis-1,3-butanedione and 3-amino-1-propanol, (H₄L²), yields the complex Cu(H₄L²)(ClO₄)₂·H₂O. The crystal structure of this complex is triclinic, R = 0.0521, 5602 reflections. The species is dimeric leading to a binuclear copper(II) complex in which the well- separated (8.93 Å intramolecular and 5.46 Å inter- molecular) copper(II) atoms are in distorted square pyramidal geometries.     

Cadmium(II) complex formation with glutathione

Complex formation between heavy metal ions and glutathione (GSH) is considered as the initial step in many detoxification processes in living organisms. In this study the structure and coordination between the cadmium(II) ion and GSH were investigated in aqueous solutions (pH 7.5 and 11.0) and in the solid state, using a combination of spectroscopic techniques. The similarity of the Cd K-edge and L₃-edge X-ray absorption spectra of the solid compound [Cd(GS)(GSH)]ClO₄·3H₂O, precipitating at pH 3.0, with the previously studied cysteine compound {Cd(HCys)₂·H₂O}₂·H₃O⁺·ClO₄ ^? corresponds to Cd(S–GS)₃O (dominating) and Cd(S–GS)₄ four-coordination within oligomeric complexes with mean bond distances of 2.51 ± 0.02 Å for Cd–S and 2.24 ± 0.04 Å for Cd–O. For cadmium(II) solutions (C _Cd(II) ~ 0.05 M) at pH 7.5 with moderate excess of GSH (C _GSH/C _Cd(II) = 3.0–5.0), a mix of Cd(S–GS)₃O (dominating) and Cd(S–GS)₄ species is consistent with the broad ¹¹³Cd NMR resonances in the range 632–658 ppm. In alkaline solutions (pH 11.0 and C _GSH/C _Cd(II) = 2.0 or 3.0), two distinct peaks at 322 and 674 ppm are obtained. The first peak indicates six-coordinated mononuclear and dinuclear complexes with CdS₂N₂(N/O)₂ and CdSN₃O₂ coordination in fast exchange, whereas the second corresponds to Cd(S–GS)₄ sites. At high ligand excess the tetrathiolate complex, Cd(S–GS)₄, characterized by a sharp δ(¹¹³Cd) NMR signal at 677 ppm, predominates. The average Cd–S distance, obtained from the X-ray absorption spectra, varied within a narrow range, 2.49–2.53 Å, for all solutions (pH 7.5 and 11.0) regardless of the coordination geometry.     

Crystal and molecular structure of Δ-cis-α-ethylenebis-S-prolinato(1,2-diamino-ethane)cobalt(III) perchlorate dihydrate, Δ-cis-α-[Co(ss-EBP)(en)] ClO4·2H2O

The crystal and molecular structure of Δ- cis-α- ethylenebis-S-prolinato(1,2-diaminoethane)cobalt(III) perchlorate dihydrate, Δ-cis-α-[Co(SS-EBP)(en)] ClO₄· 2H₂O, was determined from three-dimensional X-ray diffractometer data. The complex crystallizes in the orthorhombic system, space group P2₁2₁2₁ with a = 7.879(4) Å, b = 13.738(9) Å, c = 19.445(2) Å, V = 2104(2) ų. With Z = 4, the observed and calculated densities are 1.60(2) and 1.605 g cm^?3, respectively. The structure was refined by the block- diagonal least-squares technique to a final R = 0.0560 for 1604 observed reflections. The geometry about the cobalt atom is roughly octahedral with the tetradentate SS-EBP (= ethylenebis-S-prolinate ion), assuming cis-α configuration in which the complex possesses two out-of-plane amino acidate (R) rings and the backbone ethylenediamine (E) ring. The E ring conformation is δ. On the other hand, the R rings have λ conformation as well as the en ring. Δ-R_NR_N?(δ_E  λ_R1  λ_R2)(λ_en)-cis-α-[Co(SS-EBP)(en)]⁺ is one of two possible isomers of this compound which have been isolated and whose absolute configurations have been tentatively assigned by spectroscopy. The crystal and molecular structure determination confirms these assignments.     

The "prismane" protein resolved: X-ray structure at 1.7 Å and multiple spectroscopy of two novel 4Fe clusters

The three-dimensional structure of the native "putative prismane" protein from Desulfovibrio vulgaris (Hildenborough) has been solved by X-ray crystallography to a resolution of 1.72?Å. The molecule does not contain a [6Fe-6S] prismane cluster, but rather two 4Fe clusters some 12?Å apart and situated close to the interfaces formed by the three domains of the protein. Cluster 1 is a conventional [4Fe-4S] cubane bound, however, near the N-terminus by an unusual, sequential arrangement of four cysteine residues (Cys 3, 6, 15, 21). Cluster 2 is a novel 4Fe structure with two μ₂-sulfido bridges, two μ₂-oxo bridges, and a partially occupied, unidentified μ₂ bridge X. The protein ligands of cluster 2 are widely scattered through the second half of the sequence and include three cysteine residues (Cys 312, 434, 459), one persulfido-cysteine (Cys 406), two glutamates (Glu 268, 494), and one histidine (His 244). With this unusual mixture of bridging and external type of ligands, cluster 2 is named the "hybrid" cluster, and its asymmetric, open structure suggests that it could be the site of a catalytic activity. X-ray absorption spectroscopy at the Fe K-edge is readily interpretable in terms of the crystallographic model when allowance is made for volume contraction at 10?K; no Fe··Fe distances beyond 3.1?Å could be identified. EPR, Mössbauer and MCD spectroscopy have been used to define the oxidation states and the magnetism of the clusters in relation to the crystallographic structure. Reduced cluster 1 is a [4Fe-4S]¹⁺ cubane with S?=?3/2; it is the first biological example of a "spin-admixed" iron-sulfur cluster. The hybrid cluster 2 has four oxidation states from (formally) all Fe^III to three Fe^II plus one Fe^III. The four iron ions are exchange coupled resulting in the system spins S?=?0, 9/2, 0 (and 4), 1/2, respectively, for the four redox states. Resonance Raman spectroscopy suggests that the bridging ligand X which could not be identified unambiguously in the crystal structure is a solvent-exchangeable oxygen.     

Preparation and characterization of (9-methyladenine)triammineplatinum(II) and trans-dihydroxo(9-methyladenine)triammineplatinum(IV) complexes

The preparation is reported of [(NH₃)₃Pt(9- MeA)] X₂ (9-MeA = 9-methyladenine) with XCl (1a) and XClO₄ (1b) and of trans-[(OH)₂Pt(NH₃)₃- (9-MeA)]X₂ with XCl (2a) and XClO₄ (2b), and the crystal structure of 1b. [(NH₃)₃Pt(C₆H₇N₅)](ClO₄)₂ crystallizes in space group P2₁/n with a = 20.810(7) Å, b = 7.697(3) Å, c = 10.567(4) Å, β = 91.57(6)°, Z = 4. The structure was refined to R = 0.054, R_w = 0.063. In all four compounds Pt coordination is through N7 of 9-MeA, as is evident from ³J coupling between H8 of the adenine ring and ¹⁹⁵Pt. Pt(II) and Pt(IV) complexes can be differentiated on the basis of different ³J values, larger for Pt(II) than for Pt(IV) by a factor of 1.57 (av). In Me₂SO-d₆, hydrogen bonding occurs between Cl^? and C(8)H of 9-MeA as weil as between Cl^? and the NH₃ groups in the case of the Pt(II) complex 1a. Protonation of the 9-MeA ligands was followed using ¹H NMR spectroscopy and pK_a values for the N1 protonated 9-MeA ligands were determined in D₂O. They are 1.9 for 1a and 1.8 for 2a, which compares with 4.5 for the non-platinated 9-MeA. Possible consequences for hydrogen bonding with the complementary bases thymine or uracil are discussed briefly. Protonation of the OH groups in the Pt(IV) complexes has been shown not to occur above pH 1.     

Spectroscopic description of an unusual protonated ferryl species in the catalase from <Emphasis Type="Italic">Proteus mirabilis</Emphasis> and density functional theory calculations on related models. Consequences for the ferryl protonation state in catalase,peroxidase and chloroperoxidase

The catalase from Proteus mirabilis peroxide-resistant bacteria is one of the most efficient heme-containing catalases. It forms a relatively stable compound II. We were able to prepare samples of compound II from P. mirabilis catalase enriched in ⁵⁷Fe and to study them by spectroscopic methods. Two different forms of compound II, namely, low-pH compound II (LpH II) and high-pH compound II (HpH II), have been characterized by Mössbauer, extended X-ray absorption fine structure (EXAFS) and UV-vis absorption spectroscopies. The proportions of the two forms are pH-dependent and the pH conversion between HpH II and LpH II is irreversible. Considering (1) the Mössbauer parameters evaluated for four related models by density functional theory methods, (2) the existence of two different Fe–O_ferryl bond lengths (1.80 and 1.66 Å) compatible with our EXAFS data and (3) the pH dependence of the α band to β band intensity ratio in the absorption spectra, we attribute the LpH II compound to a protonated ferryl Fe^IV–OH complex (Fe–O approximately 1.80 Å), whereas the HpH II compound corresponds to the classic ferryl Fe^IV=O complex (Fe=O approximately 1.66 Å). The large quadrupole splitting value of LpH II (measured 2.29 mm s^?1 vs. computed 2.15 mm s^?1) compared with that of HpH II (measured 1.47 mm s^?1 vs. computed 1.46 mm s^?1) reflects the protonation of the ferryl group. The relevancy and involvement of such (Fe^IV=O/Fe^IV–OH) species in the reactivity of catalase, peroxidase and chloroperoxidase are discussed.     

Protein recognition in ferredoxin–P450 electron transfer in the class I CYP199A2 system from Rhodopseudomonas palustris

CYP199A2 from Rhodopseudomonas palustris CGA009 is a heme monooxygenase that catalyzes the oxidation of para-substituted benzoic acids. CYP199A2 activity is reconstituted by a class I electron transfer chain consisting of the associated [2Fe–2S] ferredoxin palustrisredoxin (Pux) and a flavoprotein palustrisredoxin reductase (PuR). Another [2Fe–2S] ferredoxin, palustrisredoxin B (PuxB; RPA3956) has been identified in the genome. PuxB shares sequence identity and motifs with vertebrate-type ferredoxins involved in Fe–S cluster assembly but also 50% identity with Pux and it mediates electron transfer from PuR to CYP199A2, albeit with lower steady-state turnover activity: 99 nmol (nmol P450)^?1min^?1 for 4-methoxybenzoic acid oxidation compared with 1,438 nmol (nmol P450)^?1 min^?1 for Pux. This difference mainly arises from weak CYP199A2–PuxB binding (K _m 34.3 vs. 0.45 μM for Pux) rather than slow electron transfer (k _cat 19.1 vs. 37.9 s^?1 for Pux). Comparison of the 2.0-Å-resolution crystal structure of the PuxB A105R mutant with other vertebrate-type, P450-associated ferredoxins revealed similar protein folds but also significant differences in some loop regions. Therefore, PuxB offers a platform for studying ferredoxin–P450 recognition in class I P450 systems. Substitution of PuxB residues at key locations with those in Pux shows that Ala42, Cys43, and Ala44 in the [2Fe–2S] cluster binding loop and Met66 are important in electron transfer from PuxB to CYP199A2, whereas Phe73 and the C-terminal Ala105 were involved in both protein binding and electron transfer.     

Monomeric molybdenum(V) complexes. 4. The structure of tetraphenylarsonium oxodichloro(N-2-oxophenylsalicylideniminato)molybdate(V), [Ph4As] [MoOCl2(SalphO)]

The structure of [Ph₄As] [MoOCl₂(SalphO)], where SalphO is N-2-oxophenylsalicylideniminate dianion, has been determined by X-ray crystallography. The complex crystallizes in the monoclinic space group P2₁/n with a = 11.829(2), b = 16.149(3), c = 17.410(3) Å, β = 97.485(15)° and Z = 4. The calculated and observed densities and 1.566 and 1.573(10) g cm^?3, respectively. Block-diagonal least-squares refinement of the structure using 4722 independent reflections with I ? 3σ(I) converged at R = 0.0345 and R_w = 0.0484. The crystal contains [Ph₄As]⁺ cations and [MoOCl₂(SalphO)]^? anions. The Mo atom in the anion is in a distorted octahedral coordination environment. A planar terdentate Schiff base ligand occupies meridional positions with the N atom trans to the terminal oxo group (O_t). Two Cl atoms are cis to the O_t atom. The Mo atom is displaced by 0.33 Å from the equatorial plane toward the O_t atom. The MoO_t distance is 1.673(3) Å. The MoN bond trans to the O_t atom is 2.298(4) Å. The two MoCl bond lengths are 2.371(1) and 2.408(1) Å. The difference of 0.037 Å is significant (30 σ). Preparations of the title complex and the related complexes are also described.     

Self-exchange rates and electron transfer kinetics of horse heart ferricytochrome C with amino acid-pentacyanoferrate(II) complexes

The electron transfer reactions of horse heart cytochrome c with a series of amino acid-pentacyanoferrate(II) complexes have been studied by the stopped-flow technique, at 25°C, μ = 0.100, pH 7 (phosphate buffer). A second-order behavior was observed in the case of the Fe(CN)₅ (histidine)^3? complex, with k = 2.8 x 10⁵ M^?1 sec^?1. For the Fe(CN)₅ (alanine)^4? and Fe(CN)₅(L-glutamate)^5? complexes, only a minor deviation of the second-order behavior, close to the experimental error (k = 3.2 × 10⁵ and 1.6 x 10⁵ M^?1 sec^?1, respectively) was noted at high concentrations of the reactants (e.g., 6 × 10^?4 M). The results are in accord with recent work on the Fe(CN)₆^4?/cytochrome c system demonstrating weak association of the reactants. The calculated self-exchange rate constants including electrostatic interactions for the imidazole,L -histidine, 4-aminopyridine, glycinate, β-alaninate, andL-glutamate pentacyanoferrate(II) complexes were 3.3 × 105, 3.3 × 10⁵, 2.8 × 10⁶,4.1 × 10²,5.5 × 10², and 6.0 M^?1 sec^?1, respectively. Marcus theory calculations for the cytochrome c reactions were interpreted in terms of two nonequivalent binding sites for the complexes, with the metalloprotein self-exchange rate constants varying from 10⁴ M^?1 sec^?1 (histidine, imidazole, and 4-aminopyridine complexes) to 10⁶ M^?1 sec ^?1 (glycinate, β-alaninate, and L-glutamate complexes).     

Coordination modes of polydentate ligands. 4. The crystal and molecular structure of an oxo-bridged iron(III) complex containing a pentadentate imino-alkoxy ligand

In the presence of Fe³⁺, template condensation of the fluorinated keto-alcohol CH₃C(O)CH₂C- (CF₃)₂OH with the triamine CH₃C(CH₂NH₂)₃ leads to two products: a fully condensed, imino-alkoxy, iron(III) complex, Fe{CH₃C[CH₂NC(CH₃)CH₂C(CF₃)₂O]₃}, and a partially condensed iron(III) complex, O{FeCH₃C[CH₂NC(CH₃)CH₂C(CF₃)₂O]₂(CH₂NH₂)}₂, in which two six-coordinate iron(III) centers are linked by an oxide ion. A complete crystal and molecular structure determination of the latter has been made.Crystals are monoclinic, space group C2/c, a= 13.886(4); b=23.206(5); c=15.241(4) Å; β= 106.55(2)°; V=4708 ų; Z=4. Least-squares refinement on F of 322 variables using 2627 observations converged at a conventional agreement factor of 3.8%. The Fe to bridging oxide distance is 1.811(1) Å, the FeFe distance 3.468 Å, and the FeOFe angle 146.6(2)°. A comparison is made between this structure and those of natural hemerythrin systems.     

Spectroscopy and crystal structure of neodymium coordination compound with glycine: Nd2(Gly)6·(ClO4)6·9H2O

Neodymium complex compound with glycine: Nd₂(Gly)₆·(ClO₄₆·9H₂O was synthesized and obtained in the form of monocrystals. Absorption spectra recorded in the region of 8000–35 000 cm^-1 were measured along the crystallographic axes. Intensities of the f-f transitions were analysed on the basis of the Judd theory. The X-ray crystal structure determination of the complex is reported. Crystals are triclinic, space group P$\text{I}$, with a = 11.554(4) Å, b = 14.108(1) Å, c = 15.660(3) Å, α = 97.14(1)°, β = 102.82(2)°, γ = 105.28(1)°, V = 2355.25 ų Z = 2, M.W. = 1495.4, D_c = 2.129)(3) g cm^-3, D^m = 2.103(1) g cm^-3. The structure was solved by Patterson's method and successive Fourier syntheses giving the locations of all nonhydrogen atoms. The final R factor was 0.062 and R_w = 0.073 for 12869 reflections with |F_o| > 5σ|(F_o)|. The asymmetric unit consists of a dimeric formula unit. The coordination polyhedron of Nd atoms comprises seven oxygen atoms from glycine and two from water molecules. The neodymium-glycine bonding mode is compared with that of the calcium-glycine complex.     

Kinetics of reduction of ferrioxamine B by chromium(II), vanadium(II), and dithionite

Kinetic studies of the reduction of ferrioxamine B (Fe(Hdesf)⁺) by Cr(H₂O)₆²⁺, V(H₂O)₆²⁺, and dithionite have been performed. For Cr(H₂O)₆²⁺ and V(H₂O)₆²⁺, the rate is ?d[Fe(Hdesf)⁺]/dt = k[Fe(Hdesf)⁺][M²⁺]. For Cr(H₂O)₆²⁺, k = 1.19 × 10⁴ M^?1 sec^?1 at 25°C and μ = 0.4 M, and k is independent of pH from 2.6 to 3.5. For V(H₂O)₆²⁺, k = 6.30 × 10² M^?1 sec^?1 at 25°C, μ = 1.0 M, and pH = 2.2. The rate is nearly independent of pH from 2.2 to 4.0. For Cr(H₂O)₆²⁺ and V(H₂O)₆²⁺, the activation parameters are ΔH^≠ = 8.2 kcal mol^?1, ΔS^≠ ?12 eu and ΔH^≠ = 1.7 kcal mol^?1, ΔS^≠ = ?40 eu (at pH 2.2) respectively. Reduction by Cr(H₂O)₆²⁺ is inner-sphere, while reduction by V(H₂O)₆²⁺ is outer-sphere. Reduction by dithionite follows the rate law ?d[Fe(Hdesf)⁺]/dt =kK^{$\text{1}\text{2}$}[Fe(Hdesf)⁺][S₂O₄^2?]^{$\text{1}\text{2}$} where K is the equilibrium constant for dissociation of S₂O₄^2? into SO₂^? radicals. The value of k at 25°C and μ = 0.5 is 2.7 × 10³ M^?1 sec^?1 at pH 5.8, 3.5 × 10³ M^?1 sec^?1 at pH 6.8, and 4.6 × 10³ M^?1 sec^?1 at pH 7.8, and ΔH^≠ = 6.8 kcal mol^?1 and ΔS^≠ = ?19 eu at pH 7.8.     

Xanthine complexes with 3d metal perchlorates

Complexes of xanthine (xnH) with 3d metal perchlorates were prepared by refluxing mixtures of ligand and metal salt in ethyl acetate-triethyl orthoformate. In all cases, partial substitution of anionic xn⁻ for ClO₄⁻ groups occurs, and the solid complexes isolated also contain invariably two neutral xnH ligands per metal ion, viz. Cr(xn)₂(xnH)₂ClO₄, Fe(xn)₂(xnH)₂ClO₄·H₂O, M(xn)(xnH)₂ClO₄·H₂O (M = Fe, Co, Ni) and M(xn)(xnH)₂ClO₄· 2H₂O (M = Mn, Zn). The new complexes are generally hexacoordinated and appear to be linear chainlike polymeric species characterized by a (-Mxn-)_n single-bridged backbone. Four terminal ligands per metal ion, including two xnH groups in all cases, complete its inner coordination sphere; the remaining two terminal ligands differ from complex to complex as follows: M = Cr³⁺ xn⁻, -OClO₃; Fe³⁺ xn⁻, H₂O; Fe²⁺, Co²⁺, Ni²⁺OClO₃, H₂O; Mn²⁺, Zn²⁺ two aqua ligands. Probable binding sites of bidentate bridging xn⁻ and unidentate terminal xnH and xn⁻ are discussed.     

Hydrogen and oxygen trapping at the H-cluster of [FeFe]-hydrogenase revealed by site-selective spectroscopy and QM/MM calculations

[FeFe]-hydrogenases are superior hydrogen conversion catalysts. They bind a cofactor (H-cluster) comprising a four-iron and a diiron unit with three carbon monoxide (CO) and two cyanide (CN^?) ligands. Hydrogen (H₂) and oxygen (O₂) binding at the H-cluster was studied in the C169A variant of [FeFe]-hydrogenase HYDA1, in comparison to the active oxidized (Hox) and CO-inhibited (Hox-CO) species in wildtype enzyme. ⁵⁷Fe labeling of the diiron site was achieved by in vitro maturation with a synthetic cofactor analogue. Site-selective X-ray absorption, emission, and nuclear inelastic/forward scattering methods and infrared spectroscopy were combined with quantum chemical calculations to determine the molecular and electronic structure and vibrational dynamics of detected cofactor species. Hox reveals an apical vacancy at Fe_d in a [4Fe4S-2Fe]^3 ? complex with the net spin on Fe_d whereas Hox-CO shows an apical CN^? at Fe_d in a [4Fe4S-2Fe(CO)]^3 ? complex with net spin sharing among Fe_p and Fe_d (proximal or distal iron ions in [2Fe]). At ambient O₂ pressure, a novel H-cluster species (Hox-O₂) accumulated in C169A, assigned to a [4Fe4S-2Fe(O₂)]^3 ? complex with an apical superoxide (O₂^?) carrying the net spin bound at Fe_d. H₂ exposure populated the two-electron reduced Hhyd species in C169A, assigned as a [(H)4Fe4S-2Fe(H)]^3 ? complex with the net spin on the reduced cubane, an apical hydride at Fe_d, and a proton at a cysteine ligand. Hox-O₂ and Hhyd are stabilized by impaired O₂^– protonation or proton release after H₂ cleavage due to interruption of the proton path towards and out of the active site.     

Activation parameters for directly determined first order rate constants for outer sphere electron transfer reactions,and crystal structure of a pertinent pentaammine of CoIII

The outer sphere reductions of Co(NH₃)₅B³⁺ by Fe(CN)₅A³⁻ have been studied. The observed pseudo first order rate constants (Co complex in excess) obey the dependence k_obs=K_osk_et[Co]/(1 +K_os[Co]), as expected for outer sphere electron transfer reactions. Values of the fundamental electron transfer rate constants k_et have been determined, along with the equilibrium constant K_os for a range of reactions in which A and B are pyridyl ligands of different sizes. The first order electron transfer rate constants vary in a manner that is consistcnt with adiabatic electron transfer. The outer sphere ion pairing equilibrium constants K_os have been calculated: K_os=8.6 ± 0.1 × 10² M⁻¹ when A and B=pyridine; K_os=1.07 ± 0.09 × 10³ M⁻¹ where A=pyridine, B=1-phenyl-3-(4-pyridyl)propane; K_os=1.86 ± 0.11 × 10³ M⁻¹ when A=4,4′-bipyridine, B=pyridine; K_os=1.27 ± 0.08 × 10³ M⁻¹ when A=4,4′-bipyridine, B=4-phenylpyridine. Distances of closest approach between the metal centers in the reactive ion pairs are compared, and it is concluded that there is a common mechanism, in which the ammonia side of the cobalt complex approaches the cyano side of the iron complex in each reactive ion pair.The distance of closest approach between the two metal centers (a) was calculated from the experimental values for the ion pairing equilibrium constant K_os at 25 °C: 5.2 Å when A=4,4′-bipyridine, B=pyridine; 5.4 Å when A=4,4′-bipyridine, B=4-phenylpyridine; 5.5 Å when A=pyridine, B=1-phenyl-3-(4-pyridyl)propane; 5.7 Å when A=B=pyridine. These relatively short metal-metal distances, when compared to the X-ray structure of the compound [Co(NH₃)₅(4-phenylpyridine)]₂[S₂O₆]₃· 4H₂O, do not support an ion pair orientation in which the two substituted pyridine ligands A and B are oriented toward each other. [P2₁/c,a=7.399(3), b=22.355(10), c=13.776(4) Å, β=92.02(3)°, R=0.070.] The crystallographic results show that if the two pseudo-octahedral coordination spheres are oriented in the reactive ion pair so that an ammonia face of the cobalt complex is at hydrogen bonding distance from a cyano face on the iron complex, the metal-metal distance is 5.3 Å, a distance which is in agreement with the kinetic results.     

Structural changes that occur upon photolysis of the Fe(II)a3–CO complex in the cytochrome ba3-oxidase of Thermus thermophilus: A combined X-ray crystallographic and infrared spectral study demonstrates CO binding to CuB

The purpose of the work was to provide a crystallographic demonstration of the venerable idea that CO photolyzed from ferrous heme-a₃ moves to the nearby cuprous ion in the cytochrome c oxidases. Crystal structures of CO-bound cytochrome ba₃-oxidase from Thermus thermophilus, determined at ~ 2.8–3.2 Å resolution, reveal a Fe–C distance of ~ 2.0 Å, a Cu–O distance of 2.4 Å and a Fe–C–O angle of ~ 126°. Upon photodissociation at 100 K, X-ray structures indicate loss of Fe_a3–CO and appearance of Cu_B–CO having a Cu–C distance of ~ 1.9 Å and an O–Fe distance of ~ 2.3 Å. Absolute FTIR spectra recorded from single crystals of reduced ba₃–CO that had not been exposed to X-ray radiation, showed several peaks around 1975 cm^? 1; after photolysis at 100 K, the absolute FTIR spectra also showed a significant peak at 2050 cm^? 1. Analysis of the ‘light’ minus ‘dark’ difference spectra showed four very sharp CO stretching bands at 1970 cm^? 1, 1977 cm^? 1, 1981 cm^? 1, and 1985 cm^? 1, previously assigned to the Fe_a3–CO complex, and a significantly broader CO stretching band centered at ~ 2050 cm^? 1, previously assigned to the CO stretching frequency of Cu_B bound CO. As expected for light propagating along the tetragonal axis of the P4₃2₁2 space group, the single crystal spectra exhibit negligible dichroism. Absolute FTIR spectrometry of a CO-laden ba₃ crystal, exposed to an amount of X-ray radiation required to obtain structural data sets before FTIR characterization, showed a significant signal due to photogenerated CO₂ at 2337 cm^? 1 and one from traces of CO at 2133 cm^? 1; while bands associated with CO bound to either Fe_a3 or to Cu_B in “light” minus “dark” FTIR difference spectra shifted and broadened in response to X-ray exposure. In spite of considerable radiation damage to the crystals, both X-ray analysis at 2.8 and 3.2 Å and FTIR spectra support the long-held position that photolysis of Fe_a3–CO in cytochrome c oxidases leads to significant trapping of the CO on the Cu_B atom; Fe_a3 and Cu_B ligation, at the resolutions reported here, are otherwise unaltered. This article is part of a Special Issue entitled: Respiratory Oxidases.     

Biochemical properties of the 1 alpha, 25-dihydroxyvitamin D3 cytoplasmic receptors from human and chick parathyroid glands

Cytoplasmic receptors for 1α, 25-dihydroxyvitamin D₃ from human parathyroid adenoma tissue and rachitic chick parathyroid glands have been characterized with regard to a number of physical, chemical, and ligand binding properties. Both receptors are 3.6–3.7 S proteins with molecular weights of approximately 75,000 and Stoke's molecular radii of 36 Å. It was found that the receptors possess a cysteine residue in or near the 1α, 25-dihydroxyvitamin D₃ binding site which is critical for ligand binding activity. The receptors both have equilibrium dissociation constants for 1α, 25-dihydroxyvitamin D₃ in the range of 2 to 5 × 10^?10m at 4 °C and second-order association rate constants for their seco-steroid ligand of 1 × 10⁷, m^?1 min^?1 (0 °C). The dissociation rate constants were found to be 5.3 × 10^?4 min^?1 (4 °C) for the human receptor and 1.3 × 10^?5 min^?1 (4 °C) for the chick receptor. The great deal of similarity which exists between the cytoplasmic 1α, 25-dihydroxyvitamin D₃ receptors from avian and mammalian parathyroid glands suggests a homologous function for these molecules in the two tissues.     

Transition-metal(II) thiocyanate coordination compounds containing 4-allyl-1,2,4-triazole. Structure and magnetic properties.

The synthesis and characterisation of a series of dinuclear and polynuclear coordination compounds with 4-allyl-1,2,4-triazole are described. Dinuclear compounds were obtained for Mn(II) and Fe(II) with composition [M₂(Altrz)₅(NCS)₄], and for Co(II) and Ni(II) with composition [M₂(Altrz)₄(H₂O)(NCS)₄](H₂O)₂. The crystal structure of [Co₂(Altrz)₄(H₂O)(NCS)₄](H₂O)₂ was solved at room temperature. It crystallizes in the monoclinic space group P2₁/n. The lattice constants are a = 18.033(3) Å, b = 13.611(2) Å, c = 15.619(3) Å, β = 92.04(2)° Z = 4. One cobalt ion has an octahedrally arranged donor set of ligands consisting of three vicinal nitrogens of 1,2-bridging triazoles (CoN = 2.14–2.15 Å), one terminal triazole nitrogen (CoN = 2.12 Å) and two N-bonded NCS anions (CON = 2.08 Å). The other Co(II) ion has the same geometry, but the terminal triazole ligand is replaced by H₂O (CoO = 2.15 Å). The crystal structure is stabilised by hydrogen bonding through H₂O molecules, S-atoms of the NCS anions and the lone-pair electron of the monodentate triazole. The magnetic exchange in the Mn, Co and Ni compounds is antiferromagnetic with J-values of ?0.4 cm^?1, ?10.9 cm^?1 and ?8.7 cm^?1 respectively. The Co compound was interpreted in terms of an Ising model. For [Zn₂(Altrz)₅(NCS)₂]∞[Zn(NCS)₄], [Cu₂(Altrz)₃(NCS)₄]∞ and [Cd₂(Altrz)₃(NCS)₄]∞ chain structures are proposed. In the Cu compound thiocyanates appear to be present, bridging via the nitrogen atom, as deduced from the IR spectrum.     

Coordination chemistry of 7,9-disubstituted 6-oxopurine metal compounds. 4. Platinum(II) coordination at N(1). Molecular and crystal structure of [(ethylenediamine)bis(7,9-dimethylhypoxanthine)platinum(II)] hexafluorophosphate

The preparation and molecular and crystal structure of the complex [(ethylenediamine)bis(7,9,-dimethylhypoxanthine)platinum(II)] hexafluorophosphate, [Pt(C₂H₈N₂)(C₇H₈N₄O)₂] (PF₆)₂, are reported. The complex crystallizes in the monoclinic system, space group C2/c, with a = 12.334(2)Å, b = 10.256(2)Å, c = 22.339(3)Å, β = 101.31(1)°, V = 2771.0ų, Z = 4, D_measd = 2.087(3) g cm^?3, D_calc = 2.094 g cm^?3. Intensities for 3992 symmetry-averaged reflections were collected in the θ-2o scan mode on an automated diffractometer employing graphite-monochromatized MoKα radiation. The structure was solved by standard heavy-atom Patterson and Fourier methods. Full matrix least-squares refinement led to a final R value of 0.051. Both the ethylenediamine chelate and the PF₆^? anion are disordered. The primary coordination sphere about the Pt(II) center is approximately square planar with the bidentate ethylenediamine ligand and the N(1) atoms [Pt(II) ? N(1) = 2.020(5)Å] of two 7,9-dimethylhypoxanthine bases (related by a crystallographic twofold axis of symmetry) occupying the four coordination sites. The exocyclic O(6) carbonyl oxygen atoms of the two 7,9-dimethylhypoxanthine ligands participate in intracomplex hydrogen bonding with the amino groups of the ethylenediamine chelate [N(ethylenediamine) ? O(6) = 2.89( )Å]. The observed Pt ? O(6) intramolecular distances of 3.074(6)Å are similar to those found in other Pt(II) N(1)-bound 6-oxopurine complexes and in several Pt(II) N(3)-bound cytosine systems.