Preparation and crystal structure of [NEt4]3[Fe6W2S8(SEt)9]; structural and electrochemical comparisons with its molybdenum analogue

[NEt₄]₃[Fe₆M₂S₈(SEt)₉] (M = Mo or W) compounds are isomorphous and contain molybdenum and tungsten atoms in an essentially identical environment. These complexes undergo an irreversible one-electron oxidation at −0.46 V (Mo) and −0.51 V (W) and two one-electron reductions at −1.56 and −1.76 V (Mo) and −1.52 and −1.84 V (W), in DMSO solution versus (0.1 M). The only distinction between the behavior of these molybdenum and tungsten complexes identified thus far is that, for the former the reductions are reversible whereas for the latter they are irreversible. This difference may be relevant to the low activity found for nitrogenases reconstituted with tungsten in place of molybdenum.     

Topography of cyclodextrin inclusion complexes : Part II. The iodine-cyclohexa-amylose tetrahydrate complex; its molecular geometry and cage-type crystal structure

Iodine-cyclohexa-amylose tetrahydrate [(C₆H₁₀O₅)₆ ·I₂·d4H₂O] crystallizes in the orthorhombic space-group P2₁2₁2₁, a  14.240 Å, b  36.014 Å, c  9.558 Å. The structure was solved by heavy-atom techniques and refined by least-squares methods to a conventional discrepancy index R  0.148 for the 2872 observed data. The six d-glucose residues are in the C1 chair conformation; the conformational angles vary in magnitude from 45 to 66°, the angles O(5)-C(5)-C(6)-O(6) are close to · 70°, and the six O(4) atoms are almost coplanar (r.m. s. displacement 0.13 Å). Only four of the six O(2) ?O(3) intramolecular hydrogen bonds have formed, which renders the molecule less symmetrical and more conical-shaped than in the previously determined α-cyclodextrin-potassium acetate complex. The iodine molecule is coaxial with the cyclohexa-amylose molecule. The I-I distance is a conventional 2.677 Å. Close interactions between the iodine atoms and the host molecule comprise carbon atoms C(5) and C(6) and oxygen atoms O(4), with interatomic distances all equal to or greater than van der Waals contacts. Intermolecular, almost-linear, short contacts O ? I-I?O with I?O distances of 3.22 and 3.07 Å indicate attractive interaction.The molecules are arranged in herring-bone “cage-type” fashion, with the four water molecules as space-filling mediators; the structure is held together by an intricate network of hydrogen bonds.     

Crystal structure of methyl 2,6-dichloro-2,6-dideoxy-3,4-O-isopropylidene-α-D-altropyranoside. A pyranoside system close to a skew-boat conformation

The crystal structure of methyl 2,6-dichloro-2,6-dideoxy-3,4-O-isopropylidene-α-D-altropyranoside (1) has been determined by X-ray diffraction. The compound crystallizes in the orthorhombic system, space group P2₁2₁2₁, with unit-cell dimensions a  7.932, b  8.133, and c  20.447 Å. The structure was solved by the heavy-atom method and refined by the least-squares technique to an R value of 0.047 by using 736 intensities measured on a diffractometer. The pyranoside ring is close to a skew-boat conformation, with C-2 and C-5 being maximally displaced from the least-squares plane through the remaining four atoms. The H-1H-2 dihedral angle of  158° is in agreement with the J_1,2 value of 4.5 Hz. Thus the solid-state conformation appears to correspond with the conformation in solution. The dioxolane ring is in a twist form, with O-4 and, C-8 puckered on opposite sides of the plane of the other ring atoms. The pyranose-ring substituents are in equatorial and pseudoequatorial orientations. The hydrogen atoms at C-3 and C-4 are in a cis arrangement. The orientations of both the methoxyl group and the chloromethyl group with respect to the ring are gauche—trans. The exocyclic anomeric C-1O-1 bond-distance (1.39 Å) is the shortest CO bond in the structure. The intracyclic CO bonds are significantly different, C-1O-5 being less than C-5O-5.     

Crystal structure of Turkey egg-white lysozyme: Results of the molecular replacement method at 5 Å resolution

Turkey egg-white lysozyme differs from hen egg-white lysozyme in its primary structure in 7 of the 129 residues. We have determined the rotational and translational parameters relating the known co-ordinates of hen egg-white lysozyme molecule to the turkey lysozyme. The rotational parameters were determined using the rotation function, the translational parameters were determined by placing the properly rotated molecule systematically at all positions within the unit cell and searching for those positions producing few intermolecular contacts between the α-carbon atoms of one molecule and all its neighbors. These parameters were refined by minimizing the conventional R factor between observed and calculated structure amplitudes. The final rotational and translational parameters give an R value of 46.7% for reflections with d spacings between 6 Å and 12 Å and have 7 intermolecular contacts closer than 5 Å between the a carbon atoms of one molecule and all its neighbors. An electron density map has been calculated at 5 Å resolution; the packing of the molecules in this form appears to present the entire length of the active cleft in the vicinity of the crystallographic 6-fold axis and does not appear to be blocked by neighboring molecules.     

The preparation and crystal structure analysis of a 2:1 complex between L-lysine and copper(II) chloride

The crystal structure of bis(L-lysine)Cu(II) chloride dihydrate has been determined by X-ray analysis. The complex crystallizes in the monoclinic space group P2₁, with cell dimensions a = 5.189(1), b = 16.988(3), c = 11.482(2) Å, β = 93.57(1)°. The position of the Cu atom was found from a Patterson synthesis, the remaining atoms were located with DIRDIF. The structure was refined by least-squares to R = 0.060 and R_w = 0.065 for 2637 observed reflections. The copper(II) atom has an essentially square planar coordination with the two lysine molecules chelated via the carboxy oxygen and the α-amino nitrogen. However the two chlorine atoms form weak interactions with the metal to complete a strongly tetragonally elongated six-fold coordination. The two aliphatic chains have rather different geometries and are extended in a zig-zag mode. Extensive hydrogen bonding links the complex and the water molecules together.     

Polynuclear complexes of copper(II) with the tetra-aza ligand 3,6-bis(2′-pyridyl)pyridazine(L). Crystal structure of the bromine derivative [Cu2L(OH)Br3]n

The tetra-aza ligand 3,6-bis(2′-pyridyl)pyridazine (L), when reacting in appropriate conditions with Cu(II) halides, gives rise to polynuclear complexes of general formula [Cu₂L(OH)X₃]_n (X = Cl or Br). The bromine derivative has been studied by X-ray analysis. The crystals are twins by merohedry of class I, space group Pn (P2₁/n apparent space group), with the following cell constants: a = 13.691(5), b = 6.245(3), c = 10.298(4) Å, β = 103.92(5)°. The structure was refined by least-squares techniques to a final R factor of 0.066. The structure consists of binuclear units joined to each other through bridging bromine atoms to form a polymeric array. The two independent copper atoms of the dinuclear moiety are five-coordinated with a geometry which is intermediate between a square-pyramid and a trigonal-bipyramid.     

X-ray crystallographic studies of seal myoglobin: The molecule at 2.5 Å resolution

The three-dimensional structure of metmyoglobin from the common seal has been determined at 2.5 Å resolution. The isomorphous replacement technique has been employed using two derivatives, the mercuri-iodide and the aurichloride. Four-circle diffractometer data to a Bragg angle θ = 18.05 ° were measured for one complete set of Friedel pairs of reflexions from each type of protein crystal. Atomic positions for the individual atoms in the (HgI^?₃₎-group at the two sites of attachment were obtained from three-dimensional difference electron density maps and were further refined. A ‘best’ electron density map of the native protein based on refined heavy-atom parameters was interpreted with the help of the known amino acid sequence, and co-ordinates for all the non-hydrogen atoms were measured from the model. Those of the globin were further constrained according to the ‘modelfit’ procedure of Dodson et al. (1976). The molecule is described in detail; the conformations of the side-chains relative to the positions of the heavy atoms and to the interface between neighbouring molecules are discussed. A preliminary residue by residue comparison of the seal and sperm whale myoglobin molecules is presented in the accompanying paper.     

An inspection into the class of bis(alkyne)-undecacarbonyl-quadro-tetraruthenium complexes. Crystal structure and dynamic behaviour of Ru4(CO)11 (μ4, η2-MeC2Ph)2

The reactions of the butterfly complex Ru₄(CO)₁₂(MeC₂Ph) with several alkynes give the quasiplanar derivatives Ru₄(CO)₁₁(MeC₂Ph)(Alkyne) in almost quantitative yields.The structure of Ru₄(CO)₁₁(MeC₂Ph)₂ has been determined by X-ray methods. Crystals are monoclinic, space group C2/c, with Z = 4 in a unit cell of dimensions a 22.383(16), b 9.048(8), c 18.268(12) Å, β = 127.25(4)°. The structure has been solved from diffractometer data by Patterson and Fourier methods and refined by full-matrix least-squares to R = 0.034 for 1420 observed reflections. The complex, having an imposed C₂ symmetry, presents a tetranuclear metal cluster in which the Ru atoms are in a tetrahedrally-distorted square arrangement. Ten carbonyls are terminal and one symmetrically bridges an edge of the cluster. Each of the two alkyne ligands is σ-bonded to two Ru atoms on the opposite vertices of the cluster and π-bonded to the other two. The organometallic cluster has a Ru₄C₄ core in which the metal and carbon atoms occupy the vertices of a triangulated dodecahedron.     

Crystal and molecular structure of [N,N′-ethylenebis(methyl-2-amino-1-cyclopentenedithiocarboxylato)]copper(II): A quantitative relationship between tetrahedral distortion and redox potentials in copper N2S2 compounds and the relevance to type I copper(II) centers

The crystal and molecular structure of the copper(II) complex of the N₂S₂ tetradentate ligand, ethylenebis(methyl-2-amino-1-cyclopentenedithiocarboxylate), was solved at room temperature by a single crystal x-ray diffraction study. The complex crystallizes in the orthorhombic space group P2₁2₁2₁ with a = 7.739(1) Å, b = 13.893(2) Å, c = 17.096(3) Å, V = 1838(1) ų, ?_observed = 1.56 g cm^?3 and ?_calculated = 1.57 g cm^?3 for a molecular weight of 434.2, and Z = 4. Diffraction data were collected with a Syntex P$\text{1}$ diffractometer using graphite-monochromatized Cu (λ = 1.5418 Å) radiation. The heavy atoms were located from a Patterson synthesis; all other nonhydrogen atoms were located using difference Fourier techniques, and hydrogen atoms were placed in calculated positions. Final refinement resulted in discrepancy indices of R = 0.067 and goodness of fit of 2.92 for all 995 reflections (5° < 2θ < 100°) greater than three times their standard deviation. The molecules are monomeric and well separated. Bond distances in the two ”halvesldquo; of the ligand are sufficiently different to suggest that different resonance structures exist in each portion. This agrees with the rhombic symmetry displayed by the frozen glass esr spectrum of the compound (_xx ≠ g_yy). The dihedral angle between the planes defined by the CuN₂ and CuS₂ planes is 20.0°, indicating a rather distorted inner coordination sphere. The copper(II)-copper(I) reduction potentials found for this compound and the trimethylene and tetramethylene analogs were determined to be ?1.01, ?0.79, and ?0.64 V respectively. A quantitative relationship between tetrahedral distortion and redox potentials is obtained, and these results are discussed in terms of ”blueldquo; copper(II) sites in proteins. Trends in CuS and CuN bonding patterns in the same three compounds are discussed with regard to the short CuS (cys) bond distance in plastocyani Finally, a brief discussion of the optical spectra of these three compounds, their variation, and their significance with respect to tetrahedral symmetry in copper(II) protein sites is presented.     

Structural comparison of (CuICH3CN4·dibenzo-18-crown-6 (I) and (CuICH3CN)x (II) fluorescent copper(I) materials

The single crystal X-ray structures of (CuICH₃CN₄·dibenzo-18-crown-6 (I) and (CuICH₃CN) (II) have been determined at room temperature [(I) C₂₈H₃₆Cu₄I₄N₄o₆, monoclinic space group P2₁/n, a = 10.116(4), b = 18.092(8), c = 22.211(9) Å, β = 98.66(3)°, Z = 4; (II) C₂H₃CuIN, orthorhombic pBN2₁, a = 13.618(8), b =8.742(2), c = 4.298(2), Z = 4]. (I) exists as a distorted cube with copper and iodine at alternate corners, the fourth coordination site copper occupied by an acetonitrile molecule coordinated through nitrogen. The cluster contains no crystallographic symmetry element and CuCu distances average 2.770(5) Å. The dibenzo-18-crown-6 displays only second sphere type interactions with cluster. (II) displays a pleated double chain type structure with distorted rectangles of alternating Cu and I atoms sharing opposite edges in infinite array. Copper displays tetrahedral geometry by coordination to three iodine atoms and a nitrogen bound acetonitrile molecule.     

Nickel(II) complex with a tridentate ONS-Schiff base ligand: X-ray structure,VIS and NIR solution spectra and kinetics of ligand substitution

The ligand N-(3-thia-n-pentyl)salicylaldimine (Hsalen-SEt) and its green nickel(II) complex Ni(salen-SEt)₂ was prepared. The complex crystallizes in the orthorhombic space group Pbca with a a = 2538.3(4) pm, b = 1490.0(3) pm, c = 1163.5(2) pm and Z = 8. The coordination sphere of the nickel is a distorted octahedron with two oxygen atoms in a cis-position, two nitrogen atoms in a trans-position and two sulfur atoms in a cis-position. The two NiO distances were 197.8 and 198.1 pm, the two NiN distances 201.8 and 200.6 pm, whereas the two NiS distances are 272.0 and 266.3 pm. The magnetic susceptibility of Ni(salen-SEt)₂ was measured in the temperature range 2.6-281 K, the magnetic moment being μ_eff=3.02μ_B.The VIS and NIR solution spectra of the complex in different solvents indicate that the two tridentate ONS-ligands are coordinated as bidentate ON-ligands, the coordination geometry being square-planar (trans- N₂O₂) in non-coordinating solvents (e.g., toluene) and octahedral in coordinating solvents (e.g., pyridine), due to addition of two solvent molecules. From spectrophotometric titration the individual complex formation constants for the species Ni(salen-SEt)₂·py (K₁ and Ni(salen-SEt)₂·2py (K₂) were found to be K₁ = 1.76 ± 0.40 M^-1 and K₂ = 145 ± 34 M^-1. The kinetics of the reaction Ni(salen-SEt)₂ + H₂salen → Ni(salen) + 2Hsalen-SEt as studied in acetone by stopped-flow spectrophotometry follow the rate law, rate = (k^S = k_H2^salen H₂salen]) × [Ni(salen-SEt)₂] with k_S = 0.038 ± 0.013 s^-1 and k_H2^salen = 17.2 ± 0.4 M^-1 s^-1 at 25°C. The spectroscopic and kinetic properties of Ni(salen-SEt)₂ are compared with those of bis(N-alkyl-salicylaldiminato)nickel(II) complexes     

Studies on dithio-o-toluate copper(I) complexes with bis(diphenylphosphino)methane. Crystal structures of {di-μ3-dithio-o-toluato-S,S′,S′-bis[μ-bis(diphenylphosphino)methane-μ-dithio-o-toluato-S,S′]} tetracopper(I) and {di[μ-bis(diphenylphosphino)methane]bis(dithio-o-toluato-S,S′)} dicopper(I)

The structures of [(CuS₂CT)₂dppm]₂ (I) (T = o-tolyl; dppm = bis(diphenylphosphino)methane) and [CuS₂CTdppm]₂ (II) have been determined by X-ray methods. Crystals of I are monoclinic, space group P2₁/n, with a = 15.163(4), b = 18.691(5), c = 13.478(4) Å, β = 96.81(3)°, Z = 2; crystals of II are orthorhombic. space group Pccn, with a = 23.267(4), b = 13.016(3), c = 20.731(5) Å, Z = 4. The structures of I and II have been solved by Patterson and Fourier methods and refined by full-matrix least-squares to R = 0.082 for I and 0.092 for II. The structure of I consists of centrosymmetric tetranuclear complexes in which two pairs of Cu atoms are triply bridged by a dppm ligand and two dithiocarboxylate groups from the dithio-o-toluate ligands. These last behave differently: one of them through a sulphur atom is also bonded to a Cu atom of the other pair so forming a tetranuclear complex. The Cu atoms of each pair show different coordination: Cu(1) displays a distorted trigonal and Cu(2) a distorted trigonal pyramidal geometry. The structure of II consists of dimers, in which each copper atom, doubly bridged by two dppm ligands, completes a distorted trigonal pyramidal coordination through two sulphur atoms from dithio-o-toluate anions acting as chelating ligands. In both compounds the phenyl group of the dithio-o-toluate anions is orthogonal to the corresponding CS₂ group. Both complexes give methyldithio-o-toluate in high yields by reaction with methyl iodide.     

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.     

Syntheses of iron(III) aroyl hydrazones containing pyridoxal and salicylaldehyde. The crystal and molecular structure of two iron(III)-pyridoxal isonicotinoyl hydrazone complexes

Iron(III) complexes of three aroyl hydrazones, pyridoxal isonicotinoyl hydrazone (H₂pih), pyridoxal benzoyl hydrazone (H₂pbh), and salicylaldehyde benzoyl hydrazone (H₂sbh), were synthesized and characterized. In aqueous medium at pH 7, [Fe(pih)(Hpih)]·3H₂O is formed. In acidic methanol, a 1:1 ligand-to-metal complex is formed, [FeCl₂(H₂pih)]Cl (1), whereas in aqueous medium at low pH cis-[FeCl₂(H₂pih)(H₂O)]Cl·H₂O (2) is formed. Compounds 1 and 2 are high-spin d⁵ with μ_eff = 5.88 μ_B and 5.93 μ_B (298 K). The crystal structures of 1 and 2 show that H₂pih acts as a tridentate neutral ligand in which the phenolic and hydrazidic protons have shifted to the pyridine nitrogen atoms. The co- ordination polyhedron of 1 is ‘square’ pyramidal, whereas that of 2 is pseudo-octahedral. Compound 1 is triclinic, space group P$\text{l}$, with a = 12.704(2) Å, b = 8.655(2) Å, c = 8.820(2) Å, α = 105.42(1)°, β = 89.87(1)°, γ = 107.60(1)°, V = 888 ų, and Z = 2; 2 is monoclinic, space group P2₁/c, with a = 15.358(4) Å, b = 7.304(3) Å, c = 17.442(4) Å, β = 101.00(2)°, V = 1921 ų, and Z = 4.     

Crystal structure of the koh-amylose complex

The crystal structure of potassium hydroxide complexed amylose, obtained by heterogeneous deacetylation of amylose triacetate, has been determined through a combined stereochemical structure-refinement and X-ray diffraction-analysis. The structure crystallizes in an orthorhombic unit-cell with parameters a  8.84, b  12.31, and c (fiber repeat)  22.41 Å, and with P2₁2₁2₁ symmetry. The conformation of the amylose chain is a distorted, left-handed helix with 6 d-glucose residues per turn. Each three-residue asymmetric unit is complexed with one molecule of potassium hydroxide and three molecules of water. The K⁺ ion coordinates with four oxygen atoms of the amylose chain and with two other oxygen atoms, and this coordination is probably the cause for the more-extended amylose chain-conformation than would be predicted from a φ, ψ map. The distortions in the chain are primarily manifested by different O-6 rotations and by slightly different bridge and φ, ψ angles for the individual residues. The structure is extensively hydrogen bonded, although largely through water molecules, which accounts for its ready water solubility. The left-handed conformation of the chain in this structure is consistent with the conformations of amylose triacetate and V-amylose, both of which are left-handed.     

Stereoelectronic properties of metalloenzymes. 10. a refined model that mimics the type II copper(II) site in galactose oxidase2

A number of copper(II) complexes of tridentate ligands with various donor atoms have been studied in an attempt to duplicate the unusual reactivity patterns and accompanying spectral changes of the copper(II) center in galactose oxidase. Results indicate that in order to match the optical and electron spin resonance spectral change observed upon CN^? binding by the enzyme, an equatorial, negative ligand must be displaced in a small molecule model. The crystal and molecular structure of the best model complex was solved by a single crystal X-ray diffraction study. The compound, monoacetato-1,3-bis(2-(4-methyl-pyridyl)imino)isoindolatocopper(II), crystallizes in the centro-symmetric triclinic space group Pī with a = 7.392(3) Å, b = 13.782(5) Å, c = 23.422(12) Å, α = 92.08(3)°, β = 104.11(5)°, γ = 109.98(4)°, V = 2156(1) ų, d(obsd.)(calc.)=(1.43)(1.44) g/cm^?3 for mol wt of 466.7 and Z = 4. Diffraction data were collected with a Syntex Pl diffractometer using graphite-monochromatized Cu radiation (λ = 1.5418 Å). The copper atoms were located from a Patterson synthesis; all other nonhydrogen atoms were located via difference. Fourier techniques, and hydrogen atoms were placed in calculated positions. Final refinement resulted in discrepancy indices of R = 0.089 and “Goodness to Fit” = 3.68 for all 3608 reflections having (I) ? 3σ(I) (5°<2θ<100°). There are two unique molecules in the asymmetric unit that are monomeric and well separated. The geometry around the copper atom is approximately square pyramidal, with the coordination sphere derived from three nitrogens of the tridentate ligand, one oxygen from the acetate unit, and an oxygen atom of a water molecule occupying an axial position. The structure is surprising both in that an axial water molecule is present and that the remaining four ligand atoms to the copper atom are rather distorted from a planar configuration. The plane defined by the copper, N5, and N3 atoms intersects the plane defined by the copper, Nl, and Ol, atoms forming a “twist angle” of 25.0° (0.0° would be ideal for a planar inner coordination sphere). The stereoelectronics of the inner coordination spheres of the type II Cu(II) enzymes galactose oxidase and superoxide dismutase are discussed and appropriate comparisons are made with emphasis on the origin of spectral changes observed upon anion binding.     

The synthesis and structural characterization of (2-pyridyldiphenylphosphine oxide)platinum(IV)tetrabromide. A chelating phosphine oxide with a substantially bent PtOP angle

The complex (pyPh₂PO)PtBr₄ (pyPh₂PO is 2-pyridyldiphenylphosphine oxide) has been synthesized by three different pathways, and its structure has been established by X-ray crystallography. C₁₇H₁₄Br₄NOPPt crystallizes in the space group P2₁/c (no. 14) with cell dimensions (at 140 K) of a = 13.696(7), b= 16.653(5), c = 17.612(7) Å, β = 92.23(4)°, Z = 8 and V = 3993(3) ų. The structure was refined by block-cascade least-squares to a conventional R value of 0.048 using 3647 significant data. The structure involves a six-coordinate platinum((IV) ion with the chelated ligand bound through its nitrogen and oxygen atoms. The two crystallography independent molecules in the asymmetric unit have very similar dimensions. To our knowledge this is the first reported structure of a chelating phosphine oxide. The PtOP angles within the rings are 114.4(6)° and 117.4(6)°.     

The structure of horse methaemoglobin at 2-0 A resolution

The structure of horse methaemoglobin has been redetermined by phase extension and refinement. This has improved our knowledge of the haem geometry and the stereochemistry of the interfaces between the subunits, and confirmed the disorder of the C-terminal residues. Using new four-circle diffractometer data between the limiting spheres of radius 10 and 2.0 Å^?1, the co-ordinates determined by Perutz et al. (1968a,b) were subjected to successive cycles of real-space refinement into electron density maps calculated with observed ¦F¦ values and phases derived from the latest refined model, until the reliability index had dropped from an initial value of 0.45 to 0.23. The positions of the iron atoms relative to the planes of the porphyrin rings were refined separately, and checked by Fourier syntheses based on anomalous scattering and by difference Fourier syntheses calculated with coefficients from which the iron contributions had been removed. The general root-mean-squared error in atomic positions is 0.32 Å; the probable error in the displacement of the iron atoms from the porphyrin planes is 0.06 Å. The difference Fourier synthesis, obtained after refinement of the protein was complete, showed 41 bound water molecules per asymmetric unit and also revealed five errors in amino acid sequence, one of which was confirmed chemically.The secondary structures of the subunits are stabilized by hydrogen bonds formed by main-chain NH and CO groups either with each other or with nearby polar side-chains. There are few internal hydrogen bonds linking the various chain segments; many of the external polar side-chains help to stabilize the tertiary structure by forming hydrogen bonds with each other or through bound water molecules. Several of the helical segments are irregular and the terminal residues are disordered. The contacts between the subunits are more polar than the earlier 2.8 Å map had led us to believe, because it had failed to show up the 15 bound water molecules at the α₁β₁ and the four at the α₁β₂ contact. Their inclusion has raised the number of hydrogen bonds between neighbouring subunits at α₁β₁ from five to 17 or possibly 19, and at α₁β₂ from two to six or possibly seven. The remaining 22 water molecules are distributed over the internal cavity and the molecular surface; most of them make hydrogen bonds with at least two polar groups of the protein. Despite several amino acid differences, the structure of the α₁β₁ contact, including the bound water, is the same as in human deoxyhaemoglobin (Fermi, 1975).     

Metal complexes of 2,4-diamino-5-(3′,4′,5′-trimethoxybenzyl)pyrimidine, (trimethoprim). Part II. Synthesis,magnetic characterization and X-ray structure of [Cu2(O2CCH3)4(trimethoprim)2]·2C6H6·CH3OH

The reaction of [Cu₂(O₂CCH₃)₄·2H₂O] with trimethoprim is reported. In methanol a green solution was obtained, which, on adding benzene, yielded tetrakis(μ-acetato)bis(trimethoprim)dicopper(II) di-benzene methanol solvate. The compound crystallizes with four molecules per cell in the monoclinic space group C2/c, with a = 24.109(5), b = 15.256(3), c = 16.532(3) Å, β = 116.89(2) for λ(Mo-Kα) = 0.71073 Å. The copper atoms are bridged by four acetate groups to form the binuclear molecule [Cu₂-(O₂CCH₃)₄(TMP)₂]·2C₆H₆·CH₃OH. The TMP ligand acts as a donor molecule through one pyrimidinic nitrogen atom.     

Crystal structure of antimony(II) monothiobenzoate,Sb(PhCOS)3

The title compound crystallizes rhombohedrally in the space group R3 with a = 20.453(11) and c = 4.197(1) Å (hexagonal setting). The structure was refined by full matrix lest squares with 585 independent reflections to R equal 0.042 (R_w = 0.040). Antimony, situated on the threefold axis, is pyramidally coordinated at 2.493(3) Å to the sulfur atoms of the three ligands. Secondary bonding occurs at 2.802(8) Å to the oxygen atoms of the asymmetrically chelating ligands.