Crystal structures of two cyclic pseudopentapeptides containing ψ[CH2S] and ψ[CH2SO] backbone surrogates

The solid state conformations of cyclo[Gly–Proψ[CH₂S]Gly–D –Phe–Pro] and cyclo[Gly–Proψ[CH₂–(S)–SO]Gly–D –Phe–Pro] have been characterized by X-ray diffraction analysis. Crystals of the sulfide trihydrate are orthorhombic, P2₁2₁2₁, with a = 10.156(3) Å, b = 11.704(3) Å, c = 21.913(4) Å, and Z = 4. Crystals of the sulfoxide are monoclinic, P2₁, with a = 10.662(1) Å, b = 8.552(3) Å, c = 12.947(2) Å, β = 94.28(2), and Z = 2. Unlike their all-amide parent, which adopts an all-trans backbone conformation and a type II β-turn encompassing Gly-Pro-Gly-D -Phe, both of these peptides contain a cis Gly¹-Pro² bond and form a novel turn structure, i.e., a type II′ β-turn consisting of Gly–D –Phe–Pro–Gly. The turn structure in each of these peptides is stabilized by an intramolecular H bond between the carbonyl oxygen of Gly¹ and the amide proton of D -Phe⁴. In the cyclic sulfoxide, the sulfinyl group is not involved in H bonding despite its strong potential as a hydrogen-bond acceptor. The crystal structure made it possible to establish the absolute configuration of the sulfinyl group in this peptide. The two crystal structures also helped identify a type II′ β-turn in the DMSO-d₆ solution conformers of these peptides. © 1993 John Wiley & Sons, Inc.     

Metal-penicillamine interactions. Part 1. Preparation and crystal structure of a 1:1 complex of mercuric chloride with d-penicillamine, 2[μ3-Cl){HgSC(CH3)2CH(NH3)COO}3·3(μ2-Cl)· 2(H3O)·(H2O·Cl)3

A 1:1 complex of mercuric chloride with D-peniccillamine has been isolated and characterised as 2[(μ₃-Cl){HgSC(CH₃)₂CH(NH₃)COO}3]·3(μ₂-Cl)·2(H₃O)·(H₂O·Cl)₃. The compound crystallises in cubic space group P4₁32, with a = 18.679(5) Å and Z = 4. The structure, refined to R_F = 0.086 for 443 observed Mo-Kα diffractometer data, features a triply bridging chloride ion linking three equivalent [HgSC(CH₃)₂CH(NH₃)COO]⁺ units [Hg-Cl = 2.37(1) Å, Hg-Cl-Hg′ = 98.5(9)°]. The carboxylate groups of a pair of adjacent penicillamine ligands are strongly linked via a symmetrical O?H?O hydrogen bond of length 2.24(8) Å, and neighboring pyramidal trinuclear [μ₃-Cl){HgSC(CH₃)₂CH(NH₃)-COO}₃]²⁺ moieties are further connected by symmetrical chloride bridges [Hg-Cl = 3.06(2) Å; HgClHg′' = 79.6(7)°] to form a three-dimensional network. The voids in the lattice are filled by hydronium ions and novel planar cyclic hydrogen-bonded (H₂O·Cl^?)₃ rings of edge O-H?Cl = 2.46(4) Å.     

The crystal structure of 2-deoxy-β-d-arabino-hexopyranose at −150°

2-Deoxy-β-d-arabino-hexopyranose, C₆H₁₂O₅, is orthorhombic, P2₁2₁2₁, with cell dimensions at ?150° [20°], a = 6.484(2) [6.510(3)], b = 10.364(2) [10.427(4)], c = 11.134(3) [11.153(5)] Å, V = 748.2 [757.1] ų, Z = 4, D_x = 1.457 [1.440], and D_m = [1.455] g.cm^?3. The intensities of 1269 reflections were measured by using MoKα radiation. The structure was solved by direct methods, and refined by full-matrix least-squares, with anisotropic, thermal parameters for the carbon and oxygen atoms, and isotropic parameters for the hydrogen atoms. The pyranose has the ⁴C₁(d) conformation, with puckering parameters Q = 0.563 Å, θ = 3.9°, and ? = 350.3°. The departure from ideality is very small, and less than that in β-d-glucopyranose, Q = 0.584 Å and θ = 6.9°. The β-glycosidic, CO bond is short, 1.383(4) Å, and the OCOH torsion angle is ?87°, consistent with the anomeric effect. The hydrogen-bonding scheme consists of infinite chains, with side chains terminating at a ring-oxygen atom.     

Accommodation of a D-Phe residue into a right-handed 310-helix: Structure of Boc-D-Phe-(Aib)4-Gly-L-Leu-(Aib)2-Ome,an analogue of the amino terminal segment of antiamoebins and emerimicins

The crystal structure of the nonapeptide Boc-D -Phe-Aib-Aib-Aib-Aib-Gly-Leu-Aib-AibOMe (I), which is an analogue of the N-terminal sequence of antiamoebins and emerimicins, establishes a completely 3₁₀-helical conformation with seven successive intramolecular 4 → 1 hydrogen bonds. The average, ?, ψ values for residues 1–8 are ?59° and ?32°, respectively. Crystal parameters are C₄₇H₇₇N₉O₁₂, space group P1, a = 10.636(4) Å, b = 11.239(4) Å, c = 12.227(6) Å, α = 101.17(4)°, β = 97.22(4)°, γ = 89.80(3)°, Z = 1, R = 5.95% for 3018 data with |F₀| > 3α(F), resolution 0.93 Å. The use of the torsion angle κ = C(i ? 1)N(i)C^α(i)C^β(i), where κ = 68° for D -Phe and κ = 164° for L -Leu, confirms the opposite configurations of these residues. The ?, ψ values of ?62° and ?32° at D -Phe are unusual, since this region is characteristic of residues with L configurations. Peptide I possesses only two chiral residues of opposing configuration. The observed right-handed 3₁₀-helical structure suggests that helix sense has probably been determined by the stereo-chemical preferences of the Leu residue. © 1993 John Wiley & Sons, Inc.     

Studies on the Metabolites of Aspergillus versicolor (Vuillemin) Tiraboschi

Three new pigments, named versicolorins A, Band C, as metabolites from the mycelium of Aspergillus versicolor have been isolated. Versicolorin A, C₁₈H₁₀O₇, is fine orange yellow needles, m.p. 289°C (decomp.), [α]_D^-354°. It is an anthraquinoid pigment having three hydroxyl groups and a vinyl ether system contained in a five-membered ring. Versicolorin A trimethyl ether was hydrogenated to a dihydro-derivative, and by oxidation gave 3,5-dimethoxyphthalic acid and a hydroxy acid which may be 1,6,8-trirnethoxy-3-hydroxy anthraquinone-2-carboxylic acid. These chemical behavior and NMR data show that versicolorin A probably has the structure of (I). Versicolorin B, C₁₈H₁₂O₇, is fine orange yellow needles, m.p., 298°C (decomp.), [α]_D^-223° Its trimethyl ether is identical with that of dihydroversicolorin A. Therefore, the structure (II) could be assigned to versicolorin B. Versicolorin C, C₁₈H₁₂O₇, is orange red needles, m.p.>310°C, [α]_D O° Comparison of optical properties, IR and NMR spectra of versicolorin B and its methyl ether with those of versicolorin C and its methyl ether indicates that versicolorin C is very probably a racemate of versicolorin B.     

Potential multifunctional anti-cancer metal complexes. I. Synthesis and x-ray structural studies of some dinuclear rhodium(II) carboxylate complexes with diamine-substituted acridine ligands in terminal coordination positions

The preparations are reported of [Rh(RCO₂)₂L]₂ [where R = CH₃, C₂H₅, and CH₃OCH₂; L = 6-chloro-2-methoxy-9-[2(NR′₂)ethyl]aminoacridine (R′ = H, CH₃)]. X-ray structural studies have been carried out on two of the compounds [ R = C₂H₅, R′ = H, (1); R = CH₃, R′ = CH₃, (2)]. Compound 1 is monoclinic, space group C2/c, with a = 20.864(11), b = 15.736(4), c = 14.402(4) Å, β = 93.14(4)°, V = 4721 ų, and Z = 4; 2 is monoclinic, space group P2₁/n, a = 8.861(2), b = 23.089(10), c = 12.014(2) Å, β = 105.84(2)°, V = 2365 ų, and Z = 2. Both compounds comprise the standard dinuclear rhodium(II) carboxylate unit with the substituted acridine ligands coordinated to rhodium in the axial positions, via the NH₂ group nitrogen in 1 and the N(CH₃)₂ nitrogen in 2.The dimethyl substitution on the tertiary amine group in 2, and an associated conformational change in the diamine chain, result in an increased separation of the acridine ligand from the metal centre. There is a pronounced acridine base stacking in 1 but not in 2.     

Studies on the metal—amide bond. XVIII. The crystal and molecular structure of [N,N′-bis(2′-pyridinecarboxamido)-1,3-propane] nickel(II) monohydrate

[N,N′-bis(2′-pyridinecarboxamido)-1,3-propane] - nickel(II) monohydrate, C₁₅H₁₆N₄O₃Ni is monoclinic, space group P2₁/c, with a = 7.174(4), b = 18.590(3), c = 11.641(5) Å, β = 110.69(2)°, Z = 4. The structure was refined to R = 0.030 for 1826 diffractometer data using full-matrix least-squares methods. The N₄-ligand coordinates to the nickel atom in an irregular square plane [average Ni-N_amide 1.864(4), Ni-N_pyridine 1.912(3) Å and N_amide-Ni-N_amide 96.0(1), N_pyridine-Ni-N_pyridine 98.7(1)°] with a tetrahedral twist of 15.9° at the nickel atom. The two picolinamide units are related by an approximate two-fold axis and the enforced strain in the molecule results in significant non-planar distortions in the amide chelate rings and the pyridyl rings. The plane of the chelate molecule lies approximately perpendicular to [100] and the lattice water molecule hydrogen bonds amide oxygen atoms to form chains parallel to [101]     

Design of Peptides Using α,β-Dehydro-residues: Synthesis,Crystal Structure and Molecular Conformation of N-Boc-L-Val-ΔPhe–ΔPhe-L-Ala-OCH3

To obtain general rules of peptide design using α,β-dehydro-residues, a sequence with two consecutive ΔPhe-residues, Boc-L -Val-ΔPhe–ΔPhe- L -Ala-OCH₃, was synthesized by azlactone method in solution phase. The peptide was crystallized from its solution in an acetone/water mixture (70:30) in space group P6₁ with a=b=14.912(3) Å, c= 25.548(5) Å, V=4912.0(6) ų. The structure was determined by direct methods and refined by a full matrix least-squares procedure to an R value of 0.079 for 2891 observed [I?3σ(I)] reflections. The backbone torsion angles ?₁=?54(1)°, ψ₁= 129(1)°, ω₁=?177(1)°, ?₂ =57(1)°, ψ₂=15(1)°, ω₂ =?170(1)°, ?₃=80(1)°, ψ₃ =7(2)°, ω₃=?177(1)°, ?₄ =?108(1)° and ψ^T₄=?34 (1)° suggest that the peptide adopts a folded conformation with two overlapping β-turns of types II and III′. These turns are stabilized by two intramolecular hydrogen bonds between the CO of the Boc group and the NH of ΔPhe³ and the CO of Val¹ and the NH of Ala⁴. The torsion angles of ΔPhe² and ΔPhe³ side chains are similar and indicate that the two ΔPhe residues are essentially planar. The folded molecules form head-to- tail intermolecular hydrogen bonds giving rise to continuous helical columns which run parallel to the c-axis. This structure established the formation of two β-turns of types II and III′ respectively for sequences containing two consecutive ΔPhe residues at (i+2) and (i+3) positions with a branched β-carbon residue at one end of the tetrapeptide.     

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.     

Substituent effects on the puckering mode of the cyclobutane ring and the glycosyl bond of cis–syn photodimers

The cyclobutane ring (CB) puckering of a cis–syn DNA photodimer (cis–syn d-T[p]T) differs from that of a cis–syn RNA photodimer (cis–syn r-U [p] U) [J.-K. Kim and J. L. Alderfer (1992) Journal of Biomolecular Structure and Dynamics, Vol. 9 , p. 1705]. In cis–syn d-T [p] T, interconversion of the CB ring between CB⁺ and CB^? is observed, while in cis–syn r-U [p] U only CB^? is observed. In the CB⁺ conformation, the two thymine rings of the dimer are twisted in a right-handed fashion, as are the bases in B-form DNA. In case of CB^? they are twisted in a left-handed fashion. The C5 (base) and/or C2′ (sugar) substituents apparently affect the CB ring flexibility in cis–syn d-T [p] T and cis–syn r-U [p] U. To study the effects of the C5 substituent on CB ring flexibility, two-dimensional nuclear Overhauser effect (NOE) and ³¹P-nmr experiments were performed on cis–syn d-T [p] U, cis–syn d-U [p] T, and cis–syn d-U [p] U photodimers to investigate the CB puckering mode and overall molecular conformation and dynamics. The NOE results indicate the 5-methyl group in the photodimer induces conformational flexibility of the CB ring. In cis–syn d-T [p] U and cis–syn d-U [p] T, both CB⁺ and CB^? puckering modes are observed. This indicates interconversion between two modes takes place as observed in cis–syn d-T [p] T. In the case of cis–syn d-U [p] U, only the puckering CB^? mode is observed. All three DNA-type dimers—cis–syn d-T [p] U, cis–syn d-U [p] T, cis–syn d-U [p] U—show a characteristic flexibility of glycosidic bonds at the 5′ residue; cis–syn d-T [p] T demonstrates syn–anti interconversion for both the 3′ and 5′ sides, while the others are exclusively anti on the 3′ side. In contrast, the ribophotodimer, cis–syn r-U [p] U, lacking the C5 methyls and having a C2′-OH, demonstrates no conformational flexibility in the CB ring or in either of the glycosidic bonds. Differential flexibility of the three DNA-type dimers (cis–syn d-T [p] U, cis–syn d-U [p] T, cis–syn d-U [p] U) and the RNA dimer (cis–syn r-U [p] U) in the sugar-phosphate backbone region is also apparent from the temperature dependence of the ³¹P chemical shifts of these photodimers compared to their normal dimer analogues. Over the temperature range 18-63°C, the chemical shift change is reduced 22–42% in three DNA-type dimers, while it is reduced 71% in cis–syn r-U [p] U, suggesting the RNA-type dimer is more rigid. © 1993 John Wiley & Sons, Inc.     

Fungicidal Properties of Some Novel Trifluoromethylphenyl Amides

Trifluoromethylphenyl amides (TFMPAs) were designed and synthesized as potential pesticides. Thirty‐three structures were evaluated for fungicidal activity against three Colletotrichum species using direct bioautography assays. Active compounds were subsequently tested against C. fragariae, C. gloeosporioides, C. acutatum, Phomopsis obscurans, P. viticola, Botrytis cinerea and Fusarium oxysporum. The study identified 2‐chloro‐N‐[2,6‐dichloro‐4‐(trifluoromethyl)phenyl]acetamide ( 7a ) as showing the strongest antifungal activity, and the broadest activity spectrum in this set against Colletotrichum acutatum (at 48 and 72 h) and Phomopsis viticola (at 144 h). The presence of triethylamine in its complex with N‐[2,6‐dichloro‐4‐(trifluoromethyl)phenyl]‐2,2,3,3,3‐pentafluoropropanamide ( 7b′ ) played an important role in the bioactivity, and depending on the concentration or fungal species it showed higher or lower activity than the parent amide. X‐Ray crystallography has shown that the complex ( 7b′ ) is an ion pair, (C₁₀H₂Cl₂F₈NO)^? (C₆H₁₆N)⁺, where a proton is transferred from the amide nitrogen to the triethylamine nitrogen and then connected by hydrogen bonding to the acyl oxygen (N?H 0.893 Å; H???O 1.850 Å; N???O 2.711 Å; N?H???O 161.2(13)°). Although none of these compounds were better than standards, this work revealed some potential lead structures for further development of active novel compounds.     

Metal—phenoxyalkanoic acid interactions. Part 14. The crystal and molecular structures of diaquabis(4-fluorophenoxyacetato)copper(II) bis(4-fluorophenoxyacetic acid)dihydrate and tetra-μ-(4-fluorophenoxyacetato-0,0′)-bis[(2-aminopyrimidine)copper(II)]

The crystal structures of two copper(II) complexes of 4-fluorophenoxyacetic acid (4-FPAH) have been determined by X-ray diffraction. [Cu(4-FPA)₂(H₂O)₂]·2(4-FPAH)·2H₂O (1) is triclinic, space group P1 with Z = 1 in a cell of dimensions a = 14.808(2), b = 9.832(2), c = 6.847(2) Å, α = 87.77(2), β = 98.41(2), γ = 112.33(2)° and was refined to a residual of 0.038 for 1697 ‘observed’ reflections. The coordination sphere in this complex is tetragonally distorted octahedral comprising two waters [CuO, 1.940(3) Å], two unidentate carboxylate oxygens [CuO, 1.942(2) Å] and two ether oxygens [CuO, 2.471(2) Å]. Two adducted [4-FPAH] acid molecules are linked to the un-coordinated oxygens of the acid ligands by hydrogen bonds [2.547(4) Å]. [Cu₂(4-FPA)₄(2-aminopyrimidine)₂] (2) is triclinic, space group P1 with Z = 1 in a cell of dimensions a = 12.688(2), b = 11.422(2), c = 7.951(1) Å, α = 78.74(1), β = 107.51(1), γ = 75.78(1)°, and was refined to a residual of 0.042 for 2683 ‘observed’ reflections. (2) is a centrosymmetric tetracarboxylate bridged dimer with four similar CuO (equatorial) distances [1.967–1.987 Å; 1.977(3) Å mean] and the axial position occupied by the hetero nitrogen of the 2-aminopyrimidine ligand [CuN, 2.176(3) Å]. The Cu---Cu separation is 2.710(1) Å. Crystal data are also presented which confirm the isostructurality of complex (2) with [Cu₂(phenoxyacetate)₄(2-aminopyrimidine)₂], the Co^II, Mg^II and Mn^II4-fluorophenoxyacetate complexes with their phenoxyacetic and 4-chlorophenoxyacetic acid analogues, and of Cd^II4-fluorophenoxyacetate with Cd^II and Zn^II phenoxyacetates.     

Yellow Pigments of Aspergillus niger and Aspergillus awamori

Alkaline degradation of Aurasperone A, C₃₂H₂₆O₁₀, gave a binaphthyl (IIa), m.p. 255°C and acetone. (IIa) afforded a tetraacetate (IIb), C₃₂H₃₀O₁₂ m.p. 219°C and a tetramethyl ether (IId), C₂₈H₃₀O₈, m.p. 188°C. These facts along with the NMR spectra of aurasperone A and (IIb) confirm that aurasperone A is a dimeric 2-methyl-5-hydroxy-6,8-dimethoxy-4H-naphtho[2,3-b]pyran-4-one with asymmetric C-C linkage (7-10′ or 9-10′). The ether (IId) is not identical with 1,1′ ,3,3′ ?6,6′ ,8,8′-octamethoxy-4,4′-binaphthyl. Thus, it follows that (IId) is a 2,4′-binaphthyl and hence aurasperone A is 2,2′-dimethyl-5,5′- dihydroxy-6,6′,8,8′-tetrahydroxy-7,10′-bi[4H-naphtho[2,3-b]pyran-4-one] (I).     

Comparative structural studies of the first row early transition metal(III) chloride tetrahydrofuran solvates

Two compounds of empirical formula MCl₃- (THF)₃, M = V and Cr, have been characterized by single crystal X-ray studies. The VCl₃(THF)₃ molecule, which has a mer octahedral stereochemistry, crystallizes in the monoclinic space group P2₁/c with a= 8.847(2),b= 12.861(5),c= 15.134(3) Å, β = 91.94(2)°, V = 1721(1) ų and Z = 4. The V-Ci(1) and V-CI(2) distances have a mean value of 2.330 [3] Å while V-CI(3) = 2.297(2) Å, The VO(1) and VO(2) distances have a mean value of 2.061[8] Å while V-O(3) = 2.102(3) Å cis ClVCl angles average 92.0[5]° and cis OVO angles average 86.2[2]° . The isostmctural complex, CrCl₃(THF)₃, has a crystal structure made up of discrete octahedral mer-CrCl₃(THF)₃ molecules with the following unit cell dimensions (space group P2₁/c): a = 8.715(1), b= 12.786(3), c = 15.122(3) Å, β = 92.15(1)°, V = 1684(1) ų and Z = 4. The CrCl(1) and CrCl(2) distances have a mean value of 2.310131 Å while CrCl(3) = 2.283(2) Å. The CrO(1) and CrO(2) distances have a mean value of 2.0101171 Å while CrO(3) = 2.077(4) Å. cis ClCrCl angles average 90.9[4]° and cis OCrO angles average 86.1 [2]°. The structures of these two octahedral complexes and those previously reported for ScCl₃(THF)₃ and TiCl₃(THF)₃ are compared and certain general trends are discussed.     

Chain length dependent transition of 310- to α-helix of Boc-(Ala-Aib)n-OMe

An apolar synthetic octapeptide, Boc-(Ala-Aib)₄-OMe, was crystallized in the triclinic space group P1 with cell dimensions a = 11.558 Å, b = 11.643 Å, c = 9.650 Å, α = 120.220°, β = 107.000°, γ = 90.430°, V = 1055.889 ų, Z = 1, C₃₄H₆₀O₁₁N₈·H₂O. The calculated crystal density was 1.217 g/cm³ and the absorption coefficient ? was 6.1. All the intrahelical hydrogen bonds are of the 3₁₀ type, but the torsion angles, ? and ψ, of Ala(5) and Ala(7) deviate from the standard values. The distortion of the 3₁₀-helix at the C-terminal half is due to accommodation of the bulky Boc group of an adjacent peptide in the nacking. A water molecule is held between the N-terminal of one peptide and the C-terminal of the other. The oxygen atom of water forms hydrogen bonds with N (1) -H and N (2) -H, which are not involved in the intrahelical hydrogen bonds. The hydrogen atoms of water also formed hydrogen bonds with carbonyl oxygens of the adjacent peptide molecule. On the other hand, ¹H-nmr analysis revealed that the octapeptide took an α-helical structure in a CD₃CN solution. The longer peptides, Boc-(Ala-Aib)₆-OMe and Boc-(Ala-Aib)₈-OMe, were also shown to take an α-helical structure in a CD₃CN solution. An α-helical conformation of the hexadecapeptide in the solid state was suggested by x-ray analysis of the crystalline structure. Thus, the critical length for transition from the 3₁₀- to α-helix of Boc-(Ala-Aib)_n-OMe is 8. © 1993 John Wiley & Sons, Inc.     

Crystal and molecular structure of the (2,2′-diaminobiphenyl)(R,R-trans-1,2-diaminocyclohexane)platinum(II) complex

2,2′-Diaminobiphenyl-R,R-trans-1,2-diaminocyclohexaneplatinum(II) Chloride Trihydrate, (R,R-chxn)(dabp)Pt]Cl₂·3H₂O, crystallizes in the space group _p2₁2₁2₁ (D₂⁴, No. 19) with a = 6.219(4) Å, b = 17.633(2) Å, c = 21.523(3) Å, V = 2,360.4(8) ų, ?_calcd = 1.739 g cm^?3, ?_measd = 1.74 g cm^?3, and Z = 4. Diffraction data were collected with a Picker FACS-1 four-circle diffractometer. The structure was solved by the heavy atom method and refined by least-square calculations to residuals R = 0.0586 and weighted R = 0.0668. The 2,2′-diaminobiphenyl ligand exhibits complete stereospecificity in its coordination to platinum(II) ion with λ chiral conformation.     

Observation of a sterically unfavorable side-chain conformation in a leucyl residue: Crystal and molecular structure of L-leucyl–L-leucine · DMSO solvate

The crystal structure of a dipeptide L -leucyl–L -leucine (C₁₂H₂₄N₂O₃) has been determined. The crystals are monoclinic, space group P2₁, with a = 5.434(4) Å, b = 15.712(7) Å, c = 11.275(2) Å, β = 100.41(1)°, and Z = 2. The crystals contain one molecule of dimethyl sulfoxide (DMSO) as solvent of crystallization for each dipeptide molecule. The structure has been solved by direct methods and refined to a final R index of 0.059 for 920 reflections (sinθ/λ ? 0.60 Å^?1) with I ? 2σ (I). The trans peptide unit shows substantial degree of non-planarity (Δω = 14°). The peptide backbone adopts an extended conformation with torsion angles of ψ₁ = 138(1)°, ω₁ = 166(1)°, ?₂ = ? 149.3(7)°, ψ₂₁ = 164.2(7)°, and ψ₂₂ = ? 15(1)°. For the first leucyl residue, the side-chain conformation is specified by the torsion angles ¹χ₁ = 176.7(7)°, ¹χ₂₁ = 62(1)°, ¹χ₂₂ = ? 177.4(8)°; the second leucyl residue adopts a Sterically unfavorable conformation with ²χ₁ = 61(1)°, ²χ₂₁ = 97(1)°, and ²χ₂₂ = ?151(1)°. The packing involves head-to-tail interaction of peptide molecules and segregation of polar and nonpolar regions. The DMSO molecule is strongly hydrogen bonded to the terminal NH group. © 1994 John Wiley & Sons, Inc.     

Metal-phenoxyalkanoic acid interactions. Part 15. The crystal structures of diaquabis(phenoxyacetato)cadmium(II) and catena-μ-[aquabis(phenoxyacetato-O)lead(II)-bis(phenoxyacetato)lead(II)]

The crystal structures of the cadmium(II) and lead(II) complexes of phenoxyacetic acid (PAH) have been determined by single crystal X-ray diffraction techniques. The cadmium complex, [Cd(PA)₂(H₂O)₂] (1), space group C2, with Z = 2 in a cell of dimensions, a = 11.801(2), b = 5.484(1), c = 13.431(3) Å, β = 100.87(2)°, possesses a distorted trapezoidal bipyramidal coordination around the metal atom, involving two water oxygens [2.210(5) Å] and four carboxyl oxygens from two symmetrical bidentate phenoxyacetate ligands [2.363(4), 2.365(4) Å] with Cd lying on the crystallographic two- fold axis. The lead complex, [Pb₂(PA)₄(H₂O)]_n(2) is triclinic, space group P$\text{1}$, Z = 2, with a cell of dimensions, a = 10.135(4), b = 10.675(3), c = 19.285(9) Å, α = 114.66(3), β = 91.94(3) and γ = 114.99(3)°. (2) is a two-dimensional polymer with a repeating dimer sub-unit. The first lead [Pb(1)] has an irregular MO₈ coordination [2.34?2.96(2) Å: mean, 2.63(2) Å] involving the water molecule, two oxygens from an asymmetric bidentate carboxylate group, two from a bidentate chelate [O(ether), O(carboxylate)] group and three from bridging oxygens, one of which also provides a polymer link to another symmetry generated lead. The second lead [Pb(2)] is irregular seven-coordinate [PbO, 2.48?2.73(2) Å: mean, 2.61(2) Å] with three bonds from the bridging groups, two from an unsymmetrical bidentate carboxylate (O, O′) group and one from a second carboxyl group which also bridges two Pb(2) centres in the polymer.     

The synthesis of technetium(V) complexes containing sterically bulky thiolate ligands. The molecular structure of tetrabutylammonium oxotetrakis(2,4,6-trimethylbenzenethiolato)technetate(V)

The reactions between [TcOCl₄]⁻ and the sterically bulky thiols ArSH (Ar = 2,4,6-Me₃C₆H₂, 2,4,6- Prⁱ₃C₆H₂ and 2,6-Ph₂C₆H₃) in methanol afford complexes of formula [TcO(SAr)₄]⁻ which may be isolated as salts with bulky organic cations. The molecular structure of [Buⁿ₄N][TcO(2,4,6-Me₃C₆H₂S)₄] was determined by X-ray diffraction methods. The Tc(V) centre was found to adopt the expected square pyramidal geometry in which an oxo group occupies the apical site and the four thiolate sulphurs the basal sites. The TcO distance is 1.659(11) Å and the average TcS distance 2.38(2) Å. The average cis STcS, trans STcS and OTcS angles are respectively 82.7(6)°, 138.4(3)° and 110.8(4)°.     

The preparation and structure of tris[bis(pyrazolyl)borate]indium(III)

The compound In[(pz)₂BH₂]₃ (pz = 1-pyrazolyl, C₃H₃N₂⁻) was prepared from In(NO³)₃ and K[(pz)₂-BH₂] in water, and characterised by spectroscopic and X-ray methods. Crystals are orthorhombic,Pna2₁,a = 20.279(4),b = 8.884(2),c = 13.411(2)Å;R = 0.0285. Individual molecules contain a near-regular six-coordinate indium atom with In–N (av.) 2.241(5)Å. The pyrazolyl borate ligands are puckered, with dihedral angles between the two rings of each ligand in the range 133–144°.