Heterocyclic compounds containing antimony 1. Synthesis, physicochemical properties, crystal and molecular structure of 2-(β-hydroxyethylthio) 1,3,2-oxathiastibolane

The compound (HOCH₂CH₂S) ) (1) has been prepared by the reaction of antimony(III) isopropoxide and 2-mercaptoethanol in a 1:2 molar ratio. Reaction of 1 with MOCH₃ (where M = Li, Na and K) yields bimetallic products of the type, M[(OCH₂CH₂S) )]. All these derivatives have been characterized by elemental analysis, IR, NMR (¹H and ¹³C) spectra and molar conductivity measurements. Crystals of 1 are triclinic, space group P , with a = 6.449(2), b = 10.285(2), c = 13.494(1) Å, α = 78.08(1), β = 75.99(1), γ = 71.54(2)°, V = 815.48 ų, Z = 4, D_calc = 2.239 g cm⁻³, (Mo Kα) λ = 0.7107 Å, μ = 3.55 mm₋₁, F(000) = 528, T = 295 K, final R = 0.0189 for 2344 reflections. One of the two mercaptoethanol moieties in 1 forms a five-membered chelate ring with antimony, Sb(1)---O(11) = 2.023(2) Å and Sb(1)---S(11) = 2.434(1) Å, while the other is bonded through the S atom only, Sb(1)---S(12) = 2.434(1) Å. The angles between these primary bonds with a mean value of 90.2° suggest a basically pyramidal, or pseudo tetrahedral structure if the stereochemically active lone pair is included in the coordination sphere. Two molecules are linked by intermolecular hydrogen bridges. The presence of weak intermolecular secondary bonding, Sb(1)---O(12) = 2.567(3) Å, in the complex indicates that the overall coordination polyhedron is best described in terms of a distorted trigonal bipyramidal arrangement.     

Synthesis of [Tl4Se16]: a novel tetranuclear Tl polyselenide

(Ph₄P)₄[Tl₄Se₁₆] was prepared hydrothermally in a sealed pyrex tube by the reaction of TlCl, K₂Se₄ and Ph₄PCl in a 1:1:1 molar ratio at 110 °C for one day. The red crystals were obtained in 50% yield. Crystals of (Ph₄P)₄[Tl₄Se₁₆]: triclinic P (No. 2), Z=1, a=12.054(9), b=19.450(10), c=11.799(6) Å, α=104.63(4), β=98.86(6), γ=101.99(6)° and V=2555(3) ų at 23 °C, 2θ_max=40.0°, μ=120.7 cm⁻¹, D_calc=2.23. The structure was solved by direct methods. Number of data collected: 5206. Number of unique data having F_o²>3σ(F_o²): 1723. Final R=0.075 and R_w=0.089. [Tl₄Se₁₆]⁴⁻ consists of four, almost already linearly arranged, tetrahedral thallium centers which are coordinated by two chelating Se₄²⁻, two bridging Se₂²⁻ and four bridging Se²⁻ ligands. [Tl₄Se₁₆]⁴⁻ sits on an inversion center and possesses a central {Tl₂Se₂}²⁺ planar core. The Tl(1)–Tl(1)′ distance in this core is 3.583(6) Å. These two thallium atoms are then each linked to two cyclic Tl(Se₄) fragments via bridging Se₂²⁻ and Se²⁻ ligands forming Tl₂Se(Se₂) five-membered rings.     

Synthesis and structure of an organosamarium aryloxide complex, (C5 Me5)2 Sm(OC6HMe4-2,3,5,6)

Bis(pentamethylcyclopentadienyl)samarium bis- (tetrahydrofuranate), (C₅Me₅)₂Sm(THF)₂, reacts with 2,3,5,6-tetramethylphenol in toluene to yield (C₅Me₅)₂Sm(OC₆HMe₄-2,3,5,6). The compound crystallizes in the space group P2₁/c with a = 8.725(3) Å, b=18.821(6) Å, c=18.461(6) Å, β= 111.17(2)°, V = 2827(2) ų and D_c=1.340 g cm⁻³ for Z = 4. Molecules of the aryloxide complex are monomeric and exhibit a bent metallocene structure with a nearly linear Sm---O---C(aryloxide) linkage: Sm---O = 2.13(1) Å, O---C = 1.29(2) Å, and Sm---O---C = 172.3(13)°. Reaction of the samarium complex with phenyllithium produces the previously- characterized species (C₅Me₅)₂Sm(C₆H₅)(THF).     

Synthesis and crystal structure of the MgCl2(CH3COOC2H5)2· (CH3COOC2H5) adduct

The reaction of α-MgCl₂ with boiling ethyl acetate affords MgCI₂(CH₃COOC₂H₅)₂· (CH₃COOC₂H₅), which is obtained as crystals suitable for X-ray analysis only from the mother liquor. M=315.5, orthorhombic, space group P2₁22₁ (No. 18), a=25.077(3), b=8.616(1), c=7.345(1) Å, V=1587.0(3) ų, Z=4, D_x=1.32 g cm⁻³,λ A(Mo Kα)=0.71069 Å, μ=4.17 cm⁻¹, F(000)=664, T=298 K, observed reflections: 1667, R=0.059 and R_w=0.069. The structure is composed of polymeric chains of MgCl₂(CH₃COOC₂H₅₎₂ and the ethyl acetate molecules occupy a mutually trans position.     

The X-ray structure analysis of bis-2,2′,N,N′-bipyridyl ketone cobalt(III) nitrate dihydrate

An X-ray structural analysis of bis-2,2′,N,N′-bipyridyl ketone cobalt(III) nitrate dihydrate, CoC₂₂H₂₀N₄O₄⁺· NO₃⁻·2H₂O,M_r=559.38 g/mol, P , a=8.862(2), b=16.195(3), c=8.772(2) Å, α=103.54(2), β=95.74(3), γ=105.07°, V=1164.4(4) ų, Z=2, D_x=1.595 g/cm³, Mo Kα radiation (λ=0.71073 Å), μ=7.8 cm⁻¹ and R=0.079, revealed a Co(III) cation in a slightly distorted octahedral environment. The structure reveals that the ligand di-2-pyridyl ketone (dpk) has undergone a hydration reaction across the ketone double bond and one of the hydrate oxygen atoms coordinated to the metal forming a tridentate chelate. This new Co(dpk-hydrate)₂⁺ complex displays the least distorted geometry yet reported for either 1:1 or 1:2 (metal:ligand) complexes. A geometry optimization using the INDO model Hamiltonian as implemented in the program ZINDO was performed on the title complex with the Co³⁺ modeled as a singlet. The result of the computation corroborates the geometry of the title complex as that expected for Co³⁺.     

Molecular conformation of prostaglandin A1 monoclinic cyrstalline polymorph

The molecular conformation of the monoclinic crystalline polymorph of prostaglandin A₁ has been determined by X-ray diffraction techniques. The space group is P2₁ with a = 13.637 (2), b = 7.567 (1), I c = 10.576 (2) Å, β = 107.37 (3)°; D_c = 1.073 g·cm⁻³ for Z = 2. The molecular conformation is characterized by the nearly parallel arrangement of the C₁–C₇ and C₁₃–C₂₀ side chains, with a general flattening of the overall structure when compared with the orthorhombic polymorph. The cyclopentenone moiety assumes a C₈ envelope conformation with C₈ and O₉ displaced +0.29 Å and −0.18 Å from the C₉–C₁₀=C₁₁–C₁₂ plane respectively. Concerted, small variations of the torsion angles, primarily about the C₈–C₁₂, C₁₄–C₁₅ and C₁₆–C₁₇ bonds, bring the monoclinic and orthorhombic conformations into coincidence.     

Synthesis and magnetic properties of bis(μ-hydroxo)bis[(2,2 ′-bipyridyl)copper(II)] squarate. Crystal structure of bis(μ-hydroxo)bis[(2,2′-bipyridyl)copper(II)] squarate tetrahydrate

The compound [Cu₂(bipy)₂(OH)₂](C₄O₄)·5.5H₂O, where bipy and C₄O₄²⁻ correspond to 2,2′-bipyridyl and squarate (dianion of 3,4-dihydroxy-3-cyclo- butene-1,3-dione) respectively, has been synthesized. Its magnetic properties have been investigated in the 2–300 K temperature range. The ground state is a spin-triplet state, with a singlet-triplet separation of 145 cm⁻¹. The EPR powder spectrum confirms the nature of the ground state.Well-formed single crystals of the tetrahydrate, [Cu₂(bipy)₂(OH)₂](C₄O₄)·4H₂O, were grown from aqueous solutions and characterized by X-ray diffraction. The system is triclinic, space group P , with a = 9.022(2), b = 9.040(2), c = 8.409(2) Å, α = 103.51(2), β = 103.42(3), γ = 103.37(2)°, V = 642.9(3) ų, Z = 1, D_x = 1.699 g cm⁻³, μ(Mo Kα) = 17.208 cm⁻¹, F(000) = 336 and T= 295 K. A total of 2251 data were collected over the range 1θ25°; of these, 1993 (independent and with I3σ(I)) were used in the structural analysis. The final R and R_w residuals were 0.034 and 0.038 respectively. The structure contains squarato-O¹, O³-bridged bis(μ-hydroxo)bis[(2,2′-bipyridyl)copper(II)] units forming zigzag one-dimensional chains. Each copper atom is in a square-pyramidal environment with the two nitrogen atoms of 2,2′-bipyridyl and the two oxygen atoms of the hydroxo groups building the basal plane and another oxygen atom of the squarate lying in the apical position.The magnetic properties are discussed in the light of spectral and structural data and compared with the reported ones for other bis(μ-hydroxo)bis[(2,2′-bipyridyl)copper(II)] complexes.     

Crystallization and Preliminary X-Ray Analysis of Neuropsin, a Serine Protease Expressed in the Limbic System of Mouse Brain

Neuropsin (M_r25 032) is a serine protease expressed in the limbic system of mouse brain. It has been implicated in various neurological processes including formation of memory and may be important as a drug target in the treatment of epilepsy. The recombinant protein was produced using a baculovirus expression system and was purified. Two crystal forms were obtained by a hanging-drop vapor-diffusion method with polyethylene glycol. Preliminary X-ray crystallographic analysis revealed that crystal form I belongs to triclinic space groupP1 with unit cell dimensionsa= 97.16 Å,b= 97.12 Å,c= 46.75 Å and α = 99.17°, β = 99.77°, γ = 117.35°. Self-rotation function analysis of these data of form I indicates the position of a noncrystallographic threefold axis. There are six molecules in the crystallographic asymmetric unit. Crystal form II also belongs to triclinic space groupP1 but has unit cell dimensions ofa= 38.40 Å,b= 55.16 Å,c= 65.37 Å and α = 95.38°, β = 89.98°, γ = 110.46° with two molecules in the crystallographic asymmetric unit. Form II has a noncrystallographic twofold axis. Intensity data to 3.1 Å resolution for form I and to 2.2 Å resolution for form II have been collected.     

Metal dependent coordination of biomolecular ligand: X-ray crystal structures of copper, cobalt and calcium complexes of (3-hydroxy-5-(hydroxymethyl)-2-methylisonicotinic acid) 5-phosphate, an oxidized pyridoxal 5-phosphate

X-ray crystal analyses of divalent copper, cobalt and calcium complexes of monoanionic (3-hydroxy-5-(hydroxymethyl)-2-methylisonicotinic acid) 5-phosphate (L₁C₈H₉NO₇P⁻) revealed the chemical compositions of Cu ---L·3H₂O(1), Co ---L·5H₂O(2) and Ca·L₂·7H₂O (3) and the coordination structures which depend on the coordination abilities and chemical properties of the respective metal ions. Although 1 and 2 crystals showed similar features, i.e., presence of the metal ion at the crystallographic center of symmetry and octahedral six-coordination, the patterns of coordination with the ligand molecules differed. While direct coordination to the L carboxyl oxygen was observed in 1 crystals, all ligation positions in 2 crystals were occupied by water molecules. On the other hand, 3 crystals formed a pentagonal bipyramidal structure (seven-coordination), where oxygens of L phosphates and water molecules coordinated to the calcium ion. Each of the complex structures showed characteristic molecular packing depending on the pattern of coordination to the respective metal ion. L is monoanionic in all complex crystals, where the phosphate and carboxyl groups are deprotonated and pyridine nitrogen is protonated, and is neutralized by each metal ion. Crystal data: 1, monoclinic, space group P2₁/c, A = 5.4129(6), B = 10.515(2), C = 22.770(2) Å, β = 91.853(9)°, Z = 4, R = 0.0404 for 1834 observed reflections; 2, triclinic, space group

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, c = 6.789(3) Å, α = 96.84(3), β = 109.10(3), γ = 100.50(2)°, Z = 2, R = 0.0684 for 1605 observed reflections; 3, triclinic, , a = 10.069(2), B = 14.501(3), c = 10.051(1) Å, α = 100.75(1), β = 97.28(2), γ = 76.18(2)°, Z = 2, R = 0.0540 for 3637 observed reflections.     

The structures of bis(1H,5H-S-methylisothiocarbonohydrazidium) di-μ-chloro-octachlorodibismuthate(III) tetrahydrate and tris(1H-S-methylisothiocarbonohydrazidium) esachlorobismuthate(III)

The structures of bis(1H⁺,5H⁺-S-methylisothiocarbonohydrazidium) di-μ-chlorooctachlorodibismuthate(III) tetrahydrate: (C₂H₁₀N₄S)₂(Bi₂Cl₁₀)· 4H₂O (compound [I]) and of tris(1H⁺-S-methylisothiocarbonohydrazidium) esachlorobismuthate(III): (C₂H₉N₄S)₃(BiCl_5.67I_0.33) (compound [II]) were determined from single crystal X-ray diffractometer data. Both compounds crystallize as triclinic (P ); crystals [I] with Z = 1 formula unit in a cell of constants: A = 10.621(3), B = 9.989(5), C = 7.439(3) Å, α = 88.31(2), β = 84.51(2), γ = 68.88(2)°, final R = 0.0427 for 2229 unique reflections with I 2σ(I); crystals [II] with Z = 2 and cell dimensions: A = 14.109(4), B = 12.209(9), C = 8.206(7) Å, α = 103.54(3), β = 104.95(2), γ = 81.96(2)°, final R = 0.0411 for 3637 unique reflections (1 2σ(I)). The structure of [I] is built up of diprotonated organic cations, water molecules and dinuclear centrosymmetric [Bi₂Cl₁₀]⁴⁻ anions held together by N-HCl, N-HO, O-HCl hydrogen bonds and Van der Waals interactions. The [Bi₂Cl₁₀]⁴⁻ complex consists of two edge-sharing octahedra in which three pairs of bonds of similar length are observed (Bi-Cl_av = 2.602(5), 2.712(4), 2.855(5) Å). The structure of [II] consists of monoprotonated cations and [BiCl_5.67I_0.33]³⁻ anions held together by a tridimensional network of hydrogen bonds. Each bismuth atom is octahedrally surrounded by six chlorine atoms, one of which is statistically substituted by a iodine atom.     

Conformational flexibility in the molecular structure of rotenone, a naturally occurring insecticide

The crystal and molecular structure of rotenone, a naturally occurring insecticide with mitochondrial and mitotic spindle inhibitory properties, was determined by direct methods. The crystals were orthorhombic, space group, P2_I2_I2_I with two molecules in the asymmetric unit; a = 8.413 (1) Å, b = 19.840(1), c = 23.581(1), V = 3936 ų, Z = 8. The structure was refined by least-squares methods to a final R = 0.067. The two molecules in the asymmetric unit have different conformations about the junction between the nonaromatic rings B and C. Ring B is in a sofa conformation in both molecules, with a slight distortion toward a half-chair in I, but with C8 and C8′ on opposite sides of the planar part of the rings. This difference in conformation results in I having an extended (linear) shape while II is V-shaped. The more elongated conformation of the molecule (I) has not been reported in previous studies. Ring C also has opposite conformations in the two molecules. The angle between the planes formed by rings A and D in molecule I is 64.3°, while in molecule II it is 88.3°. Molecular mechanics techniques were used to determine the energy of the two conformations. These calculations, at room temperature, predict molecule II to be the more stable conformer. The highly flexible site in the B/C ring junction is also chemically very reactive. This flexibility and reactivity are further discussed in terms of rotenone's inhibitory activities.     

Anomalous carbonylation of [Pd(dppm)(O2CCF3)]2 to give an asymmetric μ-CO complex

The reactive palladium dimer, [Pd(dppm)(O₂CCF₃)]₂, is carbonylated to [Pd(dppm)(O₂CCF₃)]₂(μ-CO) in a reversible reaction with K = c. 7.2(2)x10⁴ atm⁻¹ (P_1/2 = c. 2.4 Torr). This is significantly larger than is expected based on the λ_max = 280 nm in the electronic spectrum. The product can be isolated in analytically pure form by crystallization under a CO atmosphere. It forms crystals in the monoclinic space group Cc with a = 18.584(5), b = 28.65(1), c = 11.164(3) Å and β = 95.16(2)°. The structure is significantly distorted. The bonding about the two palladium atoms is quite asymmetric. While one is close to a square planar geometry with a Pd---C(O) distance of 1.90(2) Å, the other is significantly pyramidalized and has a longer (2.00(2) Å) bond to the bridging CO. The Pd---Pd distance is only 2.896(2) Å, much shorter than that usually observed for formally non-bonded Pd atoms.     

Tetrachloroplatinate(II) and hexachloroplatinate(IV) salts of N,N,N′,N′-tetramethylethylenediammonium: significant differences in the conformation of the cation, as shown by infrared and x-ray diffraction data

The structures of (H₂tmen)[PtCl₄], (1), (H₂tmen = N,N,N′,N′-tetramethylethylenediammonium), [triclinic, P ; A = 7.344(3), B = 8.345(3), C = 6.216(2) Å, α = 84.53(3), β = 109.22(3), γ = 69.43(3)°, Z = 1] and (H₂tmen)[PtCl₆], (2), [monoclinic, P2₁/a; A = 14.409(4), B = 12.736(7), C = 8.601(3) Å, β = 99.58(3)°, Z = 4] were determined from diffractometric data by Patterson and Fourier methods and refined by full-matrix least-squares to R = 0.027 and 0.039 for (1) and (2) respectively. In both cases the anions and cations are joined in polymeric chains through hydrogen bonds involving the protonated nitrogens and the co-ordinated chlorine atoms. The square-planar [PtCl₄]²⁻ and octahedral [PtCl₆]²⁻ anions are centrosymmetric; the H₂tmen²⁺ cations are centrosymmetric in (1) with a N-C-C-N dihedral angle of exactly 180°, while in (2) the dihedral angle is 166°. The different symmetry of the organic moieties/C_i and C₁ in (1) and (2) respectively] results in a different i.r. spectrum which is more complex the lower the symmetry; moreover the spectrum of a KBr pellet of (2) changes with time, finally resembling that of (1) and of the (H₂tmen)Cl₂ salt.     

Structure and properties of a novel OH bridged dimetal helical complex with 6,6-dimethyl-2,2′:6′,2″:6″,2:6,2-quinquepyridine ligand

A new monohelical OH⁻ bridged dinuclear complex [Zn₂(dmqpy)(OOCCH₃)₂(μ-OH)][ClO₄] · 0.5EtOH, where dmqpy is 6,6-dimethyl-2,2′:6′,2″:6″,2:6,2-quinquepyridine, has been synthesized and characterized by X-ray crystallography: monoclinic, space group P2₁/c, a=13.670(1), b=14.751(1), c=16.782(1) Å, β=96.59(1)°, U=3361.7(4) ų, Z=4, R=0.0601. Two Zn(II) ions are in different coordination modes, one is five-coordinate with a N₃O₂ donor set and the other is N₂O₂ four-coordinate with a distorted tetrahedral geometry, and the zinc ions are bridged by a hydroxyl group. The presence of the OH⁻ bridge is further confirmed by electrospray mass and infrared spectroscopies. The solution properties of the complex were investigated by ¹H NMR spectroscopy. The results of NMR indicate that the complex has higher symmetry in solution than in the solid state.     

Binuclear complexes of a new hexadentate macrocyclic ligand. The crystal and molecular structure of [LCu2(CH3CO2)2](ClO4)2·5H2O

The 30-membered hexaaza macrocylic ligand, L (L=3,7,11,18,22,26-hexaazatricyclo-[26.2.2.2^13,16]tetratriaconta-1(31),13(33),14,16(34),28(32),29-hexaene), is capable of forming binuclear complexes with the divalent transition metal ions Ni, Cu and Zn. The two metal ions are bound by the two dipropylenetriamine units of the macrocycle. Extra coordination sites on the metal ions can be occupied by exogenous ligands such as acetate, chloride and thiocyanate. The crystal structure of one of the di-copper complexes is described: [LCu₂(CH₃CO₂)₂](ClO₄)₂·5H₂O crystallizes in the monoclinic space group P2₁/c (No. 14), with a=9.369(2), b=17.644(3), c= 27.466(3) Å, β=92.90(1)°, U=4534.7 ų and Z=4. The Cu1···Cu2 separation is 8.40(3) Å. The access for potential exogenous bridging ligands, to the cavity between the copper ions, is somewhat restricted by the two phenyl units of the macrocycle which appear almost parallel in the structure. The redox potential of the couple L(Cu²⁺)₂/L(Cu⁺)₂, recorded by cyclic voltammetry for the chloride adduct, [LCu₂Cl₂]Cl₂·5H₂O, is −0.061 V versus SCE in DMF.     

New Crystal Forms of the Motile Major Sperm Protein (MSP) ofAscaris suum

We have obtained two new crystal forms of theAscarismajor sperm protein (MSP) that mediates amoeboid cell motility in nematode sperm. We obtained crystals with C2 symmetry from bacterially expressed α-MSP witha= 216.5 Å,b= 38.6 Å,c= 32.5 Å, γ = 93.1° and also crystals with P2₁symmetry from native β-MSP witha= 63.1 Å,b= 91.7 Å,c= 72.5 Å, γ = 91.3°. A full native data set has been collected for each crystal form using synchrotron radiation. Both crystal forms diffract to 2 Å and are suitable for high-resolution structural investigation.     

Synthesis and crystal structure of a complex between lanthanum nitrate and a bicyclic polyether

Reaction between lanthanum nitrate hexahydrate and a macrobicyclic polyether in ethanol has yielded a product of overall stoichiometry 3:2. The ligand, 21R, 26S, 29R, 34S-21, 22, 23, 24, 25, 26, 29, 30, 31,32,33, 34-dodecahydro-1,4,7,14,17,20,28,35-octaoxa (2^3,29syn2^18,34syn) (7.7) orthocyclophane, L, provides 8 oxygen donor atoms. Crystals were obtained from MeOH/EtOH (50/50). Crystal structure determination on 9590 observations, R=0.059, has shown the triclinic unit cell, a = 26.562(6), b = 13.486(3), c = 12.154(3) Å, α=63.9(1), β = 100.0(1), γ = 102.0(1)° space group P , V = 3806 Å to contain, as the asymmetric unit, two complex cations (La(NO₃)₂L)⁺ and one complex anion (La- (NO₃)₅MeOH)²⁻. The lanthanum is 11-coordinated in the anion and one of the cations, in which there is one bidentate and one monodentate nitrate anion and 12-coordinated in the other cation. For the monodentate nitrate La---O = 2.448(9) Å, all other nitrate ions are bidentate (La---O = 2.594(9)−2.743(10) Å). Most La---O bonds are shorter in the 11 than in the 12-coordinated cation. There are large differences in La---O bond lengths according to the nature of the carbon atoms to which the oxygen is attached. The methanol molecule forms a hydrogen bond to one oxygen atom of the monodentate nitrate group.     

Water and ion permeability of a native collagen membrane

With the help of a ribonucleoprotein it is possible to precipitate collagen in a layer of fibers with a 700 Å period. As collagen is a constituent of many membrane systems in the body, it seemed interesting to investigate the permeability of ions and water through a native collagen membrane.The experiments were carried out with the help of an acryl glass apparatus, where an osmotic pressure, a hydrostatic pressure difference or both can be maintained between the two bulk phases separated by the membrane. The diffusion coefficients for NaCl and KCl were found to be comparable with those in other biological membranes (D_s = 9 · 10⁻⁷cm² · s⁻¹) whereas there is difference of more than three orders of magnitude in the hydraulic permeability (L_p = 6 cm⁴ · J⁻¹ · s⁻¹).Volume flow measurements caused by an osmotic gradient indicated that the reflection coefficient for NaCl and KCl is very small. In hydrostatic pressure experiments, the membrane shows a preferred direction for volume flows which seems to have something to do with the mode of preparation of the membrane.     

Permeability of bilayer lipid membranes for superoxide (O2) radicals

Egg yolk phosphatidylcholine monolamellar liposomes (1000 Å in diameter) loaded with cytochrome c were placed into an external solution, in which superoxide radicals, O₂⁻, were generated by a xanthine-xanthine oxidase system. The penetration of the superoxide radicals across the liposomal membrane was detected by cytochrome c reduction in the inner liposome compartment. The effects of modifiers and temperature on this process were studied. The permeability of liposomal membrane for O₂⁻(P′_O2⁻ = (7.6 ± 0.3) · 10^-8 cm/s), or HO₂^• (P′_HO2⁻ = 4.9 · 10^-4 cm/s) were determined. The effect of the transmembrane electric potential (K⁺ concentration gradient, valinomycin) on the permeability of liposomal membranes for O₂⁻ were investigated. It was found that O₂⁻ can penetrate across liposomal membrane in an uncharged form. The feasibility of penetration of superoxide radicals through liposomal membrane, predominantly via anionic channels, was demonstrated by the use of an intramolecular cholesterol-amphotericin B complex.     

Kinetics and mechanism of CO substitution of [η-CpFe(CO)3]PF6 with PPh3 in the presence of substituted pyridine N-oxide

The kinetics of substitution reactions of [η-CpFe(CO)₃]PF₆ with PPh₃ in the presence of R-PyOs have been studied. For all the R-PyOs (R = 4-OMe, 4-Me, 3,4-(CH)₄, 4-Ph, 3-Me, 2,3-(CH)₄, 2,6-Me₂, 2-Me), the reactions yeild the same product [η⁵-CpFe(CO)₂PPh₃]PF₆, according to a second-order rate law that is first order in concentrations of [η⁵-CpFe(CO)₃]PF₆ and of R-PyO but zero order in PPh₃ concentration. These results, along with the dependence of the reaction rate on the nature of R-PyO, are consistent with an associative mechanism. Activation parameters further support the bimmolecular nature of the reactions: ΔH^≠ = 13.4 ± 0.4 kcal mol⁻¹, ΔS^≠ = −19.1 ± 1.3 cal k⁻¹ mol⁻¹ for 4-PhPyO; ΔH^≠ = 12.3 ± 0.3 kcal mol⁻¹, ΔS^≠ = 24.7 ±1.0 cal K⁻¹ mol⁻¹ for 2-MePyO. For the various substituted pyridine N-oxides studied in this paper, the rates of reaction increase with the increasing electron-donating abilities of the substituents on the pyridine ring or N-oxide basicities, but decrease with increasing ¹⁷O chemical shifts of the N-oxides. Electronic and steric factors contributing to the reactivity of pyridine N-oxides have been quantitatively assessed.