Structure, bonding, and ligand-based reactions of zwitterionic boratoiridium(I) complexes with oxazolinyl scorpionate ligands

New tris(4,4-dimethyl-2-oxazolinyl)phenylboratoiridium(I) scorpionate-type compounds [Ir(To^M)L₂] (L₂ = η⁴-C₈H₁₂ and (CO)₂) and electrophiles form adducts that contain a bidentate IrTo^M-coordination and an N-electrophile interaction of the third oxazoline instead of the oxidative addition product. The adduct with lithium chloride gives a unique heterobimetallic Li-O-oxazoline-N-Ir bridging structure that has been identified through X-ray crystallography. Density functional theory calculations provide thermodynamic data, orbital symmetries, and orbital energies that explain the formation of the observed iridium(I) products.     

Reconstitution and Engineering of Apoptotic Protein Interactions on the Bacterial Cell Surface

The interactions between pro- and anti-apoptotic members of the Bcl-2 class of proteins control whether a cell lives or dies, and the study of these protein-protein interactions has been an area of intense research. In this report, we describe a new tool for the study and engineering of apoptotic protein interactions that is based on the flow cytometric detection of these interactions on the surface of Escherichia coli. After validation of the assay with the well-studied interaction between the Bak(72-87) peptide and the anti-apoptotic protein Bcl-x_L, the effect of both increasing and decreasing Bak peptide length on Bcl-x_L binding was investigated. Previous work demonstrated that the Bak(72-87) peptide also binds to the anti-apoptotic protein Bcl-2, albeit with lower binding affinity compared to Bcl-x_L. Here, we demonstrate that a slightly longer Bak peptide corresponding to amino acids 72-89 of Bak binds Bcl-x_L and Bcl-2 equally well. Approximate binding affinity calculations on these peptide-protein complexes confirm the experimental observations. The flow cytometric assay was also used to screen a saturation mutagenesis library of Bak(72-87) variants for improved affinity to Bcl-x_L. The best variants obtained from this library exhibit an apparent K_d to Bcl-x_L 4-fold lower than that of wild-type Bak(72-87).     

Self-association of helical peptides in a lipid environment

The self-association of two model transmembrane helical peptides, differing in their surface topography, was compared in mixed micelles containing 3-([3-cholamidopropyl]dimethylammonio)-1-propanesulfonate (CHAPS) and dimyristoylphosphatidylcholine (DMPC). One peptide, Ac-KKL₂₄KK-amide (L₂₄), has large, rotationally mobile leucine side chains and a relatively rough surface. The other peptide, Ac-KKLLLLLLAALLALLAALLALLLLLLKK-amide (L₁₈A₆), has a patch of small alanines on one side of the helix that forms a smooth surface. The aggregation state of the peptides was sampled by chemical cross-linking with bis-sulfosuccinimidyl suberate (B53). A monomer-aggregate association constant was obtained from the cross-linking results in the range of 2 × 10⁵ M^–1 to 3 × 10⁵ M^–1 for both peptides. Kinetics of formation of cross-linked dimers indicated that the ratio of dimerization constants for L₁₈A₆ to L₂₄ was between 10 and 20. This suggests that the alanine patch contributes about 1.5 Kcal/mol more stabilization free energy to dimer formation of L₁₈A₆ compared to L₂₄.     

Two star‐shaped tetranuclear Ru(II) complexes containing uncoordinated imidazole groups: synthesis,characterization, photophysical and pH sensing properties

Tetrapodal ligands H₄L¹ and H₄L² containing imidazole groups have been synthesized by the reaction of 1,10‐phenanthroline‐5,6‐dione with 1,2,4,5‐tetrakis[(4‐formylphenoxy)methyl]benzene and 1,2,4,5‐tetrakis[(3‐formylphenoxy)methyl]benzene, respectively, in presence of NH₄OAc. Two star‐shaped complexes [{Ru(bpy)₂}₄(μ₄‐H₄L¹)](PF₆)₈ and [{Ru(bpy)₂}₄(μ₄‐H₄L²)](PF₆)₈ (bpy = 2,2′‐bipyridine) have been prepared by refluxing Ru(bpy)₂Cl₂·2H₂O and each ligand in ethylene glycol. The deprotonated complexes [{Ru(bpy)₂}₄(μ₄‐L¹)](PF₆)₄ and [{Ru(bpy)₂}₄(μ₄‐L²)](PF₆)₄ have been obtained by the reaction of sodium methoxide with [{Ru(bpy)₂}₄(μ₄‐H₄L¹)](PF₆)₈ and [{Ru(bpy)₂}₄(μ₄‐H₄L²)](PF₆)₈, respectively, in methanol. The pH effects on the UV–vis light absorption and emission spectra of both complexes have been studied, and ground‐ and excited‐state ionization constants of both complexes have been derived. The photophysical properties of both complexes are strongly dependent on the solution pH. They act as proton‐induced off–on–off luminescent sensors through two successive deprotonation processes of imidazole groups, with a maximum on–off ratio of 8 in buffer solution at room temperature. Theoretical calculations for the highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LOMO) orbitals of bridging ligand are also presented for plausible explanations of the fluorescence changes. Copyright © 2015 John Wiley & Sons, Ltd.     

Purification and characterization of laccase-1 fromPleurotus florida

Pleurotus florida (ITCC 3308) produces two laccase enzymes (L₁ and L₂) in potato-dextrose media containing 0.5% yeast extract. Concentrated culture filtrate was separated on DEAE-Sephadex (A-50) column into two enzyme peaks, subsequently named L₁ and L₂. The L₁ enzyme has been purified to homogeneity by ion-exchange and gel-permeation chromatography. L₁ is a monomeric glycoprotein with a molar mass of 77 and 82 kDa as determined by SDS-PAGE and gelfiltration chromatography, respectively. The pI value of L₁ has been determined to be 4.1. The optimum reaction temperature of the enzyme is 50°C. TheK _m and some other kinetic parameters of L₁ have been determined. Cyanide and azide completely inhibit the enzyme activity. The enzyme was fully active in 1: 1 (V/V) buffer-chloroform for at least 2 h. Spectroscopic analysis revealed that the enzyme has four copper atoms, a type 1 copper, a type 2 copper and a type 3 binuclear copper.     

Magnesium and lithium complexation by 1,4,7,10-tetraazacyclododecane

The stability constants of magnesium and lithium complexes of 1,4,7,10-tetraazacyclododecane (cyclen) have been determined in 0.5 M KNO ₃ at 25 °C by means of potentiometric titration, as K_MgL = 1.77 × 10 ² and K_{Li L} < 10 ⁻², respectively. Ab initio calculations on the protonated species of the cyclen ligand have been performed in order to obtain a better understanding of experimental protonation constants, and to compare them to previous calculations on the investigated metal complexes.     

Azo-containing pyridine amide ligand. A six-coordinate nickel(II) complex and its one-electron oxidized species: Structure and properties

A new potentially tridentate ligand HL¹¹ consisting of 2-pyridinecarboxamide unit and azo functionality has been used, in its deprotonated form, to prepare a nickel(II) complex which has been structurally characterized. The ligand L¹¹(−) affords a bis-complex [Ni^II(L¹¹)₂] (1). In 1, the two L¹¹(−) ligands bind to the Ni^II center in a mer configuration. The relative orientations within the pairs of pyridyl-N, deprotonated amido-N, and azo-N atoms are cis, trans, and cis, respectively. The Ni^IIN₂(pyridyl)N′₂(amide)N″₂(azo) coordination environment is severely distorted from ideal octahedral geometry. The Ni-N_am (am = amide) bond lengths are the shortest and the Ni-N_azo bond lengths are the longest. Complex 1 exhibits a quasireversible Ni^III/Ni^II redox process. Moreover, the complex displays two ligand-centered (azo group) quasireversible redox processes. Spectroscopic (absorption and EPR) properties have been studied on coulometrically-generated nickel(III) species. To understand the nature of metal-ligand bonding interactions Density Functional Theory (DFT) calculations have been performed on 1 at the B3LYP level of theory. Calculations have also been done for closely related nickel(II) complexes of deprotonated pyridine amide ligands and comparative discussion has been made using observed results.     

Coumarin derived chromophores in the donor-acceptor-donor format that gives fluorescence enhancement and large two-photon activity in presence of specific metal ions

Two coumarin derived dyes (compounds L_a and L) have been synthesized in high yields by Schiff base condensation. On probing with a femtosecond laser, none of these compounds show any two-photon activity in the wavelength range, 760-860 nm. However, L_a in presence of Zn(II) and L in presence of Mg(II), exhibit large two-photon absorption as well as emission in the same wavelength range. Theoretical calculations at the B3LYP functional with 6-31G∗ and LanL2DZ mixed basis set under DFT formalism support experimental results.     

A dinuclear vanadium compound with 24-membered macrocycle generated via formation of S-C bonds

A dinuclear vanadium 24-membered macrocycle with double-ring, [V₂O₂L₂(μ-CH₃COO)]⁻ (H₂L = 3,3′-(1,3,4-thiadiazole-2,5-diyl)bis(sulfane-diyl) bis(4-hydroxypent-3-en-2-one)), was prepared in high yield from the reaction of VO₂(acac) with 2,5-dimercapto-1,3,4-thiadiazole dipotassium salt (K₂SSS) and acetylacetone (Hacac) at room temperature. Its closure of 24-membered macrocycle has resulted from the formation of U-shape units via S-C bonding between original SSS²⁻ and acac⁻ ligands, while the bridging CH₃COO⁻ has created from an unexpected decomposition of Hacac. The obtained two products were characterized by single crystal X-ray diffraction, XRD, ESR, TGA and magnetism analyses. A possible mechanism for formation of the bimetal 24-membered macrocycle has been discussed.     

An unusual oxo-bridged mixed-valent dinuclear rhenium complex with the Re centers in various different coordination environments

The [{ReCl₄(PPh₃)}(μ-O){Re(O)(3,5-Me₂pzH)₄}] complex has been isolated as a product of the degradation of [ReCl₃(3,5-Me₂pzH)₂(PPh₃)] during slow crystallization from chloroform in air. Its crystal and molecular structure has been determined as well as spectroscopic investigations and density functional theory (DFT) calculations have been carried out. The UV-Vis spectrum of the dirhenium complex has been discussed on the basis of the spin-allowed electronic transitions calculated for [{ReCl₄(PPh₃)}(μ-O){Re(O)(3,5-Me₂pzH)₄}] with the time-dependent DFT (TDDFT) method.     

Mononuclear rhodium and iridium compounds with pyridyl-pyrazole ligands

Neutral [MCl(L₂)(Hpzpy)], [M(L₂)(pzpy)] and cationic [M(L₂)(Hpzpy)]CF₃SO₃ rhodium(I) or iridium(I) complexes [M = Rh or Ir; L₂ = diolefin or (CO)₂; pzpy = 3-(2-pyridyl)pyrazolate] have been prepared; the pzpy and Hpzpy ligands coordinate to the metal as bidentate chelate groups through one pyrazole nitrogen and the pyridine nitrogen atom. The reactivity of these complexes towards oxidative addition reactions of halogens, methyl iodide or triflic acid and towards displacement reactions has been studied. The neutral and cationic iridium(I) complexes are modest catalysts for the hydrosilylation of phenylacetylene with triethylsilane at 60 °C. The complexes have been characterised by analytical and spectroscopic data; their configuration has been confirmed by COSY and NOESY experiments and the molecular structure of [Rh(COD)(Mepzpy)(PPh₃)]CF₃SO₃ has been established by an X-ray diffraction study.     

Study of the interaction of an α-helical transmembrane peptide with phosphatidylcholine bilayer membranes by means of densimetry and ultrasound velocimetry

We applied precise densimetry and ultrasound velocimetry methods to study the interaction of a synthetic α-helical transmembrane peptide, acetyl-K₂-L₂₄-K₂-amide (L₂₄), with model bilayer lipid membranes. The large unilamellar vesicles (LUVs) utilized were composed of a homologous series of n-saturated diacylphosphatidylcholines (PCs). PCs whose hydrocarbon chains contained from 13 to 16 carbon atoms, thus producing phospholipid bilayers of different thicknesses and gel to liquid-crystalline phase transition temperatures. This allowed us to analyze how the difference between the hydrophobic length of the peptide and the hydrophobic thickness of the lipid bilayer influences the thermodynamical and mechanical properties of the membranes. We showed that the incorporation of L₂₄ decreases the temperature and cooperativity of the main phase transition of all LUVs studied. The presence of L₂₄ in the bilayer also caused an increase of the specific volume and of the volume compressibility in the gel state bilayers. In the liquid crystalline state, the peptide decreases the specific volume at relatively higher peptide concentration (mole ratio L₂₄:PC = 1:50). The overall volume compressibility of the peptide-containing lipid bilayers in the liquid-crystalline state was in general higher in comparison with pure membranes. There was, however, a tendency for the volume compressibility of these lipid bilayers to decrease with higher peptide content in comparison with bilayers of lower peptide concentration. For one lipid composition, we also compared the thermodynamical and mechanical properties of LUVs and large multilamellar vesicles (MLVs) with and without L₂₄. As expected, a higher cooperativity of the changes of the thermodynamical and mechanical parameters took place for MLVs in comparison with LUVs. These results are in agreement with previously reported DSC and ²H NMR spectroscopy study of the interaction of the L₂₄ and structurally related peptides with phosphatidylcholine bilayers. An apparent discrepancy between ²H NMR spectroscopy and compressibility data in the liquid crystalline state may be connected with the complex and anisotropic nature of macroscopic mechanical properties of the membranes. The observed changes in membrane mechanical properties induced by the presence of L₂₄ suggest that around each peptide a distorted region exists that involves at least 2 layers of lipid molecules.     

The effect of an orally active leukotriene D4/E4 antagonist, LY171883, on antigen-induced airway responses in allergic sheep

Leukotriene (LT) D₄ is a putative mediator of allergic asthma: inhaled LTD₄ produces early and late increases in specific lung resistance (SR_L) and slows tracheal mucus velocity (TMV) similat to inhaled antigen. In this study we examined the effects of an orally active LTD₄/LTE₄ antagonist, LY171883 [1-<2-Hydroxy-3-propyl-4-<4-(1H-Tetra-zol-5-yl) Butoxy>Phenyl>Ethanonel], on early and late changes in SR_L and TMV following airway challenge with antigen in conscious allergic sheep. SR_L and TMV were measured before and up to 8 h and 24 h after antigen challenge after either LY171883 (30 mg/kg, p.o. 2 h before challenge) or placebo pretreatment. After placebo pretreatment antigen challenge resulted in significant early (483% over baseline) and late (221% over baseline) increases in SR_L (n=9). LY171883 pretreatment, however, significantly reduced the early increase in SR_L (163% over baseline) and blocked the late response. LY171883 did not prevent the antigen-induced fall in TMV from 5–8 h post challenge (n=6), but TMV recovered more rapidly in the drug trial returning to baseline values by 24 h. These results suggest that the generation of LTD₄, and its metabolite LTE₄, during airway anaphylaxis contributes to the early increase in SR_L and is important for eliciting the late increase in SR_L as well as contributing to the fall in TMV.     

The replacement of chlorides by cytosines in copper(II) complexes containing guanidine derivatives

Two copper(II) chloride complexes of amidino-O-methylurea (L¹), [Cu(L¹)Cl₂] (1), and (N-benzyl)-amidino-O-methylurea (L²), [Cu(L²)Cl₂] (2), were prepared and characterized by elemental analysis, infrared, diffuse reflectance, electron spin resonance and electrospray ionization mass spectra. Their cytosine binding abilities has been studied and found that two cytosine molecules are able to coordinate with the copper centers by replacing the chloride ligands to yield the bifunctional binding adducts [Cu(L¹)(cyt)₂]Cl₂ (1c) and [Cu(L²)(cyt)₂]Cl₂ (2c), respectively. The shift of the CO band of cytosine in both cytosine-bound products to higher energy suggested that the N(3)-cytosine atom coordinates to the copper center. The large blue shifts of the d-d absorbance maxima and the nine superhyperfine splitting from the CuN₄ chromophore were also observed in their electronic and EPR spectra. Their thermal decompositions have also supported the interaction of cytosine with complexes 1 and 2. Density functional calculations have also been performed and revealed that square planar coordination geometry is more stable for both 1c and 2c. The binding energy of 1c is found to be ∼20% lower than that of 2c, indicative of the higher binding potential of 1c.     

Syntheses, electrolytic behaviour and antifungal activities of Zn(II) complexes of isomers of 3,10-C-meso-3,5,7,7,10,12,14,14-octamethyl-1,4,8,11-tetraazacyclotetradecane (L). Crystal and molecular structure of [ZnLB(NO3)]NO3 (LB = a,e,a,e-L)

The isomeric cyclam ligands Me₈[14]anes, designated by L_A, L_B and L_C, produce, on reaction with zinc(II)nitrate, zinc(II)sulphate or zinc(II)chloride corresponding complexes, i.e. dinitrato/mononitrato-nitrate complexes [ZnL(NO₃)₂]/[ZnL(NO₃)](NO₃) (L = L_A, L_B or L_C, where the indices A, B and C refer to differing orientations of the four methyl groups on secondary carbons of Me₈[14]ane)_, the diaqua-sulphates [ZnL(H₂O)₂]SO₄ (L = L_A, L_B or L_C), and the diaqua dichloride and dichlorido complexes [ZnL(H₂O)₂]Cl₂ (L = L_A or L_C) or [ZnL_BCl₂], respectively. The complexes have been characterised on the basis of elemental analyses, IR, UV-Vis, ¹H and ¹³C NMR spectroscopies, magnetic and conductance data. The structure of [ZnL_B(NO₃)](NO₃) has been determined by X-ray crystallography. The zinc centre is coordinated to a N₄O donor set in a square-pyramidal geometry. The complexes show differing electrolytic behaviour in different solvents. In chloroform, the complexes are non-electrolytes, indicating that both anions are coordinated to Zn²⁺. Antifungal activity of the ligands and complexes against the phytopathogenic fungi Alternaria alternata and Colletotrichum corcolei have been investigated, and positive results were noted.     

On the possible roles of N-terminal His-rich domains of Cu,Zn SODs of some Gram-negative bacteria

The Cu,Zn superoxide dismutases (Cu,Zn SOD) isolated from some Gram-negative bacteria possess a His-rich N-terminal metal binding extension. The N-terminal domain of Haemophilus ducreyi Cu,Zn SOD has been previously proposed to play a copper(II)-, and may be a zinc(II)-chaperoning role under metal ion starvation, and to behave as a temporary (low activity) superoxide dismutating center if copper(II) is available. The N-terminal extension of Cu,Zn SOD from Actinobacillus pleuropneumoniae starts with an analogous sequence (HxDHxH), but contains considerably fewer metal binding sites. In order to study the possibility of the generalization of the above mentioned functions over all Gram-negative bacteria possessing His-rich N-terminal extension, here we report thermodynamic and solution structural analysis of the copper(II) and zinc(II) complexes of a peptide corresponding to the first eight amino acids (HADHDHKK-NH₂, L) of the enzyme isolated from A. pleuropneumoniae. In equimolar solutions of Cu(II)/Zn(II) and the peptide the MH₂L complexes are dominant in the neutral pH-range. L has extraordinary copper(II) sequestering capacity (K_D,Cu = 7.4 × 10^− 13 M at pH 7.4), which is provided only by non-amide (side chain) donors. The central ion in CuH₂L is coordinated by four nitrogens {NH₂,3N_im} in the equatorial plane. In ZnH₂L the peptide binds to zinc(II) through a {NH₂,2N_im,COO⁻} donor set, and its zinc binding affinity is relatively modest (K_D,Zn = 4.8 × 10^− 7 M at pH 7.4). Consequently, the presented data do support a general chaperoning role of the N-terminal His-rich region of Gram-negative bacteria in copper(II) uptake, but do not confirm similar function for zinc(II). Interestingly, the complex CuH₂L has very high SOD-like activity, which may further support the multifunctional role of the copper(II)-bound N-terminal His-rich domain of Cu,Zn SODs of Gram-negative bacteria. The proposed structure for the MH₂L complexes has been verified by semiempirical quantum chemical calculations (PM6), too.     

Stereoselective non-equivalent bis-diimine coordination to Co(II) ion: Structure, luminescence and density functional theory calculations

Stereoselective coordination affording a luminescent tcc-isomer of dihalobis(diimine)Co(II) family in which two photoactive diimine ligands (L_NO2^?) are unsymmetrical and non-equivalent is reported (L_NO2^? = (E)-3-nitro-N-(pyridine-2-ylmethylene)aniline; ? = dihedral angle between the diimine unit including pyridine ring and the phenyl ring planes; tcc refers trans-cis-cis positions with respect to pyridine nitrogen-imine nitrogen-chloride donors). The single step reaction of 2-pyridinecarboxaldehyde, 3-nitro aniline and anhydrous cobalt dichloride affords only the tcc-(L_NO2^?1)(L_NO2^?2)Co^IICl₂ as the product which is characterized by IR, Mass, UV-Vis spectra and magnetic susceptibility measurements. Single crystal X-ray structure determination has established the orientation of the two ligands in the crystal as (L_NO2^33.7)(L_NO2^79.7)Co^IICl₂·0.25CH₃OH (1). The isomer is luminescent in frozen methanol glass at 77 K with the excitation and emission maxima respectively at 370 and 525 nm. The average luminescence life time is 1.86 ns measured on exciting at 356 nm. Density functional theory (DFT) calculation shows that the non-equivalence of the two diimine ligands prevails in the gas phase also and the geometry is (L_NO2^41.0)(L_NO2^60.5)Co^IICl₂. The calculation has identified four types of closely spaced localized π^∗ orbitals as unoccupied photoactive molecular orbitals (UPMOs). TD-DFT calculations have assigned the Co(II) to π^∗ charge transfer (MLCT) and π to π^∗ of charge transfer (LLCT) as the primary origin of absorption above 340 nm in methanol. Pendant nitro groups of the ligands and their intermolecular interactions have been the driving force of the stereoselectivity of the product. The binuclear L₄Co₂Cl₄ unit of 1 has constructed the helical 2D assembly and cis-CoCl₂ mediated strong inter-helices H-bonding interactions result the channeled 3D framework. Geometry optimization of other possible ttt, ctc, ccc and cct isomers has established that in the gas phase, the tcc-isomer is more stable at least by 20-42 kJ mol⁻¹ compared to other isomers where the two diimine ligands are equivalent.