Syntheses and characterization of cobalt(II) complexes of dimeric ‘picket fence’ porphyrins

A series of dimeric picket fence porphyrinatocobalt(II) complexeses in which the length of the bridging chain controls the dioxygen affinity was newly derived from the coupling of two meso-mono- (β-o-aminophenyl)-tris-(α,α,α-o-pivaloylamidophenyl)- porphyrins with (CH₂)_n(COCl)₂ (n = 1, 3, 5 or 7). Some of the dimeric complexes form a unique ‘sandwich structure’ upon binding with certain bidentate ligands, and their dioxygen affinities are greatly increased compared with those for corresponding monomeric complexes. The relationship observed between the length of the bridging chain and the dioxygen affinity of the dimer complex having a sandwich structure is interpreted in terms of the displacement mechanism of the metal atom from a porphyrin plane.     

Optically active Ru(II) complexes with a chiral Tröger's base ligand and their interactions with DNA

Three stereoisomers of a Ru(II) complex bearing a chiral bis-phenanthroline Tr?ger's base analogue, TBphen2 (1), have been isolated from the reaction of the enantiomerically pure precursor complex Lambda- (or Delta-) cis-[Ru(phen)2(py)2]2+ (phen=1,10-phenanthroline, py=pyridine) with the racemic mixture of 1. Each stereoisomer of [Ru(phen)2TBphen2]2+ (2) has been characterized by 1H NMR and CD spectroscopy. Electrochemical studies revealed that the redox properties of 2 are not influenced by the stereochemistry, however, the electrochemical oxidation of the metallic center is irreversible because of the diazocine bridge of the TBphen2 ligand. Steady-state emission measurements in the presence of calf thymus DNA showed that the DNA binding of [Ru(phen)2TBphen2]2+ depends on the stereoisomer and is mainly controlled by the absolute configuration of the metal center of the complex. The affinity constant for the stereoisomer Delta-S-2 is 10(2) higher than that for Lambda-S-2 and rac-[Ru(phen)3]2+.     

DNA binding by Ru(II)–bis(bipyridine)–pteridinyl complexes

The interactions of five bis(bipyridyl) Ru(II) complexes of pteridinyl-phenanthroline ligands with calf thymus DNA have been studied. The pteridinyl extensions were selected to provide hydrogen-bonding patterns complementary to the purine and pyrimidine bases of DNA and RNA. The study includes three new complexes [Ru(bpy)(2)(L-pterin)](2+), [Ru(bpy)(2)(L-amino)](2+), and [Ru(bpy)(2)(L-diamino)](2+) (bpy is 2,2'-bipyridine and L-pterin, L-amino, and L-diamino are phenanthroline fused to pterin, 4-aminopteridine, and 2,4-diaminopteridine), two previously reported complexes [Ru(bpy)(2)(L-allox)](2+) and [Ru(bpy)(2)(L-Me(2)allox)](2+) (L-allox and L-Me(2)allox are phenanthroline fused to alloxazine and 1,3-dimethyalloxazine), the well-known DNA intercalator [Ru(bpy)(2)(dppz)](2+) (dppz is dipyridophenazine), and the negative control [Ru(bpy)(3)](2+). Reported are the syntheses of the three new Ru-pteridinyl complexes and the results of calf thymus DNA binding experiments as probed by absorption and fluorescence spectroscopy, viscometry, and thermal denaturation titrations. All Ru-pteridine complexes bind to DNA via an intercalative mode of comparable strength. Two of these four complexes-[Ru(bpy)(2)(L-pterin)](2+) and [Ru(bpy)(2)(L-allox)](2+)-exhibit biphasic DNA melting curves interpreted as reflecting exceptionally stable surface binding. Three new complexes-[Ru(bpy)(2)(L-diamino)](2+), [Ru(bpy)(2)(L-amino)](2) and [Ru(bpy)(2)(L-pterin)](2+)-behave as DNA molecular "light switches."     

A zinc (II) complex comprising an aminoethyl-nitropyridine-derived N,N,O-donor Schiff base ligand serves as an efficient ON–OFF probe for Cu(II)

A new fluorescent zinc (II) complex-based probe 1 encompassing a Schiff's base (E)-2-methoxy-6-((2-[5-nitropyridin-2-ylamino]ethylimino)methyl)phenol ( HL ) was designed, synthesized, and used for the highly selective detection of Cu²⁺. Ligand HL and complex 1 were characterized using various spectroscopic techniques such as ¹H, ¹³C-NMR, and FTIR spectroscopy, high-resolution mass spectronomy (HRMS), UV/visible light spectroscopy, and fluorescence studies. Ligand HL did not exhibit any considerable change in fluorescence in the presence of various cations. Notably, its Zn(II) complex 1 exhibited highly selective ‘TURN-OFF’ fluorescence signalling towards Cu²⁺ that remained uninterrupted with competing analytes. Probe 1 interacted with Cu²⁺ in 1:2 (1:Cu²⁺) stoichiometry as estimated through a Job's plot. Moreover, the selectivity of 1 was further confirmed through the interaction of the 1 + Cu²⁺ complex with some possible interfering metal ions inducing an insignificant response. Additionally, the association and quenching constant were determined to be 3.30 × 10⁴ M⁻¹ and 0.21 × 10⁵ M⁻¹ through the Benesi–Hildebrand method and Stern–Volmer plot, respectively.     

Electron-paramagnetic-resonance studies on cobalt(II) carbonic anhydrase–sulphonamide complexes

Sulphonamide adducts of three Co(II) carbonic anhydrases were investigated by e.p.r. (electron paramagnetic resonance) at helium temperatures. The highly anisotropic 9 GHz spectra exhibited only three distinct features, with g values between 6.3 and 1.5. Such spectra arise from an electronic state with effective spin S'=(1/2), indicating that the high-spin (S=3/2) ground level is split into two spin doublets differing in energy by an amount large compared with the microwave quantum, but small in relation to thermal energies at ambient temperature. This situation would occur in a tetrahedral system suffering a large rhombic distortion. Calculations based on this model accounted for apparent discrepancies in integrated spectral intensities, and yielded magnetic moments in good agreement with independent measurements, especially in the case of certain small Co(II) complexes resembling the enzyme adducts in their e.p.r. signals. Precise sets of g values, reflecting a particular co-ordination geometry, were found to be representative of each enzyme variant and the type of sulphonamide inhibitor, whether benzocyclic or heterocyclic. A series of substituted benzene sulphonamides bound to the same enzyme gave rise to closely similar spectra despite a wide range of pK(i) values. Thus benzocyclic and heterocyclic sulphonamides were evidently held in the active-site cleft in characteristic orientations irrespective of side chains that might considerably influence the total binding strength. Visible absorption spectra of various sulphonamide adducts at room temperature showed a similar pattern of inhibitor dependence to the e.p.r. spectra, suggesting a correspondence between the co-ordination structures in liquid and frozen solution. E.p.r. spectra of the sulphonamide complexes were remarkable not only for their range of g values, but also for their variations in line-width and spin-lattice relaxation behaviour. Addition of glycerol to the medium produced marked enhancement in resolution, owing to the creation of a more homogeneous frozen matrix. The non-uniform spin relaxation was probably a consequence of the large anisotropy in effective g tensor.     

Determination of the equilibrium constant for binding hydroxide to tetraazamacrocyclic—Nickel(II) complexes

The pK_b for the equilibria NiL(OH)⁺ ⇄ NiL²⁺ + OH⁻ for L1,4,7,10-tetraazacyclotridecane, 1,4,8, 12-tetraazacyclopentadecane, C-β-racemic-5,7,7,12, 14,14-hexamethyl-l,4,8,11-tetraazacyclotetradecane and C-β-racemic-1,4,5,7,7,8,11,12,14,14,-decamethyl- 1,4,8,112-tetraazacyclotetradecane are 0.95, 1.9, 0.2 and 0.65 respectively. The results are compared with data for analogous complexes reported earlier. The results indicate that the main factors affecting these equilibrium constants are the in plane ligand field strengths of the square planar complexes and steric factors.     

Thermodynamics of substrate binding to the metal site in homoprotocatechuate 2,3-dioxygenase: Using ITC under anaerobic conditions to study enzyme–substrate interactions

BackgroundExtradiol dioxygenases are a family of nonheme iron (and sometimes manganese) enzymes that catalyze an O₂-dependent ring-opening reaction in a biodegradation pathway of aromatic compounds. Here we characterize the thermodynamics of two substrates binding in homoprotocatechuate 2,3-dioxygenase (HPCD) prior to the O₂ activation step.MethodsThis study uses microcalorimetry under an inert atmosphere to measure thermodynamic parameters associated with catechol binding to nonheme metal centers in HPCD. Several stopped-flow rapid mixing experiments were used to support the calorimetry experiments.ResultsThe equilibria constant for 4-nitrocatechol and homoprotocatechuate binding to the iron(II) and manganese(II) forms of HPCD range from 2 × 10⁴ to 1 × 10⁶, suggesting there are distinctive differences in how the enzyme–substrate complexes are stabilized. Further experiments in multiple buffers allowed us to correct the experimental ΔH for substrate ionization and to fully derive the pH and buffer independent thermodynamic parameters for substrate binding to HPCD. Fewer protons are released from the iron(II) dependent processes than their manganese(II) counterparts.ConclusionsCondition independent thermodynamic parameters for 4-nitrocatechol and homoprotocatechuate binding to HPCD are highly consistent with each other, suggesting these enzyme–substrate complexes are more similar than once thought, and the ionization state of metal coordinated waters may be playing a role in tuning redox potential and in governing reactivity.General significanceSubstrate binding to HPCD is a complex set of equilibria that includes ionization of substrate and water release, yet it is also the key step in O₂ activation. This article is part of a Special Issue entitled Microcalorimetry in the BioSciences — Principles and Applications, edited by Fadi Bou-Abdallah.     

The effects of structural variations of thiophene-containing Ru(II) complexes on the acid–base and DNA binding properties

A phenylthiophenyl-bearing Ru(II) complex of [Ru(bpy)₂(Hbptip)](PF₆)₂ {bpy?=?2,2′-bipyridine, Hbptip?=?2-(4-phenylthiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline} was synthesized and characterized by elemental analysis, ¹H NMR spectroscopy, and electrospray ionization mass spectrometry. The ground- and excited-state acid–base properties of the complex were studied by UV–visible absorption and photoluminescence spectrophotometric pH titrations and the negative logarithm values of the ground-state acid ionization constants were derived to be pK _a1?=?1.31?±?0.09 and pK _a2?=?5.71?±?0.11 with the pK _a2 associated deprotonation/protonation process occurring over 3 pK _a units more acidic than thiophenyl-free parent complex of [Ru(bpy)₂(Hpip)]²⁺ {Hpip?=?2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline}. The calf thymus DNA-binding properties of [Ru(bpy)₂(Hbptip)]²⁺ in Tris–HCl buffer (pH 7.1 and 50?mM NaCl) were investigated by DNA viscosities and density functional theoretical calculations as well as UV–visible and emission spectroscopy techniques of UV–visible and luminescence titrations, steady-state emission quenching by [Fe(CN)₆]^4?, DNA competitive binding with ethidium bromide, DNA melting experiments, and reverse salt effects. The complex was evidenced to bind to the DNA intercalatively with binding affinity being greater than those for previously reported analogs of [Ru(bpy)₂(Hip)]²⁺, [Ru(bpy)₂(Htip)]²⁺, and [Ru(bpy)₂(Haptip)]²⁺ {Hip?=?1H-imidazo[4,5-f][1,10]phenanthroline, Htip?=?2-thiophenimidazo[4,5-f][1,10]phenanthroline, Haptip?=?2-(5-phenylthiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline}.     

Structural Thermodynamics of myr-Src(2–19) Binding to Phospholipid Membranes

Many proteins are anchored to lipid bilayer membranes through a combination of hydrophobic and electrostatic interactions. In the case of the membrane-bound nonreceptor tyrosine kinase Src from Rous sarcoma virus, these interactions are mediated by an N-terminal myristoyl chain and an adjacent cluster of six basic amino-acid residues, respectively. In contrast with the acyl modifications of other lipid-anchored proteins, the myristoyl chain of Src does not match the host lipid bilayer in terms of chain conformation and dynamics, which is attributed to a tradeoff between hydrophobic burial of the myristoyl chain and repulsion of the peptidic moiety from the phospholipid headgroup region. Here, we combine thermodynamic information obtained from isothermal titration calorimetry with structural data derived from ²H, ¹³C, and ³¹P solid-state nuclear magnetic resonance spectroscopy to decipher the hydrophobic and electrostatic contributions governing the interactions of a myristoylated Src peptide with zwitterionic and anionic membranes made from lauroyl (C12:0) or myristoyl (C14:0) lipids. Although the latter are expected to enable better hydrophobic matching, the Src peptide partitions more avidly into the shorter-chain lipid analog because this does not require the myristoyl chain to stretch extensively to avoid unfavorable peptide/headgroup interactions. Moreover, we find that Coulombic and intrinsic contributions to membrane binding are not additive, because the presence of anionic lipids enhances membrane binding more strongly than would be expected on the basis of simple Coulombic attraction.     

Thermodynamics of metalligand bond formation XXXV. Lewis acidity of tin(IV) halides

Thermodynamic data have been obtained for the reactions in benzene solution at 30°C of tin tetrahalides with the following Lewis bases: triphenylarsine, tetrahydrothiophene, tetrahydrofuran, pyridine-N-oxide, dimethylsulphoxide, dimethylformamide and acetonitrile.Triphenylarsine forms a 1:1 adduct of low stability with tin tetrachloride and fails to react with the other halides. The other bases when they react form both 1:1 and 1:2 adducts, the reactivity decreasing in the order SnCl₄ > SnBr₄ > SnI₄. THT and THF do not react with SnI₄; CH₃CN does not react with SnBr₄ or SnI₄. When it is possible to determine data separately for formation of successive adducts it is found that K₁ > K₂ for SnCl₄ and SnBr₄ but K₁ < K₂ for SnI₄.     

pH-dependence of the specific binding of Cu(II) and Zn(II) ions to the amyloid-β peptide

Metal ions like Cu(II) and Zn(II) are accumulated in Alzheimer's disease amyloid plaques. The amyloid-β (Aβ) peptide involved in the disease interacts with these metal ions at neutral pH via ligands provided by the N-terminal histidines and the N-terminus. The present study uses high-resolution NMR spectroscopy to monitor the residue-specific interactions of Cu(II) and Zn(II) with (15)N- and (13)C,(15)N-labeled Aβ(1-40) peptides at varying pH levels. At pH 7.4 both ions bind to the specific ligands, competing with one another. At pH 5.5 Cu(II) retains its specific histidine ligands, while Zn(II) seems to lack residue-specific interactions. The low pH mimics acidosis which is linked to inflammatory processes in vivo. The results suggest that the cell toxic effects of redox active Cu(II) binding to Aβ may be reversed by the protective activity of non-redox active Zn(II) binding to the same major binding site under non-acidic conditions. Under acidic conditions, the protective effect of Zn(II) may be decreased or changed, since Zn(II) is less able to compete with Cu(II) for the specific binding site on the Aβ peptide under these conditions.     

Fe(III)–resorcylate as a spectrophotometric probe for phospholipid–cation interactions

A simple spectrophotometric microplate assay that allows quantification of the interaction between phospholipids and metal ions or other small cationic compounds has been developed. The assay is based on the competition of the phospholipids for the Fe³⁺ ion in the purple-colored Fe(III)–γ-resorcylate complex and for other cations. To compare the binding affinities of several cation–phospholipid interactions, K_0.5 values were derived from binding curves constructed by determination of the absorbance of the Fe(III)–γ-resorcylate at 490 nm as a function of the cation concentration. The assay was used to analyze the binding of lanthanide ions, calcium ions, and amines (hydrochlorides of ethanolamine, spermidine, and hexyltrimethylammonium chloride) to small unilamellar vesicles (SUVs) and mixed micelles containing anionic lipids such as phosphatidic acid and phosphatidyl-p-nitrophenol. The method was evaluated by fluorescence measurements with Eu³⁺ ions as tracer. Lanthanide ions such as La³⁺ and Ce³⁺ ions showed K_0.5 values smaller by one to two orders of magnitude compared with Ca²⁺ ions. In the presence of increasing amounts of detergents such as Triton X-100, the method also reflected transitions from SUVs to micelles. The binding capacity for metal ions was higher for phospholipid-containing micelles than for the corresponding SUVs.     

Illuminating the binding interactions of galactonoamidines during the inhibition of β-galactosidase (E. coli)

Several galactonoamidines were previously identified as very potent competitive inhibitors that exhibit stabilizing hydrophobic interactions of the aglycon in the active site of β-galactosidase (Aspergillus oryzae). To elucidate the contributions of the glycon to the overall inhibition ability of the compounds, three glyconoamidine derivatives with alteration in the glycon at C-2 and C-4 were synthesized and evaluated herein. All amidines are competitive inhibitors of β-galactosidase (Escherichia coli) and show significantly reduced inhibition ability when compared to the parent. The results highlight strong hydrogen-bonding interactions between the hydroxyl group at C-2 of the amidine glycon and the active site of the enzyme. Slightly weaker H-bonds are promoted through the hydroxyl group at C-4. The inhibition constants were determined to be picomolar for the parent galactonoamidine, and nanomolar for the designed derivatives rendering all glyconoamidines very potent inhibitors of glycosidases albeit the derivatized amidines show up to 700-fold lower inhibition activity than the parent.