Showing papers on "Schiff base published in 2011"

Crystal structure of metarhodopsin II.

Abstract: Structural studies of active states of the visual pigment rhodopsin, a G protein-coupled receptor, have previously been limited to apoprotein or opsin forms that do not contain the agonist all-trans-retinal. Two groups now report structures that reveal more details of the transformations involved in rhodopsin activation. Choe et al. solve the X-ray crystal structure of the metarhodopsin II intermediate of the photoreceptor rhodopsin, and Standfuss et al. determine the structure of a constitutively active mutant of rhodopsin bound to a peptide derived from the C-terminus of the G protein transducin. Here the X-ray crystal structure of the metarhodopsin II intermediate of the photoreceptor rhodopsin is solved. Comparison of this structure with previously published structures enabled the proposal of how retinal translocation and rotation induce the conformational changes observed in this structure. G-protein-coupled receptors (GPCRs) are seven transmembrane helix (TM) proteins that transduce signals into living cells by binding extracellular ligands and coupling to intracellular heterotrimeric G proteins (Gαβγ)1. The photoreceptor rhodopsin couples to transducin and bears its ligand 11-cis-retinal covalently bound via a protonated Schiff base to the opsin apoprotein2. Absorption of a photon causes retinal cis/trans isomerization and generates the agonist all-trans-retinal in situ. After early photoproducts, the active G-protein-binding intermediate metarhodopsin II (Meta II) is formed, in which the retinal Schiff base is still intact but deprotonated. Dissociation of the proton from the Schiff base breaks a major constraint in the protein and enables further activating steps, including an outward tilt of TM6 and formation of a large cytoplasmic crevice for uptake of the interacting C terminus of the Gα subunit3,4,5. Owing to Schiff base hydrolysis, Meta II is short-lived and notoriously difficult to crystallize. We therefore soaked opsin crystals with all-trans-retinal to form Meta II, presuming that the crystal’s high concentration of opsin in an active conformation (Ops*)6,7 may facilitate all-trans-retinal uptake and Schiff base formation. Here we present the 3.0 A and 2.85 A crystal structures, respectively, of Meta II alone or in complex with an 11-amino-acid C-terminal fragment derived from Gα (GαCT2). GαCT2 binds in a large crevice at the cytoplasmic side, akin to the binding of a similar Gα-derived peptide to Ops* (ref. 7). In the Meta II structures, the electron density from the retinal ligand seamlessly continues into the Lys 296 side chain, reflecting proper formation of the Schiff base linkage. The retinal is in a relaxed conformation and almost undistorted compared with pure crystalline all-trans-retinal. By comparison with early photoproducts we propose how retinal translocation and rotation induce the gross conformational changes characteristic for Meta II. The structures can now serve as models for the large GPCR family.

Development and applications of fluorescent indicators for Mg2+ and Zn2+.

Abstract: In a study of the spectroscopic behavior of two Schiff base derivatives, salicylaldehyde salicylhydrazone (1) and salicylaldehyde benzoylhydrazone (2), Schiff base 1 has high selectivity for Zn(2+) ion not only in abiotic systems but also in living cells. The ion selectivity of 1 for Zn(2+) can be switched for Mg(2+) by swapping the solvent from ethanol-water to DMF (N,N-dimethylformamide)-water mixtures. Imine 2 is a good fluorescent probe for Zn(2+) in ethanol-water media. Many other ions tested, such as Li(+), Na(+), Al(3+), K(+), Ca(2+), Cr(3+), Mn(2+), Fe(3+), Co(2+), Ni(2+), Cu(2+), Ag(+), Cd(2+), Sn(2+), Ba(2+), Hg(2+), and Pb(2+), failed to induce any spectral change in various solvents. The selectivity mechanism of 1 and 2 for metal ions is based on a combinational effect of proton transfer (ESPT), C═N isomerization, and chelation-enhanced fluorescence (CHEF). The coordination modes of the complexes were investigated.

Synthesis of new chiral heterocyclic Schiff base modulated Cu(II)/Zn(II) complexes: their comparative binding studies with CT-DNA, mononucleotides and cleavage activity.

Abstract: New Schiff base ligand L derived from the condensation reaction of 2-amino-3-formylchromone with (R)-2-amino-2-phenylethanol was synthesized and characterized which involves combination element of ammine functionality and naturally occurring heterocyclic chromone, 4H-benzopyran-4-one. Subsequently, their complexes 1 and 2 with Cu(NO 3 ) 2 and Zn(NO 3 ) 2 , respectively were prepared. The DNA binding studies of the ligand L and complexes 1 and 2 with CT-DNA as compared to classical anticancer drug cisplatin were carried out by employing different optical methods viz, UV–vis, fluorescence, circular dichroism and viscosity measurements. Furthermore, the absorption studies, 1 H and 31 P with mononucleotides were also monitored to examine the base specific interactions of the transition metal complexes which revealed a higher propensity of copper(II) complex 1 for 5′-GMP while for zinc(II) complex 2 towards 5′-TMP involving groove binding mechanism of the complexes towards DNA. The complex 1 exhibits a remarkable DNA cleavage activity with pBR322 DNA in presence of different activators and cleavage reaction involves various oxygen species suggesting the involvement of active oxygen species for the DNA scission.

Ferrocene-Conjugated L-Tryptophan Copper(II) Complexes of Phenanthroline Bases Showing DNA Photocleavage Activity and Cytotoxicity

Abstract: Ferrocene-conjugated l-tryptophan (l-Trp) reduced Schiff base (Fc-TrpH) copper(II) complexes [Cu(Fc-Trp)(L)](ClO4) of phenanthroline bases (L), viz. 2,2′-bipyridine (bpy in 1), 1,10-phenanthroline (phen in 2), dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq in 3), and dipyrido[3,2-a:2′,3′-c]phenazine (dppz in 4), were prepared and characterized and their photocytotoxicity studied. Cationic reduced Schiff base (Ph-TrpH) complexes [Cu(Ph-Trp)(L)(H2O)](ClO4) (L = phen in 5; dppz in 6) having the ferrocenyl moiety replaced by a phenyl group and the Zn(II) analogue (7) of complex 4 were prepared and used as control species. The crystal structures of 1 and 5 with respective square-planar CuN3O and square-pyramidal CuN3O2 coordination geometry show significantly different core structures. Complexes 1–4 exhibit a Cu(II)–Cu(I) redox couple near −0.1 V and the Fc+–Fc couple at ∼0.5 V vs SCE in DMF–0.1 M [Bun4N](ClO4) (Fc = ferrocenyl moiety). The complexes display a copper(II)-based d–d band near 600 nm and a Fc-centered b...

Fluorescent zinc(II) complex exhibiting "on-off-on" switching toward Cu2+ and Ag+ ions.

Abstract: Binuclear zinc(II) and copper(II) complexes based on a new Schiff base ligand N,N′-bis(2-hydroxybenzilidene)-2,4,6-trimethylbenzene-1,3-diamine (H2L) have been synthesized. The ligand H2L and complexes under investigation have been characterized by elemental analyses, spectral (FT-IR, 1H, 13C NMR, ESI-MS, electronic absorption, emission), and electrochemical studies. The structures of H2L and complexes [{Zn(C23H18N2O2)}2] (1) and [{Cu(C23H18N2O2)}2]·H2O (2) have been determined crystallographically. Selective “On-Off-On” switching behavior of the fluorescent complex 1 has been studied. The fluorescence intensity of 1 quenches (turns-off) upon addition of Cu2+, while enhances (turns-on) in the presence of Ag+ ions. The mechanisms of “On-Off-On” signaling have been supported by 1H NMR, ESI-MS, electronic absorption, and emission spectral studies. Job’s plot analysis supported 1:1 and 1:2 stoichiometries for Cu2+ and Ag+ ions, respectively. Association and quenching constants have been estimated by the Benes...

Naphthaldehyde–Urea/Thiourea Conjugates as Turn-On Fluorescent Probes for Al3+ Based on Restricted C=N Isomerization

Abstract: We have synthesized naphthaldehyde–carbonohydrazone (NC) and naphthaldehyde–thiocarbonohydrazone (NTC) Schiff base ligands by the condensation reaction of 2-hydroxy naphthaldehyde with urea and thiourea, respectively. Both these Schiff base ligands show blue turn-on fluorescence in the presence of aluminum(III), as a result of a restricted C=N isomerization mechanism. Aluminum(III) can be detected in the presence of most of the competing metal ions by using NC and NTC. The association constant (log Ka) of NC and NTC for Al3+ in acetonitrile was calculated to be 14.5 and 14.1, respectively.

Ring-Opening Polymerization of Cyclic Esters and Trimethylene Carbonate Catalyzed by Aluminum Half-Salen Complexes

Abstract: A series of ONO-tridentate Schiff base ligands derived from chiral and achiral amino alcohols and amino acids were synthesized and reacted with AlEt(3) to provide dimeric aluminum complexes. These complexes were tested for the ring-opening polymerization (ROP) of rac-lactide at 70 °C in toluene, producing poly(lactide) with up to 82% isotacticity. The most active of these aluminum complexes was chosen to perform ring-opening homopolymerizations of rac-lactide, trimethylene carbonate (TMC), rac-β-butyrolactone (rac-β-BL), δ-valerolactone (δ-VL), and e-caprolactone (e-CL). Kinetic parameters were investigated, and each polymerization was found to be first order with respect to monomer concentration. Fractional orders were observed with respect to catalyst concentration, indicating catalyst aggregation during the polymerization processes. Activation parameters were determined for all monomers, with their ΔG(‡) values at 90 °C being in the order rac-lactide ≈ rac-β-BL > δ-VL > TMC ≈ e-CL. Fineman-Ross and kinetic studies of the copolymerization of rac-lactide and δ-VL both indicate that the rate of rac-lactide enchainment is higher than that of δ-VL, resulting in a tapered copolymer. In addition, single crystals of one of these aluminum complexes were grown in the presence of rac-lactide and characterized using X-ray crystallography. The unit cell contains two lactide monomers, one D- and one L-lactide, adding further proof that polymerization takes place via an enantiomorphic site control mechanism.

New class of organic-inorganic hybrid aggregates based on polyoxometalates and metal-Schiff-base.

Abstract: By combination of heteropolymolybdates and metal–Schiff-base complexes, two new organic–inorganic hybrid compounds {Mn(salen)2(H2O)2[AlMo6(OH)6O18]}[arg]·16H2O (1) and {Mn(salen)2(H2O)2[CrMo6(OH)6O18]}[arg]·11H2O (2) (salen = N,N′-ethylene-bis(salicylideneiminate) have been successfully isolated. Compounds 1 and 2 were structurally characterized by elemental analyses, IR spectra, thermogravimetric analysis, and single-crystal X-ray diffraction. To the best of our knowledge, compounds 1 and 2 represent the first single-crystal structures of metal–Schiff-base decorated polyoxometalates (POMs) compounds. Photocatalytic experiments indicated that both 1 and 2 exhibit high catalytic activity for photodegradation of RhB with UV irradiation. In addition, the magnetic properties of 1 were also investigated.

Catalytic asymmetric ring-opening of meso-aziridines with malonates under heterodinuclear rare earth metal Schiff base catalysis.

Abstract: Catalytic asymmetric ring-opening of meso-aziridines with malonates is described. The combined use of two rare earth metal sources with different properties promoted the desired ring-opening reaction. A 1:1:1 mixture of a heterobimetallic La(O-iPr)(3)/Yb(OTf)(3)/Schiff base 1a (0.25-10 mol %) efficiently promoted the reaction of five-, six-, and seven-membered ring cyclic meso-aziridines as well as acyclic meso-aziridines with dimethyl, diethyl, and dibenzyl malonates, giving chiral cyclic and acyclic γ-amino esters in 99-63% yield and >99.5-97% ee.

Synthesis, Characterization and Biological Activity of Schiff Bases

Abstract: Three new series of biologically active amino substituted Schiff bases with general formula, R1N=CHR2. Here R1 = 2-amino-benzthiazole, 4-amino-salicylic acid and 4-aminophenol. R2 =4-chloro- benzaldehyde, 2-chloro-benzaldehyde, salicylaldehyde, vanillin and benzaldehyde were synthesized by the reaction of three different amino substituted compounds and substituted aldehydes in ethanol. Such compounds were characterized by different physico-chemical techniques like, melting point, elemental analysis, multinuclear NMR (1H, 13C). The free ligands and their metal complexes have been screened for their in vitro biological activities against bacteria, fungi and yeast. The metal complexes show more potent activities compared with Schiff base ligands.

Chirality and Diastereoselection of Δ/Λ-Configured Tetrahedral Zinc Complexes through Enantiopure Schiff Base Complexes: Combined Vibrational Circular Dichroism, Density Functional Theory, 1H NMR, and X-ray Structural Studies

Abstract: The metal-centered Δ/Λ-chirality of four-coordinated, nonplanar Zn(A∧B)2 complexes is correlated to the chirality of the bidentate enantiopure (R)-A∧B or (S)-A∧B Schiff base building blocks [A∧B = ...

The use of Cu and Zn salicylaldimine complexes as catalyst precursors in ring opening polymerization of lactides: ligand effects on polymer characteristics

Abstract: A range of monomeric tetra-coordinate copper (II) and zinc (II) complexes based on N,O-bidentate salicylaldimine Schiff base ligands has been synthesized and characterized using various spectroscopic techniques. These complexes were then evaluated as initiators in ring-opening polymerization of lactides at both 70 degrees C and 110 degrees C. The effect of structural changes in the complexes on the ability of these compounds to initiate lactide polymerization as well as the impact on the chemical and physical characteristics of the polymers obtained indicate that the coordination geometry of the metal complex, M-O bond length and substituents on the Schiff base ligand all play a role in the catalyst activity. Electronic factors were dominant in the case of the copper complexes while steric factors prevailed in the case of Zn initiators. Both the Zn and Cu complexes exhibit characteristics of living ring opening polymerization. Copyright (C) 2010 John Wiley & Sons, Ltd. (Less)

Synthesis and characterization of cerium and yttrium alkoxide complexes supported by ferrocene-based chelating ligands.

Abstract: Two series of Schiff base metal complexes were investigated, where each series was supported by an ancillary ligand incorporating a ferrocene backbone and different N=X functionalities. One ligand is based on an imine, while the other is based on an iminophosphorane group. Cerium(IV), cerium(III), and yttrium(III) alkoxide complexes supported by the two ligands were synthesized. All metal complexes were characterized by cyclic voltammetry. Additionally, NMR, Mossbauer, X-ray absorption near-edge structure (XANES), and absorption spectroscopies were used. The experimental data indicate that iron remains in the +2 oxidation state and that cerium(IV) does not engage in a redox behavior with the ancillary ligand.

Novel Organic‐Inorganic Hybrid Mesoporous Silica Supported Oxo‐Vanadium Schiff Base for Selective Oxidation of Alcohols

Abstract: Novel organic-inorganic hybrid mesopo- rous materials containing diimine moieties inside the pore wall were used for the grafting of an oxo- vanadium Schiff base complex at the mesopore sur- face. The prepared catalyst was found to be highly active and selective for the oxidation of a variety of primary, secondary and a-hydroxy carbonyl com- pounds to the corresponding aldehydes, ketones and 1,2-dicarbonyl compounds using tert-butyl hy- droperoxide as oxidant under mild reaction condi- tions. After completion of the reaction, the catalyst could easily be recovered by simple filtration and could be reused several times without significant change in the catalytic efficiency. Importantly, the catalyst did not show any leaching during the reac- tion, confirming that the prepared catalyst was truly heterogeneous in nature.

Synthesis, Characterization and Biological Activities of Cu(II), Co(II), Mn(II), Fe(II), and UO2(VI) Complexes with a New Schiff Base Hydrazone: O-Hydroxyacetophenone-7-chloro-4-quinoline Hydrazone

Abstract: The Schiff base hydrazone ligand HL was prepared by the condensation reaction of 7-chloro-4-quinoline with o-hydroxyacetophenone. The ligand behaves either as monobasic bidentate or dibasic tridentate and contain ONN coordination sites. This was accounted for be the presence in the ligand of a phenolic azomethine and imine groups. It reacts with Cu(II), Ni(II), Co(II), Mn(II), UO(2) (VI) and Fe(II) to form either mono- or binuclear complexes. The ligand and its metal complexes were characterized by elemental analyses, IR, NMR, Mass, and UV-Visible spectra. The magnetic moments and electrical conductance of the complexes were also determined. The Co(II), Ni(II) and UO(2) (VI) complexes are mononuclear and coordinated to NO sites of two ligand molecules. The Cu(II) complex has a square-planar geometry distorted towards tetrahedral, the Ni(II) complex is octahedral while the UO(2) (VI) complex has its favoured heptacoordination. The Co(II), Mn(II) complexes and also other Ni(II) and Fe(III) complexes, which were obtained in the presence of Li(OH) as deprotonating agent, are binuclear and coordinated via the NNNO sites of two ligand molecules. All the binuclear complexes have octahedral geometries and their magnetic moments are quite low compared to the calculated value for two metal ions complexes and thus antiferromagnetic interactions between the two adjacent metal ions. The ligand HL and metal complexes were tested against a strain of Gram +ve bacteria (Staphylococcus aureus), Gram -ve bacteria (Escherichia coli), and fungi (Candida albicans). The tested compounds exhibited high antibacterial activities.