Reversible photochromism of novel silver(I) coordination complexes with 1,2-bis(2′-methyl-5′-(2″-pyridyl)-3′-thienyl)perfluorocyclopentene in crystalline phase

Three novel silver(I) complexes with 1,2-bis(2-methyl-5′-(2″-pyridyl)-3′-thienyl)perfluorocyclopentene (BM-2-PTP) were synthesized by the reaction of Ag(CF₃SO₃) or Ag(CF₃COO) with BM-2-PTP in benzene at different temperatures. The structures of these metal complexes were revealed by X-ray crystallographic analyses and the correlation between crystal structures and photochromic performance was discussed. In complexes 1 and 2, silver(I) is three-coordinated to two nitrogens from distinct ligand molecules as well as one oxygen from anions to form a 1-D polymeric structure. On the other hand, complex 3 contains two crystallographic independent Ag(I) with different coordination environments, and the adjacent BM-2-PTP molecules are connected by Ag-CF₃CO₂-Ag chains to afford a 1-D double chain structure. The difference in structures of three complexes shows the interesting anionic effect on coordination and the subtleness of crystal engineering. It is noted that complex 3 underwent reversible photochromic reaction in crystalline state despite the unfavorable framework to the rotation of thiophene groups.     

Zinc coordination polymers with 1-(3-aminopropyl)-imidazole as bridging bidentate unit

The reaction of Zn(SCN)₂ with one or two equivalents of 1-(3-aminopropyl)-imidazole (api) yields the coordination polymers [Zn(SCN)₂(api)]_n (1) and [Zn(SCN)₂(api)₂]_n (2), respectively. Single-crystal X-ray diffraction analysis reveals one-dimensional polymeric chain structures for both compounds. The structure of 1 consists of tetrahedral Zn(SCN)₂(api)₂ units linked by one molecule 1-aminopropyl imidazole in an unsymmetric mode, i.e., each metal center is coordinated by an imidazole nitrogen as well as a nitrogen of the aminopropyl group. The metal ions in 2 display an octahedral coordination geometry with each Zn(SCN)₂(api)₂ unit linked by two molecules of the imidazole, thus, exhibiting two imidazole and two amino groups in the coordination sphere. The polymers were further characterized by IR-, ¹H NMR- and ¹³C NMR-spectroscopy.     

A MLCT-switched photochromic copper(II) coordination polymer with 1,2-bis(2′-methyl-5′-(4″-pyridyl)-3′-thienyl)perfluorocyclopentene in crystalline phase

A novel Cu(II) coordination polymer with photochromic 1,2-bis(2′-methyl-5′-(4″-pyridyl)-3′-thienyl)perfluorocyclopentene (BM-4-PTP) was prepared and crystallographically characterized. Its photochromic behavior as well as magnetic property was investigated in crystalline phase. In complex [Cu(BM-4-PTP)Br₂(DMF)₂] (1), each copper atom is bridged by two N atoms of BM-4-PTP, two Br atoms from anions and two O atoms of DMF in an slightly distorted octahedral geometry. The basal planar center is in turn linked by bidentate ligand forming a 1-D polymeric chain. Free ligand showed typical spectral changes upon appropriate optical excitation, indicating the reversible photochromism in crystalline phase. Complex 1 occurred reversible photoisomerization not only through the π-π^∗ transition but also the MLCT transition in crystalline phase. On the other hand, the magnetic property of complex 1 has been investigated by means of ESR. The spectra slightly and reversibly changed in response to UV and visible light supporting the normal photoreactivity in crystalline phase.     

Synthesis, structures and luminescence properties of two new Zn(II) coordination compounds incorporating the 5-(4-pyridyl)tetrazolate spacer ligand

The hydrothermal reaction of ZnCl₂ with 5-(4-pyridyl)tetrazole afforded the 2D metal-organic coordination framework [Zn(OH)(4-ptz)] (1) and the mononuclear complex [Zn(4-ptz)₂(H₂O)₄]?2(H₂O) (2) [4-ptz = 5-(4-pyridyl)-tetrazolate]. Compound 1 consists of a zig-zag bidimensional network formed by rectangular (4,4) grid sheets. Molecules of 2 form a 3D extended network of hydrogen bonding involving water molecules and the tetrazolate ligand. In addition, compound 1 exhibits strong fluorescence at room temperature in the solid state.     

Enzymatic synthesis of theanine with L-glutamine-Zn(II) complexes

Theanine, a unique amino acid found in tea plants, has many important physiological functions. Theanine can be enzymatically synthesized via the γ-glutamyltranspeptidation reaction. In this study, we described a new method of theanine synthesis using the L-glutamine-Zn(II) (Zn(Gln)₂) complex instead of glutamine as the donor, which successfully reduced the side autotranspeptidation reaction and led to higher yield of theanine. We prepared the Zn(Gln)₂ complexes and showed that they are stable in liquid bulk under 9.0 pH. After using the Zn(Gln)₂ in the γ-glutamyltranspeptidation reaction, we utilized HPLC and Mass spectrometry analysis to demonstrated that Zn(Gln)₂ was an more effective γ-glutamyl donor than glutamine. The autotranspeptidation reaction was restrained effectively. As a result, the theanine yield and the conversion rate for glutamine were vastly improved. In a reaction mixture containing 48 mM of Zn(Gln)₂, 1.6 M ethylamine, and 0.5 U/mL GGT, the final concentration of theanine obtained was 61.3 mM after incubation at 37°C for three hours. The conversion rate for glutamine was 63.8%, which showed a 16.9% increase as compared to when using glutamine alone as the donor substrate.     

Three zinc(II) complexes constructed from aromatic dicarboxylate and 1,4-bis((2-(pyridin-2-yl)-1H-imidazol-1-yl)methyl)benzene: Syntheses, crystal structures and luminescent properties

To investigate the effect of organic anions on the coordination frameworks, we synthesized three new complexes, namely, Zn(DPA)(bpimb)_0.5(H₂O) (1), Zn(BDC)(bpimb)_0.5 (2) and Zn₂(SDBA)₂(bpimb)·H₂O (3) (H₂DPA = diphenic acid; H₂BDC = isophthalic acid; H₂SDBA = 4,4′-dicarboxybiphenylsulfone), which were obtained by the reactions of 1,4-bis((2-(pyridin-2-yl)-1H-imidazol-1-yl)methyl)benzene (bpimb) as main ligand, and several aromatic polycarboxylate as organic anions with Zn(NO₃)₂·6H₂O. Single-crystal structure analysis shows that complex 1 is a one-dimensional chain structure, which is further interlinked into a higher-dimensional supramolecular framework by hydrogen-bonding interactions. In 2, BDC bridge Zn(II) atoms to give dimeric units, which are further linked by bpimb ligands to form sql nets. In 3, SDBA ligands and bpimb ligands connect Zn(II) ions into catenane-like two-dimensional layers. These catenane-like two-dimensional layers stack in an ABAB fashion to form a 3D supramolecular network. The distinct structures indicate three kinds of carboxylic ligands with different lengths and angles play fundamental roles in the formation of the final products. In addition, the luminescence measurements reveal that three complexes exhibit strong fluorescent emissions in the solid state at room temperature.     

Synthesis and structural investigation of biologically active complexes of 4-acetyl-2-(acetylamino)-5-dimethyl-Δ2-1,3,4-thiadiazole with Zn(II), Hg(II), Cd(II) and Cu(II)

4-Acetyl-2-(acetylamino)-5-dimethyl-Δ2-1,3,4-thiadiazole (AAT) has been used to obtain the complexes of the general formula [M(AAT)X₂]·H₂O where M(II) = Zn, Hg, Cd and Cu, and X  Cl or $\text{1}\text{2}$ SO₄. The complexes have been characterized on the basis of their elemental analysis, molar conductance, magnetic susceptibility and spectral data. Probable structures for the complexes have been proposed on the basis of their physico-chemical properties. The fungitoxicity of AAT and the isolated complexes has been tested on pathogenic fungi.     

Characteristics of complexation reaction of tetraphenylporphine with zinc(II) at solid-liquid interface

Fast complexation was found in the solid-liquid system using silica gel adsorbing Zn(ll) (ZnSiO₂) and organic solution of 5,10,15,20-tetraphenylporphine (H₂tpp). The reaction rate is mainly controlled by affinity of H₂tpp for silica gel, but the more complicated effect of the ZnSiO₂ surface is clarified.     

Synthesis and chemical characterization of Cu, Ni and Zn complexes of 3,5-bis(2′-pyridyl)-1,2,4-oxadiazole and 3-(2′-pyridyl)5-(phenyl)-1,2,4-oxadiazole ligands

The synthesis and structural characterization of Ni^II, Cu^II and Zn^II complexes of two chelating 1,2,4-oxadiazole ligands, namely 3,5-bis(2′-pyridyl)-1,2,4-oxadiazole (bipyOXA) and 3-(2′-pyridyl)5-(phenyl)-1,2,4-oxadiazole (pyOXA), is here reported. The formed hexacoordinated metal complexes are [M(bipyOXA)₂(H₂O)₂](ClO₄)₂ and [M(pyOXA)₂(ClO₄)₂], respectively (M = Ni, Cu, Zn). X-ray crystallography, ¹H and ¹³C NMR spectroscopy and C, N, H elemental analysis data concord in attributing them an octahedral coordination geometry. The two coordinated pyOXA ligands assume a trans coplanar disposition, while the two bipyOXA ligands are not. The latter result is a possible consequence of the formation of H-bonds between the coordinated water molecules and the nitrogen atom of the pyridine in position 5 of the oxadiazole ring. The expected splitting of the d metal orbitals in an octahedral ligand field explains the observed paramagnetism of the d⁸ and d⁹ electron configuration of the nickel(II) and copper(II) complexes, respectively, as determined by the broadening of their NMR spectra.     

Zinc(II) complexes of 2-acetyl pyridine 1-(4-fluorophenyl)-piperazinyl thiosemicarbazone: Synthesis, spectroscopic study and crystal structures - Potential anticancer drugs

2-Acetyl pyridine thiosemicarbazone containing an 1-(4-fluorophenyl)-piperazinyl ring incorporated at N(4)-position, HAcPipPheF (1) and the zinc(II) complexes [Zn(AcPipPheF)₂] (2) and [Zn(OAc)(AcPipPheF)]₂ (3) have been prepared and structurally characterized by means of vibrational and NMR (¹H and ¹³C) spectroscopy. The crystal structures of the compounds 1-3 have been determined by X-ray crystallography. The metal coordination geometry of [Zn(AcPipPheF)₂] is described as distorted octahedral configuration in a trans-N-cis-N-cis-S configuration. In [Zn(OAc)(AcPipPheF)]₂ one of the acetato group exhibits monoatomic bridge and the other bridges in a bidentate manner. The zinc(1) metal ion is coordinated in a distorted octahedral configuration while the metal coordination of Zn(2) is described as distorted square pyramidal. Biomedical studies revealed that, compounds 1-3 displayed potent anticancer activity. The antiproliferative activity of 1-3 was found to be considerably stronger than that of cis-platin. The IC₅₀ values range from 26 to 90 nM, against all cell lines tested, while for cis-platin the IC₅₀ values range from 2 to 17 μM and for the zinc salt, ZnCl₂, the IC₅₀ values range from 81 to 93 μM. The complex 3 shows the highest activity against all four cancer cell lines and the highest selectivity against K562 and MDA-MB-453 cancer cell lines. The compounds inhibited tumor cell proliferation by arresting the cell cycle progression at the S phase.     

Zinc(II) and copper(II) 1D coordination polymeric complexes of a reduced Schiff base ligand

The reaction of Zn(ClO₄)₂ · 6H₂O and Cu(ClO₄)₂ · 6H₂O with H₃Sas (H₃Sas = N-(2-hydroxybenzyl)-L-aspartic acid in water afforded the complexes [Zn₆(Sas)₄(H₂O)₈]·5H₂O (1) and [Cu(HSas)(H₂O)] (2), respectively, which were characterized by infrared spectroscopy, elemental analysis, thermogravimetry and single-crystal X-ray crystallography. In 1, the pentanuclear clusters formed by four H₃Sas ligands and five Zn(II) metal ions are bridged by the “[Zn(H₂O)₄]²⁺” cations to form 1D polymeric chains. While in 2, the mononuclear [Cu(HSas)(H₂O)] repeating units form a 1D zigzag chain and further extended by strong intermolecular hydrogen bonds to form a 2D sheet. The different coordination geometries of Cu(II) and Zn(II) show significant influence on the polymeric structures.     

Preparation and in vitro photodynamic activity of amphiphilic zinc(II) phthalocyanines substituted with 2-(dimethylamino)ethylthio moieties and their N-alkylated derivatives

Treatment of 4,5-dichlorophthalonitrile with 2-(dimethylamino)ethanethiol hydrochloride and K₂CO₃ afforded 4,5-bis[2-(dimethylamino)ethylthio]phthalonitrile or a heterocycle-fused phthalonitrile depending on the reaction temperature. The latter has been spectroscopically and structurally characterized. Both compounds underwent mixed cyclization with 3 equiv of unsubstituted phthalonitrile in the presence of Zn(OAc)₂·2H₂O and 1,8-diazabicyclo[5.4.0]undec-7-ene to give the corresponding 2,3-disubstituted zinc(II) phthalocyanines. N-methylation or pentylation of the bis[2-(dimethylamino)ethylthio] substituted analogue resulted in the formation of the respective dicationic phthalocyanines. For comparison, the octa-substituted analogues were also prepared by base and zinc-promoted self-cyclization of 4,5-bis[2-(dimethylamino)ethylthio]phthalonitrile followed by N-methylation. The spectroscopic and basic photophysical properties of these di- and octa-substituted phthalocyanines were examined in N,N-dimethylformamide. All of them remained essentially non-aggregated, showed moderate fluorescence emission, and could generate singlet oxygen, except the heterocycle-fused analogue, of which the singlet excited state was reductively quenched by the amino substituent. The photocytotoxicity of these compounds was also evaluated against HepG2 human hepatocarcinoma cells and HT29 and T84 human colon adenocarcinoma cells. The disubstituted amphiphilic phthalocyanines are particularly potent with IC₅₀ values down to 0.08 μM. Fluorescence microscopic studies revealed that the non-ionic derivative has selective affinity to the mitochondria of HT29 cells, while its di-N-methylated analogue shows preferential localization in the cell membrane.     

Isostructural M(RL)2(hfac)2 complexes with RL = 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)pyrimidine

5-(4-(N-tert-Butyl-N-aminoxylphenyl))pyrimidine (RL, 4PPN) forms crystallographically isostructural and isomorphic pseudo-octahedral M(RL)₂(hfac)₂ complexes with M(hfac)₂, M = Zn, Cu, Ni, Co, and Mn. Multiple close contacts occur between sites of significant spin density of the organic radical units. Magnetic behavior of the Zn, Cu, Ni, Co complexes appears to involve multiple exchange pathways, with multiple close crystallographic contacts between sites that EPR (of 4PPN) indicates to have observable spin density. Powder EPR spectra at room temperature and low temperature are reported for each complex. Near room temperature, the magnetic moments of the complexes are roughly equal to those expected by a sum of non-interacting moments (two radicals plus ion). As temperature decreases, AFM exchange interactions become evident in all of the complexes. The closest fits to the magnetic data were found for a 1-D Heisenberg AFM chain model in the Zn(II) complex (J/k = (?)7 K), and for three-spin RL—M—RL exchange in the other complexes (J/k = (?)26 K, (?)3 K, (?)6 K, for Cu(II), Ni(II), and Co(II) complexes, respectively).     

A fluorescent ligand rationally designed to be selective for zinc(II) over larger metal ions. The structures of the zinc(II) and cadmium(II) complexes of N,N-bis(2-methylquinoline)-2-(2-aminoethyl)pyridine

The metal ion coordinating properties of the ligands N,N-bis(2-methylquinoline)-2-(2-aminoethyl)pyridine (DQPEA) and N,N-bis(2-methylquinoline)-2-(2-aminomethyl)pyridine (DQPMA) are presented. DQPEA and DQPMA differ only in that DQPEA forms six-membered chelate rings that involve the pyridyl group, whereas DQPMA forms analogous five-membered chelate rings.These two ligands illustrate the application of a ligand design principle, which states that increase of chelate ring size in a ligand will result in increase in selectivity for smaller relative to larger metal ions. The formation constants (log K₁) of DQPEA and DQPMA with Ni(II), Cu(II), Zn(II), Cd(II) and Pb(II) are reported. As expected from the applied ligand design principle, small metal ions such as Ni(II) and Zn(II) show increases in log K₁ with DQPEA (six-membered chelate ring) relative to DQPMA (five-membered chelate ring), while large metal ions such as Cd(II) and Pb(II) show decreases in log K₁ when the chelate ring increases in size. In order to further understand the steric origin of the destabilization of complexes of metal ions of differing sizes by the six-membered chelate ring of DQPEA, the structures of [Zn(DQPEA)H₂O](ClO₄)₂ (1) [triclinic, , a = 9.2906(10), b = 10.3943(10), c = 17.3880(18) Å, α = 82.748(7)°, β = 88.519(7)°, γ = 66.957(6)°, Z = 4, R = 0.073] and [Cd(DQPEA)(NO₃)₂] (2) [monoclinic, C2/c, a = 22.160(3), b = 15.9444(18), c = 16.6962(18) Å, β = 119.780(3)°, Z = 8, R = 0.0425] are reported. The Zn in (1) is five-coordinate, with a water molecule completing the coordination sphere. The Cd(II) in (2) is six-coordinate, with two unidentate nitrates coordinated to the Cd. It is found that the bonds to the quinaldine nitrogens in the DQPEA complexes are considerably stretched as compared to those of analogous TPyA (tri(pyridylmethyl)amine) complexes, which effect is attributed to the greater steric crowding in the DQPEA complexes. The structures are analyzed for indications of the origins of the destabilization of the complex of the large Cd(II) ion relative to the smaller Zn(II) ion. A possible cause is the greater distortion of the six-membered chelate ring in (2) than in (1), as evidenced by torsion angles that are further away from the ideal values in (2) than in (1). Fluorescence properties of the DQPMA and DQPEA complexes of Zn(II) and Cd(II) are reported. It is found that the DQPEA complex of Zn(II) has increased fluorescence intensity compared to the DQPMA complex, while for the Cd(II) complex the opposite is found. This is related to the greater strain in the six-membered chelate ring of the Cd(II) DQPEA complex as compared to the Zn(II) complex, with resulting poorer overlap in the Cd-N bond, and hence greater ability to quench the fluorescence in the Cd(II) complex.     

Diamagnetic 8-amidoquinoline and 8-(trialkylsilylamido)quinoline complexes of divalent nickel and zinc

Transamination of Zn[N(SiMe₃)₂]₂ with 8-(triisopropylsilylamino) (1a) and 8-(tert-butyldimethylsilylamino)quinoline (1b) yields monomeric heteroleptic bis(trimethylsilyl)amido zinc 8-(trialkylsilylamido)quinoline (2a, 2b). The reaction of Zn[N(SiMe₃)₂]₂ with 8-aminoquinoline leads to the formation of heteroleptic dimeric 8-amidoquinoline zinc bis(trimethylsilyl)amide (3). This compound shows fluxionality on the NMR time scale due to a transannular trans-cis isomerization process and also a strong temperature dependency of the ratio of these two isomers. Variation of the stoichiometry allows the synthesis and isolation of homoleptic zinc bis(8-amidoquinoline) (4) with the zinc atoms in a distorted tetrahedral environment. The metallation of 8-aminoquinoline with (tmeda)NiMe₂ yields diamagnetic nickel bis(8-amidoquinoline) (5) with a nickel center showing a distorted square planar coordination sphere. Dinuclear zinc complexes are accessible employing tetra-dentate di(8-aminoquinolyl)diphenylsilane (6). The metallation of 6 with dimethylzinc yields dimeric zinc di(8-amidoquinolyl)diphenylsilane (7) with tetrahedrally coordinated zinc atoms. A comparison of the molecular structures of 1a, 2b, 3, 4, 5, 6, and 7 shows nearly no dependency of the structural data of the quinolyl unit on the substitution pattern of the amino function. The metal-nitrogen bond lengths depend on the coordination numbers of the metal and the nitrogen atoms (terminal or bridging position) as well as the electrostatic attraction between the metal cations and the amino bases.     

Different crystal forms of Zn(II) compound with diethyl (pyridin-3-ylmethyl)phosphonate (3-pmpe) ligand: Zn(3-pmpe)Cl2

The synthesis and characterization of the new didentate ligand diethyl (pyridin-3-ylmethyl) phosphonate (3-pmpe) and three of its Zn(II) complexes are described. IR and X-ray analyses show that in the reaction of ZnCl₂ with 3-pmpe in methanol three crystalline polymorphs are formed: [Zn(3-pmpe)Cl₂]₂ (1) and [Zn(3-pmpe)Cl₂]_n (2 and 3). In these crystals 3-pmpe acts as a didentate N,O-bridging ligand and Zn(II) are in a slightly distorted tetrahedral ZnNOCl₂ environment. Zn²⁺ ions in 1 are doubly bridged by the 3-pmpe ligands, resulting in the formation of dinuclear species. In polymeric compounds 2 and 3 Zn²⁺ ions are singly bridged by the 3-pmpe, resulting in the formation of one-dimensional chains. Small differences in the conformation of the ligand in 1 and 2 have been found. The infrared spectra are in agreement with the structural data.     

Direct conversion of cellulose to 1-(furan-2-yl)-2-hydroxyethanone in zinc chloride solution under microwave irradiation

Conversion of cellulose to 1-(furan-2-yl)-2-hydroxyethanone has been demonstrated in concentrated zinc chloride solution under microwave irradiation. Compared with the conventional oil-bath heating mode, microwave irradiation significantly reduced the reaction time and increased the yield of 1-(furan-2-yl)-2-hydroxyethanone. A typical degradation reaction with cellulose produced 1-(furan-2-yl)-2-hydroxyethanone in 12.0% molar yield in ZnCl₂ solution (ZnCl₂–H₂O ratio = 2.25:1, w/w) with microwave irradiation at 600 W for 5 minutes at 135 °C.     

Synthesis,Biological Evaluation and in Silico Studies of 3-Hydroxy-N-(2-(substituted phenyl)-4-oxothiazolidin-3-yl)-2-napthamide Derivatives

In the present study, a series of 3-hydroxy-N-(2-(substituted phenyl)-4-oxothiazolidin-3-yl)-2-napthamide derivatives were synthesized, characterized and evaluated for theirin vitroactivity, i. e., antimicrobial, antioxidant and anti-inflammatory. The target compounds were synthesized by condensation reaction of 3-hydroxy-2-naphthoic acid hydrazide with substituted benzaldehydes which were subjected to cyclization reaction with thioglycolic acid and ZnCl₂ to get target compounds. The synthesized 3-hydroxy-N-(2-(substituted phenyl)-4-oxothiazolidin-3-yl)-2-napthamide derivatives were examined for their antimicrobial activity and 3-hydroxy-N-(4-oxo-2-(3,4,5-trimethoxyphenyl)thiazolidin-3-yl)-2-naphthamide ( S20 ) exhibited the highest antimicrobial potential. The N′-(2,3-dichlorobenzylidene)-3-hydroxy-2-naphthohydrazide ( S5 ) displayed good antifungal potential against Rhizopus oryzae, whereas N′-(2,3-dichlorobenzylidene)-3-hydroxy-2-naphthohydrazide ( S20 ) showed the highest antioxidant potential and N-(2-(2,6-dichlorophenyl)-4-oxothiazolidin-3-yl)-3-hydroxy-2-naphthamide ( S16 ) displayed the highest anti-inflammatory activity. The results of molecular docking studies revealed that existence of hydrogen bonding and hydrophobic interactions with their respective proteins. In silico ADMET studies were carried out by Molinspiration, Pre-ADMET and OSIRIS property explorer to predict the pharmacokinetic behaviour of synthesized 3-hydroxy-N-(2-(substituted phenyl)-4-oxothiazolidin-3-yl)-2-napthamide derivatives.     

Crystal structures and catalytic activities of Zn(II) compounds containing btp ligands

The structures of new compounds containing Zn(II) ions and btp (2,6-bis(N′-1,2,4-triazolyl)pyridine) ligands have been determined. With coordinating chloride and bromide anions, Zn(II) produces chains 2 and 3 containing C-H?X(Cl or Br) weak interactions to build crystal structures and they are face-to-face aligned. With non-coordinating anions, Zn(II) produces monomeric compounds 4 and 5 containing H-bonds between anions and water ligands and face-to-face π-π interactions to build crystal structures. The previously reported polymeric compound 1 containing [Zn(NO₃)(H₂O)₂(btp)₂]⁺ units bridged by btp ligands was found to carry out the catalytic transesterification of a range of esters with methanol at room temperature under mild conditions, whereas it did not catalyze the ring opening of epoxide (easier reaction) with methanol. These results indicate that the polymeric compound 1 shows selective reactivity. In addition, the catalyst 1 has shown even better catalytic activity than the corresponding Zn(NO₃)₂ salt.     

Horse liver alcohol dehydrogenase derivatives containing nickel(II) and cobalt(lI) in the noncatalytic metal binding site

The noncatalytic zinc in horse liver alcohol dehydrogenase was selectively replaced by nickel(II). This novel species, Zn(c)₂Ni(n)₂⁴ horse liver alcohol dehydrogenase (where c denotes the catalytic and n denotes the noncatalytic site) was compared to Zn(c)₂Co(n)₂ horse liver alcohol dehydrogenase with respect to its absorption, circular dichroism and magnetic circular dichroism spectra, as well as its magnetic moment. For Zn(c)₂Co(n)₂ horse liver alcohol dehydrogenase (prepared according to refs. 1 and 2) the extinction coefficients were redetermined in the UV, visible and near-infrared region and the molar ellipticities in the range 300-800 nm. The average magnetic moment was determined by the NMR method as 4.5-5.0 B.M. The results confirm a tetrahedral structure in the zinc-cobalt enzyme. In contrast, the spectroscopic data and the zero magnetic moment support a planar geometry for the nickel(Il) bound in the noncatalytic site. Zn(c)₂Ni(n)₂ horse liver alcohol dehydrogenase is very temperature-sensitive and precipitates after short exposure to room temperature. Stored in the cold it has the same activity as the native enzyme. The results indicate that the protein is flexible in the loop region binding the noncatalytic metal ion and that it may retain catalytic activity even in a partially distorted conformation.