Complexes withN Research Articles

Crystal structure of the novel amino-acid racemase isoleucine 2-epimerase from Lactobacillus buchneri.

Crystal structures of Lactobacillus buchneri isoleucine 2-epimerase, a novel branched-chain amino-acid racemase, were determined for the enzyme in the apo form, in complex with pyridoxal 5'-phosphate (PLP), in complex with N-(5'-phosphopyridoxyl)-L-isoleucine (PLP-L-Ile) and in complex with N-(5'-phosphopyridoxyl)-D-allo-isoleucine (PLP-D-allo-Ile) at resolutions of 2.77, 1.94, 2.65 and 2.12 Å, respectively. The enzyme assembled as a tetramer, with each subunit being composed of N-terminal, C-terminal and large PLP-binding domains. The active-site cavity in the apo structure was much more solvent-accessible than that in the PLP-bound structure. This indicates that a marked structural change occurs around the active site upon binding of PLP that provides a solvent-inaccessible environment for the enzymatic reaction. The main-chain coordinates of the L. buchneri isoleucine 2-epimerase monomer showed a notable similarity to those of α-amino-ℇ-caprolactam racemase from Achromobactor obae and γ-aminobutyrate aminotransferase from Escherichia coli. However, the amino-acid residues involved in substrate binding in those two enzymes are only partially conserved in L. buchneri isoleucine 2-epimerase, which may account for the differences in substrate recognition by the three enzymes. The structures bound with reaction-intermediate analogues (PLP-L-Ile and PLP-D-allo-Ile) and site-directed mutagenesis suggest that L-isoleucine epimerization proceeds through abstraction of the α-hydrogen of the substrate by Lys280, while Asp222 serves as the catalytic residue adding an α-hydrogen to the quinonoid intermediate to form D-allo-isoleucine.

New Cu (II) and Co(II) octaazamacrocyclic complexes with 2-amino-3-phenyl-propanoic acid

New cationic Cu(II) and Co(II) complexes with N,N',N'',N'''-tetrakis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane (tpmc) and the anion of 2-amino-3-phenyl-propanoic acid (S-phenylalanine) were prepared. The complexes were analyzed and characterized by elemental analysis, conductometric, polarimetric, magnetic and cyclic voltammetric measurements, as well as by spectroscopic data (UV/Vis, IR). Both complexes are binuclear with the general formula [M2(S-Phe)tpmc](ClO4)3?nH2O; S-PheH = S-phenylalanine, M(II) = Cu, n = 7; Co, n = 0. Based on previously reported data for some familiar complexes and the present results, pentacoordinated geometry was proposed. Both of the central metal ions are coordinated with two pyridyl and two cyclam nitrogens and bridged with -N-(CH2)3-N- portions of the cyclam ring and oxygen atoms of the S-phenylalaninate ion. Antimicrobial screening of the complexes, solvent, starting salts and ligands alone was performed against fungi, mould and some bacteria. In certain cases, enhanced activity of Co(II) complex towards bacteria compared with the relevant free ligands and starting salts was detected.

Synthesis, spectroscopic characterization (IR,1H,13C and119Sn NMR, electrospray mass spectrometry) and toxicity of new organotin(IV) complexes withN,N′,O- andN,N′,S-scorpionate ligands

New organotin(IV) derivatives containing the anionic ligands bis(3,5-dimethylpyrazol-1-yl)dithioacetate [LCS 2 ] - and bis(3,5-dimethylpyrazol-1-yl)acetate [LCO 2 ] - have been synthesized from reaction between (CH 3 ) 2 SnCl 2 and lithium salts of the ligands. Mononuclear complexes of the type {[LCX 2 ](CH 3 ) 2 SnCl} (X = S or O) have been obtained and fully characterized by elemental analyses and FT-IR in the solid state and by NMR ( 1 H, 13 C and 119 Sn) spectroscopy, conductivity measurements and electrospray ionization mass spectrometry in solution. The acute toxicity of new organotin(IV) derivatives on rat was studied, comparing their effect with those of dimethyltin chloride (CH 3 ) 2 SnCl 2 . The comparison of LD 50 of organotin(IV) complexes and (CH 3 ) 2 SnCl 2 administered intraperitoneally, as a single dose, evaluated in vivo on rats, showed that toxicity decreases as follows: (CH 3 ) 2 SnCl 2 > LCO 2 > LCS 2 . The effect of these organotin(IV) complexes on DNA was evaluated in vitro and in vivo on rats treated with different doses of these compounds (1/20 LD 50 and 1/100 LD 50 ). The lymphocyte DNA status was assessed by the comet assay, a rapid and sensitive single-cell electrophoresis technique, used to detect primary DNA damage in individual cells. After 36 h from the start of treatment the two new organotin(IV) derivatives induced a significant rise in comet assay parameters, indicating an increasing presence of damaged DNA.

Synthesis and crystal structures of copper and manganese complexes withN,N -dibenzyl-1,10-phenanthroline-2,9-dimethylamine

The new ligand, N,N′-dibenzyl-1,10-phenanthroline-2,9-dimethylamine trihydrochloric acid monohydrate (L · 3HCl·H2O), has been synthesized and characterized. Complexes of the ligand (L) with CuII and MnII were prepared and their crystal structures were determined. The copper ion is five-coordinated via three nitrogen atoms of the ligand and two chloride ions, while the manganese ion is six-coordinated by three nitrogens of the ligand, two chloride ions and one water molecule. In both complexes, the ligand behaves in a tri-dentate fashion and the uncoordinated nitrogen of the other arm is protonated.

XAS, ESR and Potentiometric Studies of Three DinuclearN,N′‐para‐Xylylenebis(tetraazamacrocycle)copper(II) Complexes − X‐ray Crystal Structure of [N,N′‐p‐Xylylenebis(cyclen)]copper(II)

AbstractDicopper complexes withN,N′‐p‐xylylenebis(cyclam or cyclen) and with the heteroditopicN,N′‐p‐xylylenebis(cyclam‐cyclen) were synthesized. An X‐ray study of theN,N′‐p‐xylylenebis(cyclen)dicopper complex showed that the copper(II) ion is five‐coordinate with an H2O molecule in apical position. With this ligand, a polymeric chain was also obtained in the presence of KSCN. The terminal donor atoms of the bridging NCS−anion are coordinated in apical position to the square‐pyramidal copper(II) ion. Two alternating kinds of Cu2L4+moieties are present in the chain, the first with two N4S chromophores and the second with two N5chromophores. EXAFS and XANES results are in agreement with a five‐coordinate copper ion in the cyclen unit and a six‐coordinate copper ion in the cyclam unit. Thermodynamic constants were determined by potentiometry. The existence of dinuclear Cu2L4+species (ligand/metal ratio < 1) and mononuclear CuLHn(2+n)+species (ligand/metal ratio > 1) were confirmed by an ESR study at variable pH. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Hydrolysis of phosphodiester with hydroxo- or carboxylate-bridged dinuclear Ni(ii) and Cu(ii) complexes

A hydroxo- or carboxylate-bridged dinuclear Ni(II) complexes with N,N,N′,N′-tetrakis{(6-methyl-2-pyridyl)methyl }-1,3-diaminopropan-2-ol has been synthesized as models for Ni(II)-substituted phosphotriesterase, which are more active catalysts for hydrolysis of phosphodiester than the corresponding dinuclear Cu(II) and Zn(II) complexes.

Integrals in involution for groups of linear symplectic transformations and natural mechanical systems with homogeneous potential

We prove that if a complex Hamiltonian system withn degrees of freedom hasn functionally independent meromorphic first integrals in involution and the monodromy group of the corresponding variational system along some phase curve hasn pairwise skew-orthogonal two-dimensional invariant subspaces, then the restriction of the action of this group to each to these subspaces has a rational first integral. The result thus obtained is applied to natural mechanical systems with homogeneous potential, in particular, to then-body problem.

Theoretical investigation into electron density redistribution in theS 0→S 1 transition for (H2O) n ,n=2–6, complexes with hydrogen bonding

The association of theH 2 O molecule and the effect of UV radiation on complexes with hydrogen bonds were studied. Complete ab initio calculations of the S0 and S1 states were performed for the(H 2 O) n ,n=2–6, complexes. Changes in the effective atomic charges, bond orders, and electron densities on the bonds were analyzed. The analysis and the calculated cross sections of potential surfaces along theOH bonds reveal that one of theOH complex bonds is in decay. The electron excitation is shown to be localized at a molecule of the(H 2 O) n ,n=2–6, complex, while for theH 2 O complexes withn>2 the excitation involves, for the most part, a freeOH f bond of the water molecule that does not participate in the formation of the hydrogen bondO...H. A direct correlation is observed between the behavior of theOH f bond and the changes in the atomic charges Q(O) and Q(Hf) reflecting the electron density redistribution.

Spectra of optical phonons and low-energy electronic transitions in C60/TMPD and C60/TPA single crystals

Single Crystals of C60/TMPD and C60/TPA have been grown from a chlorobenzene solution. Optical transmission spectra of single crystals of fullerene complexes withN,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) and triphenylamine (TPA) are studied in the spectral range from 600 to 16000 cm−1. Splitting of the intramolecular vibration of C60 is observed at 1428 cm−1, which is likely caused by freezing of the rotation of the C60 molecules due to their interaction with amines. Single crystals of C60/TMPD differ from those of C60/TPA by a decrease in the vibration frequency at 1428 cm−1, vibrations of the C-C bonds of the TMPD molecule, and the redistribution of the forces of the oscillators of the vibrations of the C-N bonds. These peculiarities are interpreted to be the result of partial electron transfer from TMPD to C60 in the C60/TMPD single crystals. The electron transfer in the C60/TPA system is less pronounced.

Synthesis of a C60 complex withN,N,N?,N?-tetramethyl-p-phenylenediamine and its crystal structure

The complex of fullerene C60 withN,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) was synthesized and studied by X-ray analysis.

Laser ionization spectroscopy of Ag(NH3) n clusters

The ionization and the fragmentation properties of Ag(NH3) n heteroclusters have been studied. The measured ion yield spectra near the threshold for complexes withn=2−20 indicate strong differences of the cluster geometry in the electronic ground and in the ionized state, respectively. For smaller clusters (n≤6) in the neutral ground state the nonsolvated Ag atom is localized near the surface of the complex. A modification of the surface structure at the transition to larger clusters (n≥7) is suggested by significant changes in the ionization and fragmentation behaviour. Preliminary results of ab initio calculations confirm the conclusion that due to the correlation interaction between the valence electron and the 4d electrons of the Ag atom no solvation effects occur.

Equilibrium and hydrolysis of ?-amino acid esters in mixed-ligand complexes withN-(acetamido)-iminodiacetatecopper(II)

The formation equilibria of the binary and ternary complexes of CuII withN-(acetamido)-iminodiacetic acid (ADA) and amino acids or their esters were investigated potentiometrically. The chelation mode was ascertained by conductivity measurements. The kinetics of the base hydrolysis of amino acid esters in the presence of the copper(II)-ADA complex were studied. The rate and catalysis constants were estimated.

Synthesis, structure and magnetic studies of copper(II)-nickel(II) complexes withN,N′-bis(2-aminopropyl)oxamidocopper(II)

Three novel heterobinuclear complexes have been prepared and identified as [Cu(oxap)Ni(L)2](ClO4)2·ξH2O, where oxap denotes theN,N′-bis(2-aminopropyl)oxamido dianion and L denotes 2,2′-bipyridine (bipy), 1,10-phenanthroline (phen) or 5-nitro-1,10-phenanthroline (NO2-phen). The crystal structure of [Cu(oxap)Ni(phen)2]-(ClO4)2·2H2O has been determined. Crystal data: triclinic, space group $$P\bar 1$$ , a=12.079 (6), b=12.409 (4), c=17.261 (8) A, α=70.91 (2), β=86.72 (4), γ=89.19 (3)o. At room temperature, Z=2. The CuII is in a square planar environment and the NiII is in an octahedral environment. The Cu−Ni distance is 5.292 A. The temperature dependences of the magnetic susceptibilities of [Cu(oxap)Ni(phen)2]-(ClO4)2·2H2O and [Cu(oxap)Ni(bipy)2](ClO4)2 have been studied in the 4.2–300 K range, giving the exchange integral J=−92.4 cm−1 for bipy and J=−94.3 cm−1 for phen. These results are commensurate with antiferromagnetic interactions between the adjacent metal ions.

Mössbauer study of the hyperfine and magnetic properties of organo-di-iron electron reservoirs containing a polyaromatic bridging ligand

Organo di-iron electron reservoirs Fe(CP*)2(Ar)n+ withn=2, 1, 0, where Cp* is C5(CH3)5 and where Ar are the following bridges: biphenyl, dihydrophenanthrene, triphenylene, have been studied by Mossbauer spectroscopy in the solid state. Complexes withn=2, with 36e− in the coordination spheres of the metals, exhibit the usual diamagnetic behaviour of 18e−, FeII mono-iron systems. Complexes withn=1, 37e−, are delocalized mixed valence (FeIIFeI) with a spin 1/2; the magnetic hyperfine interaction, measured under an external field, shows equal delocalization of the 37th e− on the two iron centers and the two bridging carbon atoms of the biphenylene. Complexes withn=0, formally with 38e−, have a practically temperature-independent quadrupole splitting, and isomer shift values which constrast with the expected behaviour of independent FeI, 19e− centers. This indicates that the 37th and 38th electrons are mostly located on the polyaromatic bridge. Spectra obtained in an external field show a negligible magnetic hyperfine interaction and support this conclusion. In the case of biphenyl and dihydrophenanthrene bridges, this electron localization can be related to a strong intramolecular chemical coupling, evidenced by other spectroscopic and X-ray data [1].