Carboxylate-bridged copper(II)-lanthanide(III) complexes [(Cu(3)Ln(2)(oda)(6)(H(2)O)(6)).12H(2)O](n)(Ln = Dy,Ho, Er,Y; oda = oxydiacetate)

The hydrothermal reaction of Ln(2)O(3) (Ln = Dy and Ho), Cu(OAc)(2).2H(2)O, and oxydiacetic acid in the approximate mole ratio of 1:3:8 resulted in the formation of two new members of the isostructural series of polymers formulated as [(Cu(3)Ln(2)(oda)(6)(H(2)O)(6)).12H(2)O](n), crystallizing in the hexagonal crystal system, space group P6/mcc (No. 192). Temperature-dependent magnetic susceptibilities and EPR spectra are reported for the heterometallic compounds Cu-Dy 1, Cu-Ho 2, Cu-Er 3, and Cu-Y 4. The results are discussed in terms of the structure of the compounds, the electronic properties of the lanthanide ions, and the exchange interactions between the magnetic ions.     

Metal ion catalysis in the beta-elimination reactions of N-[2-(4-pyridyl)ethyl]quinuclidinium and N-[2-(2-pyridyl)ethyl]quinuclidinium in aqueous solution

Catalysis of the beta-elimination reaction of N-[2-(4-pyridyl)ethyl]quinuclidinium (1) and N-[2-(2-pyridyl)ethyl]quinuclidinium (2) by Zn(2+) and Cd(2+) in OH(-)/H(2)O (pH = 5.20-6.35, 50 degrees C, and mu = 1 M KCl) has been studied. In the presence of Zn(2+), the elimination reactions of both isomers occur from the Zn(2+)-complexed substrates (C). The equilibrium constants for the dissociation of the Zn(2+)-complexes are as follows: K(d) = 0.012 +/- 0.003 M (isomer 1) and K(d) = 0.065 +/- 0.020 M (isomer 2). The value of k(C)(H2O) for isomer 1 is 4.81 x 10(-6) s(-1). For isomer 2 both the rate constants for the "water" and OH(-)-induced reaction of the Zn(2+)-complexed substrate could be measured, despite the low concentration of OH(-) in the investigated reaction mixture [k(C)H2O)= 1.97 x 10(-6) s(-1) and k(C)(OH-)= 21.9 M(-1) s(-1), respectively]. The measured metal activating factor (MetAF), i.e., the reactivity ratio between the complexed and the uncomplexed substrate, is 8.1 x 10(4) for the OH(-)-induced elimination of 2. This high MetAF can be compared with the corresponding proton activating factor (Alunni, S.; Conti, A.; Palmizio Errico, R. J. Chem. Soc., Perkin Trans. 2 2000, 453), PAF = 1.5 x 10(6) and is in agreement with an E1cb irreversible mechanism (A(xh)D(E)* + D(N)) (Guthrie, R. D.; Jencks, W. P. Acc. Chem. Res. 1989, 22, 343). A value of k(C)(H2O)>or= 23 x 10(-7) s(-1) is estimated for the Cd(2+)-complexed isomer 2, while catalysis by Cd(2+) has not been observed for isomer 1.     

Thermal denaturation of bovine serum albumin and its oligomers and derivativespH dependence

In a previous paper, we report a preliminary DSC study on bovine (BSA) and human (HSA) serum albumins. However, at accurate HPLC analysis the commercial proteins show three peaks: Fraction V-I, probably globulins (as declared by the producers), Fraction V-II (about 15–18% of the product) and Fraction V-III that represents pure BSA or HSA. A hypothesis is that the Fraction II is a covalent dimer, or trimer or a mixture of both, generated during the scalf-life of the commercial product. Denaturation enthalpies of the purified Fraction V-III and Fraction V-II of BSA, have been determined calorimetrically, at changing thepH, and the results of both compared with those obtained on the untreated protein. Few calorimetric experiments have been also carried on a BSA monomer derivative with sulphidril group protected. Computer program have been developed for the deconvolution of exo- and endothermic effects and for the analysis of thermal denaturation profiles.     

The chemistry and the stereochemistry of poly(N-alkyliminoalanes) : V. The preparation and crystal structure of the pentamer [(HalN-i-Pr)2(H2AlNH-i-Pr)3]

The compound [(HAlN-i-Pr)₂(H₂AlNH-i-Pr)₃] has been prepared and the crystal and molecular structure determined by an X-ray analysis, carried out with three-dimensional data collected on a diffractometer. The molecule is made up of a cyclohexane-type ring, [(HAlN-i-Pr)₂(H₂AlNH-i-Pr)], in skewboat conformation, on each side of which is bonded an -H₂AlNH-i-Pr- bridging unit between a nitrogen atom and an aluminum atom of the ring. The molecule lies on a binary axis of the crystal, but this symmetry is fulfilled only by a statistical orientation of the asymmetric molecular units (the statistical model is not however completely defined). The AlN bond lengths range from 1.901 to 1.985 Å; the average NC bond length is 1.527(9) Å. Main crystal data are: monoclinic space group C2/c; a = 10.15(2), b = 21.64(3), c = 12.84(2) Å, β = 111.9(5)°; Z = 4; calculated density 1.095 g/cm³. The structure was solved by direct methods and block-matrix least-squares converged to an R value of 5.6%.     

Selectivity in cycloadditions-IX: Cycloadditions of nitrile oxides to indoles. Reactivity and regiochemistry

Cycloadditions of benzonitrile oxide and mesitonitrile oxide to N-methylindole and indole yield the acid sensitive cycloadducts 1 a-d with high regioselectivity. With N-carbethoxyindole the stable cycloadducts 1 e,f and minor amounts of the regioisomeric 2 e,f are isolated. The electron withdrawing substituent reduces both the regioselectivity and the reactivity of the cycloadditions.Frontier orbital considerations, based on MINDO/3 calculations, allow elucidation of the observed changes in reactivity and regiochemistry.     

ICP-MS multi-element analysis of wine samples - a comparative study of the methodologies used in two laboratories

A comparison of the performance of the methodologies used in two distinct laboratories (Lab A and Lab B) for multi-element analysis in different wines was carried out. ICP-MS apparatus (quadrupole mass analyzers) of different brands as well as different wine pre-treatments were used. At Lab A, a pre-treatment by UV-irradiation was performed. At Lab B, a micro-concentric nebulizer was used for direct analysis of the wine. Twenty-six elements (Li, V, Co, Ni, Cu, Zn, Ga, As, Rb, Sr, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Pb) were measured in common at the two labs in three different wine samples (red and white Bordeaux table wines and Port wine) and the results were compared. The two methodologies provided similar LODs and similar precisions, with RSDs of 0.5-5%, for most of the elements. The recovery percentages were 85-120% at Lab A for the three wines, and 78-119% at Lab B for the Bordeaux wines, validating the accuracy of the methods used. Comparable results were obtained at both labs for ten elements (Li, V, Co, Ni, Cu, Zn, Rb, Sr, Ba, and Pb) in the three selected wines; the differences were lower than 10% in most cases. For REEs, the differences observed were slightly higher, but still in the acceptable range due to the sub-ppb levels involved. The results obtained for As and Ga were not comparable, due to methodological influence. A comparison through linear least-squares adjustment indicated that the results obtained by the two labs were linearly correlated (correlation coefficient =0.997) but statistically different as the slope was slightly, but significantly different from one, for a confidence level of 95% (the intercept was statistically identical to zero in any case). In the future, strictly more identical results can be achieved by using a reference wine sample.     

Molecular orbital theory of the hydrogen bond. 24. Ground-state water–uracil complexes

Ab initio SCF calculations with the STO -3G basis set have been performed to investigate the structural, energetic, and electronic properties of mixed water–uracil dimers formed at the six hydrogen-bonding sites in the uracil molecular plane. Hydrogen-bond formation at three of the carbonyl oxygen sites leads to cyclic structures in which a water molecule bridges N₁? H and O₂, N₃? H and O₂, and N₃? H and O₄. Open structures form at O₄, N₁? H, and N₃? H. The two most stable structures, with energies of 9.9 and 9.7 kcal/mole, respectively, are the open structure at N₁? H and the cyclic one at N₁? H and O₂. These two are easily interconverted, and may be regarded as corresponding to just one “wobble” dimer. At 1 kcal/mole higher in energy is another “wobble” dimer consisting of an open structure at N₃? H and a cyclic structure at N₃? H and O₄. The third cyclic structure at N₃? H and O₂ collapses to the “wobble” dimer at N₃? H and O₄. The two “wobble” dimers are significantly more stable than the open dimer formed at O₄, which has a stabilization energy of 5.4 kcal/mole. Uracil is a stronger proton donor to water through N₁? H than N₃? H, owing to a more favorable molecular dipole moment alignment when association occurs through H₁. Hydration of uracil by additional water molecules has also been investigated. Dimer stabilization energies and hydrogen-bond energies are nearly additive in most 2:1 water:uracil structures. There are three stable “wobble” trimers, which have stabilization energies that vary from 7 to 9 kcal/mole per water molecule. Hydrogen-bond strengths are slightly enhanced in 3:1 water:uracil structures, but the cooperative effect in hydrogen bonding is still relatively small. The single stable water–uracil tetramer is a “wobble” tetramer, with two water molecules which are relatively free to move between adjacent hydrogen-bonding sites, and a stabilization energy of approximately 8 kcal/mole per water molecule. Within the rigid dimer approximation, successive hydration of uracil is limited to the addition of one, two, or three water molecules.     

Improvement of plant parameters of the ROBO gamma irradiation facility due to design modification

Two industrial scale, “ROBO” type ⁶⁰Co gamma irradiation facilities have recently been put into operation in Syria and Peru, and the dosimetry commissioning of both plants have been carried out to determine dose distribution within products and to calculate plant parameters such as efficiency, dose uniformity ratio and throughput. There are some design modifications between the two plants in connection with the location of the carriers with respect to the source plaque and also to each other. The effect of these construction modifications on the plant parameters is discussed in the analysis of the dose distribution data measured in the carriers with depth and height among the four irradiation rows on both sides of the source plaque. The plant parameters were also calculated for different product densities using the technical data of the facilities, and the calculated and measured results were compared to each other.     

Influence of aromatic substituents on metal(II)salen catalysed, asymmetric synthesis of α-methyl α-amino acids

The influence of substituents on both the aromatic rings of the catalyst, and the benzylidene unit of the substrate are investigated in the (salen)copper(II) catalysed asymmetric benzylation of alanine derivatives. Catalysts with electron-donating, and electron-withdrawing substituents of various sizes and at various locations on the aromatic rings of the salen ligand were prepared, but all exhibited inferior enantioselectivity to the parent (salen)copper(II) complex. In contrast, the introduction of halogenated substituents onto the aromatic ring of the N-benzylidene alanine methyl ester substrate was found to enhance the enantioselectivity of the alkylation with a para-chloro substituent giving optimal results. A new procedure for the preparation of the catalysts which avoids the need for chromatography on sephadex LH20 is reported, and the optimal catalyst obtained in this way was found to be a cobalt(salen) complex.