A new approach to analytical Mössbauer spectroscopy

A fast and accurate method for the determination of oxygen to metal ration (O/M) in U₃O₈ is described. The method involves dissolution of the sample in 85% phosphoric acid containing sulfate ions and measurement of the U(IV) absorbance directly and after reducing the U(VI) in the sample to U(IV) with Fe(II). The O/M is calculated from the ratio of the two absorbances. Elaborate calibrations for U(IV) and U(VI) are avoided and the sample requirement is a minimum. The method gives the oxygen to metal ratio with an accuracy better than ±0.004 O/M units and precision better than ±0.004 O/M units.     

Isolation and structure elucidation of a new amphidinol with a truncated polyhydroxyl chain from Amphidinium klebsii

A new member of amphidinols (AM7) possessing a polyene-polyhydroxyl structure with the shortest carbon backbone and sulfate ester was isolated from the cultured dinoflagellate, Amphidinium klebsii. AM7 showed hemolytic and antifungal activities. The structure was elucidated on the basis of 2D NMR data in combination with CID MS/MS experiments.     

Kinetics of radical polymerization—XLIV. Kinetic investigation of the polymerization of methyl methacrylate in viscous medium

This paper deals with the homopolymerization of the system MMA-AIBN-DBF at 50°. The dependences of initiation and overall rates of the polymerization on the concentrations of monomer and the viscous solvent were studied. The absolute rate constants for chain propagation and termination were determined. The termination rate constant of the system MMA-DBP was found to be inversely proportional to the viscosity of the medium.     

Microspeciation in the copper(II)-L-histidylglycine system. An ESR study by the two-dimensional computer simulation method

Twelve ESR-active (and one inactive) copper(II) complexes of L-histidylglycine (HL) were characterized via their formation (micro)constants and ESR parameters obtained by two-dimensional ESR spectroscopic evaluation in aqueous solution. In strongly acidic media, the ligand is coordinated through its N-terminal donor groups: the complex [CuLH(2)](3+) involves monodentate imidazole binding, whereas [CuLH](2+) involves bidentate ligation through the amino and imidazole N atoms. This histamine-like bonding mode also predominates in the isomers of [CuL(2)], formed at ligand excess near pH 7: in the major 4N isomer, both ligands occupy two equatorial sites, while in the 3N isomer, the second dipeptide is coordinated equatorially by the amino and axially by the imidazole groups. At above pH 3-4, deprotonation of the peptide group also starts: in approximately 60% of the molecules of [CuL](+), the peptide group is deprotonated, while in the minor isomer histamine-like coordination occurs. At higher pH, the active dimer [Cu(2)L(2)H(-2)], the mixed hydroxo complexes (the inactive [Cu(2)L(2)H(-3)](-) and the active [CuLH(-2)](-)), and the bis complexes [CuL(2)H](+) and [CuL(2)H(-1)](-) all involve tridentate equatorial ligation of the backbone by the amino and deprotonated peptide N and the carboxylate O atoms. In the active dimer, the neutral imidazole groups form bridges between CuLH(-1) units. In [CuL(2)H](+), the second ligand is bound equatorially via its imidazole group; in [CuL(2)H(-1)](-), the L ligand occupies the fourth equatorial site and an axial site through its amino and imidazole N atoms, respectively.     

Unexpected effect of charge density of the aromatic guests on the stability of calix[6]arene-phenol host-guest complexes

The pi-pi interaction-based inclusion complexation of calix[6]arene hexasulfonate as host with neutral aromatic guest molecules was studied in aqueous media. To vary the electron density on the guest's aromatic rings, the phenol parent compound was functionalized in the para-position with different electron-withdrawing groups, such as NO2 and Cl, as well as H and CH3 groups. To study the interaction between calixarene and the guests, PL, DSC, and quantum-chemical methods were used. The results indicate 1:1 stoichiometry for all examined host-guest complexes. Although the enthalpy change predicts strong interaction between the host and the guest, the Gibbs free energy change of the complex formation is small, resulting in a relatively low complex stability. This property is due to the high and negative entropy change during the complex formation. Comparing the thermodynamic parameters observed on the series of the guests, we observed a decrease of the enthalpy change when the electron density on the guest's aromatic ring increased. However, the Gibbs free energy and therefore, the stability of the complexes increased when the enthalpy change lowered. These unexpected results are based on the enthalpy-entropy compensation effect and probably due to the quite different entropy change related to the high and low electron density on the aromatic rings of different guest molecules. Using molecular dynamic calculations, a redistribution of the electron density of calixarene rings, followed by the reordering of the solvent molecules, was identified as a background of this unexpected entropy change at molecular level.     

Mössbauer study of bridged ferrocenyl ketones

The effect of the carbonyl group on Mössbauer parameters can be seen in the keto-derivatives of methylene-bridged ferrocenes. The disturbing influence of ring tilt and bond shortness on the electron withdrawal of the carbonyl group is shown by some trimethylene-bridged ferrocenes. The results are interpreted by a change of electron population in ring-based orbitals and by the absence of coplanarity of the carbonyl group with the cyclopentadienyl ring.     

Theoretical conformational analysis for neurotransmitters in the gas phase and in aqueous solution. Norepinephrine

The natural neurotransmitter (R)-norepinephrine takes the monocationic form in 93% abundance at the physiological tissue pH of 7.4. Ab initio and DFT/B3LYP calculations were performed for 12 protonated conformers of (R)-norepinephrine in the gas phase with geometry optimizations up to the MP2/6-311++G level, and with single-point calculations up to the QCISD(T) level at the HF/6-31G-optimized geometries. Four monohydrates were studied at the MP2/6-31G//HF/6-31G level. In the gas phase, the G1 conformer is the most stable with phenyl.NH(3)(+) gauche and HO(alc).NH(3)(+) gauche arrangements. A strained intramolecular hydrogen bond was found for conformers (G1 and T) with close NH(3)(+) and OH groups. Upon rotation of the NH(3)(+) group as a whole unit about the C(beta)-C(alpha) axis, a 3-fold potential was calculated with free energies for barriers of 3-12 kcal/mol at the HF/6-31G level. Only small deviations were found in MP2/6-311++G single-point calculations. A 2-fold potential was calculated for the phenyl rotation with free energies of 11-13 kcal/mol for the barriers at T = 310 K and p = 1 atm. A molecular mechanics docking study of (R)-norepinephrine in a model binding pocket of the beta-adrenergic receptor shows that the ligand takes a conformation close to the T(3) arrangement. The effect of aqueous solvation was considered by the free energy perturbation method implemented in Monte Carlo simulations. There are 4-5 strongly bound water molecules in hydrogen bonds to the conformers. Although hydration stabilizes mostly the G2 form with gauche phenyl.NH(3)(+) arrangement and a water-exposed NH(3)(+) group, the conformer population becomes T > G1 > G2, in agreement with the PMR spectroscopy measurements by Solmajer et al. (Z. Naturforsch. 1983, 38c, 758). Solvent effects reduce the free energies for barriers to 3-6 and 9-12 kcal/mol for rotations about the C(beta)-C(alpha) and the C(1)(ring)-C(beta) axes, respectively.     

On the Reactions Occurring in the CaO?Cr2O3?O2 system

The reactions occurring in the system CaO? Cr₂O₃? O₂ have been investigated. In the mixture of two oxides heated in air to higher temperature calcium chromate and chromite are being formed. The rate and the ratio of the two reactions is very sensitive to the composition of gaseous atmosphere, especially to the partial pressure of oxygen. The formation of spinel was accelerated by a small amount of oxygen but was partly or fully suppressed at a higher oxygen pressure on the expense of chromate formation. The oxidation of CaO? Cr₂O₃ was also studied kinetically. Detailed measurements in different gaseous atmospheres were performed with doped CaO and Cr₂O₃. The results were interpreted in the light of the defect structure of the reacting oxides.     

Comparative studies of the molecular structure of 1-R-1,2,3,4,5,6-hexamethylbenzenonium ions: Molecular and crystal structure of 1-chloromethyl-1,2,3,4,5,6-hexamethylbenzenonium tetrachloroaluminate

1-Chloromethyl-1,2,3,4,5,6-hexamethylbenzenonium tetrachloroaluminate is investigated in an X-ray diffraction study. The crystals are monoclinic: a=8.910(2), b=12.290(3), c=17.654(4) Å, β=111.57(2)°, space group P2₁/c. The AlCl ₄ ^? anion has an almost perfect tetrahedral configuration; the average length of the Al?Cl bond is 2.130(3) Å. For cations, the symmetry is close to C_s. The C(2)?C(6) atoms lie in a plane (within ±0.012(5) Å), and the C(1) atom deviates from the plane by 0.113(5) Å to the side that is opposite to the CH₂Cl group. Effects of the environment, as well as the effect of the nature of the R group, on the structure of 1-R-1,2,3,4,5,6-hexamethylbenzenonium are considered.     

Study of the determination of acetylcholine after enzymatic hydrolysis by triangle programmed coulometric flow titration

A new method for the determination of acetylcholine is introduced and studied. The method uses enzymatic hydrolysis of acetylcholine carried out in a flow-through analytical reactor column, while for the detection triangle programmed coulometric flow titration is employed. The flow-through coulometric titration system and the preparation of the enzyme reactor are described. The operation conditions for the hydrolysis and for the titration were optimized. The flow-through coulometric titration technique offers a better precision for the determination of acetylcholine in small concentration ranges than methods based on the detection of pH change. Working conditions of the system can be easily adjusted for different sample concentration ranges. The limit of detection for the determination of acetylcholine was found to be 8x10(-5) M. The method described could be advantageously used for indirect determination of acetylcholinesterase inhibitors.