Labile species of Pb, Zn and Cd determined by anodic stripping staircase voltammetry and their toxicity to tetrahymena

The amounts of free ions and labile complexes of Pb, Zn and Cd have been determined in a complex organic growth medium by use of anodic stripping staircase voltammetry, ASSV. It was possible to determine the labile fractions of metals and to obtain good correlation with data on the toxicity of the metals to Tetrahymena when using a specific reduction potential (E(s)) for each element, - 600, - 800 and - 1200 mV vs. SCE for Pb, Zn and Cd, respectively. The labile fractions in the organic growth medium were less than 1% for Pb (Pb precipitated), 15% for Zn (as labile complexes) and 30-40% for Cd (as free ions) for total heavy metal concentrations of 0.5mM-2mM. The toxicity to Tetrahymena decreased in the order Cd Zn Pb. The effect of Pb was greater than predicted by ASSV, probably because Tetrahymena additionally ingests the lead-containing precipitate and therefore is exposed to concentrations of Pb exceeding those of the soluble species found in the medium. The results stress the importance of using different specific reduction potentials when different elements are compared, instead of one potential common to all elements. The use of a fixed potential may lead to erroneous conclusions regarding the concentrations of labile species in solution.     

Construction of orbital angular momentum eigenfunctions for many-particle systems by harmonic projection

Formulas are derived which allow the direct construction of total orbital angular momentum eigenfunctions for many-particle systems without the use of Clebsch–Gordan coefficients. One of the equations is closely analogous to Dirac' identity for the total spin operator. This equation describes the action of L² on a function of the particle coordinates in terms of a class operator of the symmetric group and a "contraction operator." A general projection operator for constructing symmetric eigenfunctions of L² is presented.     

Field desorption mass spectrometry of 1-methylpyridinium salts

Quaternary pyridinium salts were investigated by field desorption, field ionization and electron impact mass spectrometry. Thermal decomposition of the salts under field desorption conditions was found to give very abundant volatile products, identified as the dihydro analogues and the ‘methides’ of the cations. The formation of the volatile compounds, especially those with molecular weights corresponding to those of the (cation ?1), (cation +1) and (cation +13) was studied in detail by deuterium and ¹³C labelling experiments.     

Time-resolved and steady-state fluorescence spectroscopic studies of the human lens with comparison to argpyrimidine,pentosidine and 3-OH-kynurenine

The intrinsic fluorescence from the human lens on excitation in the UV region, referred to as blue lens autofluorescence, increases with age or in the presence of diabetes. The present study reveals that the relative contribution of compounds responsible for the blue autofluorescence appears to be a constant with age. Three potential candidates for the blue fluorescence were also studied with respect to fluorescence spectroscopic properties. These were argpyrimidine and pentosidine, both advanced glycation end products, and 3-hydroxykynurenine (3-OH-kynurenine), a photooxidative derivative of tryptophan. It was shown that the spectral properties of argpyrimidine and pentosidine are compatible with the observed blue fluorescence of the human lens, whereas the fluorescence from 3-OH-kynurenine is negligible.     

Host-guest chemistry of the chromium-wheel complex [Cr8F8(tBuCO2)16]: prediction of inclusion capabilities by using an electrostatic potential distribution determined by modeling synchrotron X-ray structure factors at 16 K

The structure and detailed electron density distribution (EDD) of the large octanuclear chromium-wheel host complex [Cr8F8(tBuCO2)16] (1) has been determined from synchrotron X-ray structure factors collected at 16(5) K. The complex has a central cavity with a minimum entry distance between carbon atoms of the pivalate methyl groups (pivalic acid = tBuCO2H) of 4.027(4) A on one side of the molecule and 7.273(4) A on the other. The screened side of the molecule can be "opened" by rotation of methyl groups to create a strained host structure, which is compensated for by improved host-guest and host-solvent interaction. The EDD of the 272-atom complex (1144 e-) was determined by multipole modeling based on the experimental structure factors. 3d orbital populations on the Cr atoms and topological analysis of the EDD show that the covalent part of the metal-ligand interactions consists mainly of sigma donation from the ligands, but that overall the interactions are predominantly electrostatic. The electrostatic potential (EP) has been calculated from the experimental EDD. Knowledge of the geometry of the naked complex 1 as well as the EP in the central cavity of this molecule allows us to deduce which characteristic properties guest molecules must have to be accepted into the void. To probe these predictions, a series of complexes of 1 with different guest inclusions were synthesized (2 = 1 + N,N'-dimethylformamide (DMF), 3 = 1 + N,N'-dimethylacetamide (DMA), 4 = 1 + DMA + DMF, 5 = 1 + 2CH3CN), and their structures were examined by using X-ray diffraction data measured at 120(1) K. Results of these studies indicate that in the crystalline state, the optimal guest molecule should be linear and possess a permanent dipole. Attempts to crystallize the host complex with cations incorporated into the cavity were fruitless, although electrospray ionization mass spectrometry showed that a [1 + potassium]+ entity pre-exists in solution and can be transferred intact into the gas phase.