Interference of Humic Substances in the Spectrophotometric Determination of Bromate by Phenothiazines in Natural Waters

Abstract

The spectrophotometric method of bromate (BrO₃ ^?) determination by phenothiazines was applied to natural water samples and the interferences due to the presence of inorganic and humic substances were investigated. Common ions present in natural waters did not interfere and only the less abundant NO₂ ^? and Fe²⁺ exhibited strong interferences. Interferences of the two latter ions, if they existed, could be controlled and the method proved to be accurate and with a low detection limit. However, it was found that the presence of soluble humic substances resulted in positive interference, rendering the method unsuitable for bromate determination in natural waters and restricted its use in pure bromate solutions. This interference can be attributed to the electron acceptor groups invariably existing in the humic molecules. Since humic substances can remain in the water even after its ozonation, they will also contribute to a positive interference in bromate determination in potable waters.     

Thermally Induced Synthesis of Anthracene-, Pyrene- and Naphthalene-Fused Porphyrins

The synthesis of π-extended porphyrins containing anthracenyl moieties still represents an important challenge. Here, we report on the synthesis of a series of unsubstituted naphthyl-, pyrenyl- and anthracenyl-fused zinc porphyrin derivatives. To this aim, meso-substitued porphyrins are synthesized and the fusion of the PAHs (Polycyclic Aromatic Hydrocarbon) on the β-positions are performed through thermally induced dehydro-aromatization. The fused zinc-porphyrin derivatives are fully characterized and their optical absorption and photoluminescence properties are reported. We also demonstrate that zinc can be removed from the porphyrin core, giving rise to pure C, H, N materials. This work constitutes the first step towards the synthesis of the fully-fused tetra-anthracenylporphyrin.     

Highly stereoselective hydroxy-directed Diels-Alder reaction

The successful stereocontrol of the Diels-Alder reaction of semicyclic dienes possessing a secondary and tertiary allylic magnesium alkoxide alcohol functionality and activated dienophiles such as methyl acrylate, methacrolein, acrolein, and N-phenylmaleimide is described.     

S K-edge XAS and DFT calculations on square-planar NiII-thiolate complexes: effects of active and passive H-bonding

S K-edge XAS for a low-spin NiII-thiolate complex shows a 0.2 eV shift to higher pre-edge energy but no change in Ni-S bond covalency upon H-bonding. This is different from the H-bonding effect we observed in high-spin FeIII-thiolate complexes where there is a significant decrease in Fe-S bond covalency but no change in energy due to H-bonding (Dey, A.; Okamura, T.-A.; Ueyama, N.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 2005, 127, 12046-12053). These differences were analyzed using DFT calculations, and the results indicate that two different types of H-bonding interactions are possible in metal-thiolate systems. In the high-spin FeIII-thiolate case, the H-bonding involves a thiolate donor orbital which is also involved in bonding with the metal (active), while in the low-spin NiII-thiolate, the orbital involved in H-bonding is nonbonding with respect to the M-S bonding (passive). The contributions of active and passive H-bonds to the reduction potential and Lewis acid properties of a metal center are evaluated.     

Aromatic Gain in a Supramolecular Polymer

The synergy of aromatic gain and hydrogen bonding in a supramolecular polymer is explored. Partially aromatic bis(squaramide) bolaamphiphiles were designed to self‐assemble through a combination of hydrophobic, hydrogen‐bonding, and aromatic effects into stiff, high‐aspect‐ratio fibers. UV and IR spectroscopy show electron delocalization and geometric changes within the squaramide ring indicative of strong hydrogen bonding and aromatic gain of the monomer units. The aromatic contribution to the interaction energy was further supported computationally by nucleus‐independent chemical shift (NICS) and harmonic oscillator model of aromaticity (HOMA) indices, demonstrating greater aromatic character upon polymerization: at least 30 % in a pentamer. The aromatic gain–hydrogen bonding synergy results in a significant increase in thermodynamic stability and a striking difference in aggregate morphology of the bis(squaramide) bolamphiphile compared to isosteres that cannot engage in this effect.     

Design and spectroscopic characterization of peptide models for the plastocyanin copper-binding loop

The Cu(II)- and Co(II)-binding properties of two peptides, designed on the basis of the active site sequence and structure of the blue copper protein plastocyanin, are explored. Peptide BCP-A, Ac-Trp-(Gly)(3)-Ser-Tyr-Cys-Ser-Pro-His-Gln-Gly-Ala-Gly-Met-(Gly )(3)-His-(Gly)(2)-Lys-CONH(2), conserves the Cu-binding loop of plastocyanin containing three of the four copper ligands and has a flexible (Gly)(3) linker to the second His ligand. Peptide BCP-B, Ac-Trp-(Gly)(3)-Cys-Gly-His-Gly-Val-Pro-Ser-His-Gly-Met-Gly-CONH(2), contains all four blue copper ligands, with two on either side of a beta-turn. Both peptides form 1:1 complexes with Cu(II) through His and Cys ligands. BCP-A, the ligand loop, binds to Cu(II) in a tetrahedrally distorted square plane with axial solvent ligation, while BCP-B-Cu(II) has no tetrahedral distortion in aqueous solution. In methanolic solution, distortion of the square plane is evident for both BCP-Cu(II) complexes. Tetrahedral Co(II) complexes are observed for both peptides in aqueous solution but with 4:2 peptide:Co(II) stoichiometries as estimated by ultracentrifugation. Cu(II) reduction potentials for the aqueous peptide-Cu(II) complexes were measured to be +75 +/- 30 mV vs NHE for BCP-A-Cu(II) and -10 +/- 20 mV vs NHE for BCP-B-Cu(II). The results indicate that the plastocyanin ligand loop can act as a metal-binding site with His and Cys ligands in the absence of the remainder of the folded protein but, by itself, cannot stabilize a type 1 copper site, emphasizing the role of the protein matrix in protecting the Cu binding site from solvent exposure and the Cys from oxidation.     

Stereocontrolled synthesis of carbocycles via four successive pericyclic reactions

Herein, we report a highly diastereoselective construction of fused carbocycles using four successive pericyclic reactions.     

A simple and fast liquid chromatography–tandem mass spectrometry method for measurement of underivatized <Emphasis Type="SmallCaps">l</Emphasis>-arginine,symmetric dimethylarginine,and asymmetric dimethylarginine and establishment of the reference ranges

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthase and an established biomarker for endothelial function, while symmetric dimethylarginine (SDMA), an emerging biomarker for renal function, has been shown to outperform creatinine-based equations for estimated glomerular filtration rate. In order to study these analytes for clinical research, a fast and simple method for measuring arginine (ARG), SDMA, and ADMA in plasma by liquid chromatography–tandem mass spectrometry (LC-MS/MS) has been developed. Plasma (50 μL) was mixed with 50 μL of internal standard of ¹³C-arginine and d₇-ADMA followed by protein precipitation with methanol containing 1% ammonium acetate (300 μL). After centrifugation, the supernatant (100 μL) was mixed with 300 μL of acetonitrile with 1% formic acid, and the mixture was injected onto a silica column monitored by a mass spectrometer. The analytical cycle time was 5.0 min. The method was linear from 5.7 to 489.7 μM for ARG, 0.06 to 5.15 μM for SDMA, and from 0.34 to 5.65 μM for ADMA, with an accuracy of 99.0–120.0%. Total coefficients of variation for all analytes ranged from 2.7% to 7.7% for three concentration levels. The effects of hemolysis, lipemia, uremia, icterus, specimen tube types, storage at different temperature, and freeze/thaw were thoroughly investigated. Reference ranges were established using 51 well-defined reference subjects (12 men and 39 women, age 19–64 years): 53.1–129.7 μM for ARG, 0.32–0.65 μM for SDMA, and 0.36–0.67 μM for ADMA. In conclusion, the validated LC-MS/MS method described here offers a fast and reliable ARG, SDMA, and ADMA quantitation in plasma with minimum sample preparation.     

Tetradentate N2S2 vanadyl(IV) coordination complexes: synthesis, characterization, and reactivity studies

The versatile N(2)S(2) tetradentate ligands (bme-daco)(2-), (bme-dach)(2-), and (ema)(4-) are known to accommodate many divalent transition-metal ions (M = Ni(II), Pd(II), Pt(II), Pb(II), Zn(II), Cd(II), Cu(II), and Fe(II)) while maintaining reactivity at the S-thiolate sites of the respective N(2)S(2)M complexes. The vanadyl ion, of interest for its pharmacological possibilities and its spin-label reporter properties for bioinorganic studies, also shows an affinity for such mixed nitrogen/sulfur-donor environments. Thus, (V≡O)(2+) analogues of a well-characterized series of N(2)S(2)Ni complexes have been prepared as mimics of possible N(2)S(2)(V≡O) formed from in vivo binding sites of the tripeptide motif, Cys-X-Cys. The nucleophilicity of the S-thiolate in these systems is explored with alkylating agents. IR [ν(VO)], electronic spectral, and electron paramagnetic resonance measurements are presented. X-ray diffraction studies of (bme-daco)(V≡O), (bme-dach)(V≡O), and [Et(4)N](2)[(ema)(V≡O)] further characterize the vanadyl complexes. A comparison of the spectral properties with the product of vanadyl interaction with the CGC tripeptide, the biological analogue of the tetraanionic N(2)S(2) ligand, is given.