Removal of arsenic from aqueous solution: A study of the effects of pH and interfering ions using iron oxide nanomaterials

Nanophase Fe₃O₄ and Fe₂O₃ were synthesized through a precipitation method and were utilized for the removal of either arsenic (III) or (V) from aqueous solution as a possible method for drinking water treatment. The synthesized nanoparticles were characterized using X-ray diffraction, which showed that the Fe₃O₄ and the Fe₂O₃ nanoparticles had crystal structures of magnetite and hematite, respectively. In addition, Secherrer's equation was used to determine that the grain size nanoparticles were 12 ± 1.0 nm and 17 ± 0.5 nm for the Fe₂O₃ and Fe₃O₄, respectively. Under a 1 h contact time, batch pH experiments were performed to determine the optimum pH for binding using 300 ppb of either As(III) or (V) and 10 mg of either Fe₃O₄ or Fe₂O₃. The binding was observed to be pH independent from pH 6 through pH 9 and a significant drop in the binding was observed at pH 10. Furthermore, batch isotherm studies were performed using the Fe₂O₃ and Fe₃O₄ to determine the binding capacity of As(III) and As(V) to the iron oxide nanomaterials. The binding was found to follow the Langmuir isotherm and the capacities (mg/kg) of 1250 (Fe₂O₃) and 8196 (Fe₃O₄) for As(III) as well as 20,000 (Fe₂O₃) and 5680 (Fe₃O₄) for As(III), at 1 and 24 h of contact time, respectively. The As(V) capacities were determined to be 4600 (Fe₂O₃), 6711(Fe₃O₄), 4904 (Fe₂O₃), and 4780 (Fe₃O₄) mg/kg for nanomaterials at contact times of 1 and 24 h respectively.     

Determination of the usage of shade structures via a dosimetry technique

A measurement system is described that allows an objective review and evaluation of the amount of use by different population groups of provided shade structures. It employs the comparison of the erythemal UV exposure measured with dosimeters to either the vertex or forehead to that in full sun. The technique has been developed using three shade structures and found to provide a linear relationship with an R(2) of 0.99 between the exposure ratio and the time spent in the shade for the solar zenith angle range of 19-53° and for both low- and high-cloud levels. It provides an objective determination of the amount of shade use by population groups that have set periods of time outdoors.     

Silver containing sol–gel derived silica thin films: effect of aluminum incorporation on optical,microstructural and bactericidal properties

Silver containing silica (Ag–SiO₂) thin films with and without aluminum (Al) were prepared on soda-lime-silica glass by spin coating of aqueous sols. The coating sol was formed through mixing tetraethyl orthosilicate [Si(OC₂H₅)₄]/ethanol solution with aqueous silver nitrate (AgNO₃) and aluminum nitrate nonahydrate [(AlNO₃)₃·9H₂O] solutions. The deposited films were calcined in air at 100, 300 and 500 °C for 2 h and characterized using x-ray diffraction, UV-visible and x-ray photoelectron spectroscopy. The effect of Al incorporation and calcination treatment on microstructure and durability of the films, and chemical/physical state of silver in the silica thin film have been reported. The bactericidal activity of the films was also determined against Staphylococcus aureus via disk diffusion assay studies before and after chemical durability tests. The investigations revealed that the optical, bactericidal properties and chemical durability of Ag–SiO₂ films can be improved by Al addition. The Al-modified Ag–SiO₂ thin films do not exhibit any coloring after calcination in the range of 100–500 °C, illustrating that silver is incorporated within the silica gel network in ionic form (Ag⁺). Al incorporation also improved the overall durability and antibacterial endurance of Ag–SiO₂ thin films.     

Site-specific control of N7-metal coordination in DNA by a fluorescent purine derivative

A synthetic strategy that utilizes O6‐protected 8‐bromoguanosine gives broad access to C8‐guanine derivatives with phenyl, pyridine, thiophene, and furan substituents. The resulting 8‐substituted 2′‐deoxyguanosines are push–pull fluorophores that can exhibit environmentally sensitive quantum yields (Φ=0.001–0.72) due to excited‐state proton‐transfer reactions with bulk solvent. Changes in nucleoside fluorescence were used to characterize metal‐binding affinity and specificity of 8‐substituted 2′‐deoxyguanosines. One derivative, 8‐(2‐pyridyl)‐2′‐deoxyguanosine (2PyG), exhibits selective binding of Cu^II, Ni^II, Cd^II, and Zn^II through a bidentate effect provided by the N7 position of guanine and the 2‐pyridyl nitrogen atom. Upon incorporation into DNA, 2‐pyridine‐modified guanine residues selectively bind to Cu^II and Ni^II with equilibrium dissociation constants (K_d) that range from 25 to 850 nM ; the affinities depend on the folded state of the oligonucleotide (duplex>G‐quadruplex) as well as the identity of the metal ion (Cu>Ni?Cd). These binding affinities are approximately 10 to 1 000 times higher than for unmodified metal binding sites in DNA, thereby providing site‐specific control of metal localization in alternatively folded nucleic acids. Temperature‐dependent circular‐dichroism studies reveal metal‐dependent stabilization of duplexes, but destabilization of G‐quadruplex structures upon adding Cu^II to 2PyG‐modified oligonucleotides. These results demonstrate how the addition of a single pyridine group to the C8 position of guanine provides a powerful new tool for studying the effects of N7 metalation on the structure, stability, and electronic properties of nucleic acids.     

Yersiniabactin synthetase: a four-protein assembly line producing the nonribosomal peptide/polyketide hybrid siderophore of Yersinia pestis

Yersiniabactin synthetase comprises four proteins, YbtE, HMWP1, HMWP2, and YbtU, encompassing seventeen functional domains, twelve catalytic and five carrier, to select, activate, and incorporate salicylate, three cysteines, and one malonyl moiety into the iron chelator yersiniabactin (Ybt). In the present study, yersiniabactin has been reconstituted in vitro from the 4 protein assembly line by the use of eight biosynthetic precursors. The rate of one turnover, comprising 22 chemical operations performed by the assembly line to release the completed Ybt molecule, was determined at 1.4 min(-1). During the course of Ybt production, the elongating acyl-S-enzyme chain was shown to transfer across a nonribosomal peptide synthetase/polyketide synthase (NRPS/PKS) interprotein interface and then a PKS/NRPS intraprotein interface. This study on the Ybt synthetase assembly line represents the first complete in vitro reconstitution of a nonribosomal peptide/polyketide hybrid system.     

On the Vertices of the d-Dimensional Birkhoff Polytope

Let us denote by Ω _n the Birkhoff polytope of n×n doubly stochastic matrices. As the Birkhoff–von Neumann theorem famously states, the vertex set of Ω _n coincides with the set of all n×n permutation matrices. Here we consider a higher-dimensional analog of this basic fact. Let $\varOmega^{(2)}_{n}$ be the polytope which consists of all tristochastic arrays of order n. These are n×n×n arrays with nonnegative entries in which every line sums to 1. What can be said about $\varOmega ^{(2)}_{n}$ ’s vertex set? It is well known that an order-n Latin square may be viewed as a tristochastic array where every line contains n?1 zeros and a single 1 entry. Indeed, every Latin square of order n is a vertex of $\varOmega^{(2)}_{n}$ , but as we show, such vertices constitute only a vanishingly small subset of $\varOmega^{(2)}_{n}$ ’s vertex set. More concretely, we show that the number of vertices of $\varOmega ^{(2)}_{n}$ is at least $(L_{n})^{\frac{3}{2}-o(1)}$ , where L _n is the number of order-n Latin squares. We also briefly consider similar problems concerning the polytope of n×n×n arrays where the entries in every coordinate hyperplane sum to 1, improving a result from Kravtsov (Cybern. Syst. Anal., 43(1):25–33, 2007). Several open questions are presented as well.     

An upper bound on the number of high-dimensional permutations

What is the higher-dimensional analog of a permutation? If we think of a permutation as given by a permutation matrix, then the following definition suggests itself: A d-dimensional permutation of order n is an n×n×...×n=[n] ^d+1 array of zeros and ones in which every line contains a unique 1 entry. A line here is a set of entries of the form {(x ₁,...,x _i?1,y,x _i+1,...,x _d+1)|n≥y≥1} for some index d+1≥i≥1 and some choice of x _j ∈ [n] for all j ≠ i. It is easy to observe that a one-dimensional permutation is simply a permutation matrix and that a two-dimensional permutation is synonymous with an order-n Latin square. We seek an estimate for the number of d-dimensional permutations. Our main result is the following upper bound on their number $$\left( {(1 + o(1))\frac{n} {{e^d }}} \right)^{n^d } .$$ We tend to believe that this is actually the correct number, but the problem of proving the complementary lower bound remains open. Our main tool is an adaptation of Brégman’s [1] proof of the Minc conjecture on permanents. More concretely, our approach is very close in spirit to Schrijver’s [11] and Radhakrishnan’s [10] proofs of Brégman’s theorem.     

Benzylic coupling constants in toluene derivatives by J doubling in the frequency domain and DFT calculations

The recently modified J doubling in the frequency domain method (MJDFDM) allowed the determination of ^{4, 5, 6}J in toluene, a series of 4‐mono‐ and 3,5‐disubstituted toluene derivatives, as well as in 4‐picoline. The methyl and aromatic signals were subjected to successive deconvolution processes, which at the end led to singlet signals and afforded the corresponding coupling constant values with a high degree of accuracy. Density functional theory calculation of benzylic coupling constants by addition of the Fermi contact, the spin‐dipole, the diamagnetic spin‐orbit, and the paramagnetic spin‐orbit terms revealed good agreement between predicted and measured values when the B3LYP/aug‐cc‐pVTZ level of theory was used. Evaluation of the substituent effect over the coupling constant was made for all studied compounds and some limitations of the methodology were evidenced. Copyright © 2009 John Wiley & Sons, Ltd.