Theoretical and spectroscopic study of the effect of ring substitution on the adsorption of anisole on platinum

The adsorption of anisole, 3,5-dimethylanisole, and 3,5-bis-(trifluoromethyl)-anisole on Pt(111) was studied theoretically and compared to the adsorption of benzene using relativistically corrected density functional theory. A cluster of 31 platinum atoms was used to simulate the surface. The three anisoles were found to be less strongly adsorbed than the parent molecule benzene, 3,5-bis-(trifluoromethyl)-anisole showing weakest adsorption, with an adsorption energy of only one-third that of benzene. The theoretical study was complemented by in situ ATR-IR spectroscopy of the adsorption of the anisole derivatives on a polycrystalline Pt film. The spectroscopic study indicated that the adsorption strength of the anisoles follows the same order as predicted by the calculations. In addition, catalytic hydrogenation tests showed that the propensity to aromatic ring hydrogenation can also be correlated to the mode and strength of adsorption of the anisoles. The degree of saturation followed the same order as the adsorption strength found by the calculations and indicated by spectroscopy. Although 3,5-dimethyl substitution on anisole resulted in only a partial loss of adsorption energy and reactivity toward ring hydrogenation as compared to anisole, the substitution by CF(3) groups led to a large loss of adsorption energy and complete loss of reactivity toward aromatic ring saturation. Along with the study of the substituent effect on the adsorption of aromatic molecules, the correlation between adsorption and propensity to saturation of aromatic substrates could be corroborated.     

Extended weak bonding interactions in DNA: pi-stacking (base-base), base-backbone, and backbone-backbone interactions

We report on several weak interactions in nucleic acids, which, collectively, can make a nonnegligible contribution to the structure and stability of these molecules. Fragments of DNA were obtained from previously determined accurate experimental geometries and their electron density distributions calculated using density functional theory (DFT). The electron densities were analyzed topologically according to the quantum theory of atoms in molecules (AIM). A web of closed-shell bonding interactions is shown to connect neighboring base pairs in base-pair duplexes and in dinuleotide steps. This bonding underlies the well-known pi-stacking interaction between adjacent nucleic acid bases and is characterized topologically for the first time. Two less widely appreciated modes of weak closed-shell interactions in nucleic acids are also described: (i) interactions between atoms in the bases and atoms belonging to the backbone (base-backbone) and (ii) interactions among atoms within the backbone itself (backbone-backbone). These interactions include hydrogen bonding, dihydrogen bonding, hydrogen-hydrogen bonding, and several other weak closed-shell X-Y interactions (X, Y = O, N, C). While each individual interaction is very weak and typically accompanied by perhaps 0.5-3 kcal/mol, the sum total of these interactions is postulated to play a role in stabilizing the structure of nucleic acids. The Watson-and-Crick hydrogen bonding is also characterized in detail at the experimental geometries as a prelude to the discussion of the modes of interactions listed in the title.     

Mechanism for the elimination of aromatic molecules from polyenes in tandem mass spectrometry

We report here a general mechanism for the elimination of aromatic molecules from polyene containing natural products of several compound classes in tandem mass spectrometry.     

Enhancement of antibacterial efficiency at silver electrodeposited on coconut shell activated carbon by modulating pulse frequency

The natural antibacterial activity of silver represents an alternative to deal with the ever increasing pathogenic breakouts related with contaminated water resources. In this study, silver was deposited on the surface of activated carbon (AC) particles via pulsed electrodeposition (PED) employing an electrochemical reactor operating at fixed and fluidized bed regimes. Silver-coated activated carbon (Ag/AC) particles were prepared at different current pulse frequency values. Antimicrobial properties of the produced material were tested against two well-known foodborne pathogens, i.e., Escherichia coli O157:H7 and Salmonella typhimurium. The results demonstrate a strong influence of the applied current pulse frequency on both the bactericidal efficiency and the specific surface of silver deposited on the activated carbon. Antibacterial results demonstrate up to eight orders of magnitude decrease in the CFU cm^?3 (colony-forming units per cm³) against both microorganisms in just 20 min contact time. Additional chronoamperometry transient data were fitted to the Scharifker-Hills nucleation model for the electrodeposition of silver at a rotating disk electrode, revealing an instantaneous nucleation growth processes. The Ag/AC particles were characterized by field emission scanning electron microscopy (FE-SEM), specific surface area (S_BET), Raman spectroscopy, energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD), demonstrating the existence of crystalline phase formation of a preferential (200) plane growth with silver and silver oxide being present.

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Bond length and the electron density at the bond critical point: X--X, Z--Z, and C--Z bonds (X = Li-F, Z = Na-Cl)

The aim was to investigate the relationship between the bond length and the electron density at the bond critical point in homonuclear X--X and Z--Z and heteronuclear C--Z bonds (X = Li-F, Z = Na-Cl). The d,rho(c) pairs were obtained from 472 target bonds in DFT-optimized (B3LYP/6-311+G(d,p)) small molecular species. These species were selected arbitrarily but with a view to maximize the range widths WR for each atom combination. It was found that (i) with one clear exception, the d(A - A) means (A = X or Z) correlate linearly with the bond lengths d(A(2)) of the respective diatomic molecules; (ii) the d(A - A) means correlate parabolically with n, the formal number of valence electrons in the atoms of the bond; and (iii) with increasing sample size N the ratio WR(rho(c))/WR(d) appears to converge toward a representation f [WR(rho(c))/WR(d)](N-->infinity) characteristic of A. Detailed analysis of the d,rho(c) relationship has shown that by and large simple power regression accounts best for the DFT data. The regression coefficients of d = arho(c) (-b) and rho(c) = alphad(-beta) (b, beta > 0) vary with n in a seemingly irregular manner but one that is consistent with simple chemical notions. The d(A(2)) can be approximated in terms of multilinear MO electron occupancies.     

Negative ion 'chip-based' nanospray tandem mass spectrometry for the analysis of flavonoids in glandular trichomes of Lychnophora ericoides Mart. (Asteraceae)

This paper reports a method for the analysis of secondary metabolites stored in glandular trichomes, employing negative ion 'chip-based' nanospray tandem mass spectrometry. The analyses of glandular trichomes from Lychnophora ericoides, a plant endemic to the Brazilian 'cerrado' and used in traditional medicine as an anti-inflammatory and analgesic agent, led to the identification of five flavonoids (chrysin, pinocembrin, pinostrobin, pinobanksin and 3-O-acetylpinobanksin) by direct infusion of the extracts of glandular trichomes into the nanospray ionisation source. All the flavonoids have no oxidation at ring B, which resulted in a modification of the fragmentation pathways compared with that of the oxidised 3,4-dihydroflavonoids already described in the literature. The absence of the anti-inflammatory and antioxidant di-C-glucosylflavone vicenin-2, or any other flavonoid glycosides, in the glandular trichomes was also demonstrated. The use of the 'chip-based' nanospray QqTOF apparatus is a new fast and useful tool for the identification of secondary metabolites stored in the glandular trichomes, which can be useful for chemotaxonomic studies based on metabolites from glandular trichomes.     

Comparative analysis of the gas-phase reactions of cylindrospermopsin and the difference in the alkali metal cation mobility

Cylindrospermopsin (CYN) belongs to a group of toxins produced by several strains of freshwater cyanobacteria. It is a compact zwitterionic molecule composed of a uracil section and a tricyclic guanidinium portion with a primarily hepatotoxic effect. Using low multi-stage and high-resolution mass spectrometry, the gas-phase reactions of this toxin have been investigated. Our data show that collision-induced dissociation (CID) spectra of CYN are dominated by neutral losses, and three major initial fragmentation pathways are clearly distinguishable. Interestingly, comparative analysis of protonated and cationizated molecules showed a significant difference in the balance of the SO3 and terminal ring elimination. These data indicate that the differential ion mobility of H+, Li+, Na+ and K+ leads to different fragmentation pathways, giving rise to mass spectra with different profiles.     

Guest-induced modification of a magnetically active ultramicroporous, gismondine-like, copper(II) coordination network

A novel ultramicroporous coordination polymer, namely [Cu(F-pymo)2(H2O)1.25]n (1, F-pymo = 5-fluoropyrimidin-2-olate), has been prepared and structurally characterized. 1 displays a zeolitic gismondine (GIS) topology, with ca. 2.9 A wide helical channels which, in the thermally activated counterpart (1'), account for a 13% void volume and are responsible for the observed selective solid-gas adsorption properties toward H2, N2, and CO2. At 77 K 1' behaves as a molecular sieve, selectively adsorbing H2 over N2, possibly due to size-exclusion reasons. At variance, although CO2 molecules are slightly larger than the pore size, they are readily incorporated by 1' at temperatures as high as 433 K. Variable-temperature X-ray powder diffraction (TXRPD) studies, in the temperature range 303-473 K, show that dehydration is reversible and has almost negligible effects on the network. At variance, the uptake of CO2 occurs through a transient phase and channels expansion. While the gas storage capacity of 1' is not very high-H2, 0.56 wt % and 0.010 kg H2/L at 90 K and 900 Torr, and CO2, 7.6 wt % at 273 K and 900 Torr-the guest molecules achieve very high densities, comparable to that of the liquid for H2 (0.023 vs 0.021 molecules A-3) and to that of the solid for CO2 (0.014 vs 0.022 molecules A-3). In addition, we have also studied the effect of the perturbation exerted by the guest molecules on its magnetic properties. The results show that while dehydration of 1 has negligible effect on its spin-canted antiferromagnetic behavior, CO2 incorporation in the pores is responsible for an increment of the transition temperature at which the weak ferromagnetic ordering takes place from 22 to 29 K.     

Crystal structure and cytotoxic activities of a bis(pyrrolyl-imine) gold(III) complex

A gold(III) complex with N,N′-ethylenebis(pyrrol-2-yl-methyleneamine) (H₂pyren) was synthesized and characterized by physicochemical and spectroscopic measurements. Density functional theory (DFT) studies and cytotoxic assays were performed. Infrared, mass spectrometry, and ¹H, ¹³C, and {¹⁵N,¹H} nuclear magnetic resonance analyses indicate that pyren is deprotonated and gold(III) is four coordinate in a square planar environment, with the pyrrole and imine nitrogens as donors. The structure was confirmed by powder X-ray diffraction and confirmed as a minimum of the potential energy surface by DFT. Cytotoxic activity of [Au(pyren)]⁺ was active against three tumorigenic cell lines with IC₅₀ values of 35 μM. Interaction studies with CT-DNA by fluorescence and competition with ethidium bromide (EB) showed a quenching of the emission band of DNA with a Stern–Volmer quenching constant value of (3.0 ± 0.1) × 10⁴ M^?1 and a decrease in fluorescence quenching of EB-DNA system, respectively, confirming that DNA is a possible target for the complex via an intercalative binding, which was confirmed by DNA conformational changes observed with circular dichroism spectroscopy.     

Determination of human erythropoietin by on-line immunoaffinity capillary electrophoresis: a preliminary report

Several CE methodologies have been described for the analysis of rHuEPO in concentrated solutions, but the inherently limited concentration sensitivity of CE precludes the detection of EPO at the levels found in biological fluids. In this work, we have investigated an on-line immunoaffinity solid-phase extraction capillary electrophoresis (IA-CE) methodology for the selective preconcentration of EPO in diluted solutions. The preliminary results obtained using a custom-made immunoaffinity sorbent prepared from an anti-human EPO polyclonal antibody and glutaraldehyde–glass beads show the potential of this novel approach. The summarized findings are discussed in detail as a starting point for our ongoing investigations.