Fischer carbene mediated covalent grafting of a peptide nucleic acid on gold surfaces and IR optical detection of DNA hybridization with a transition metalcarbonyl label

Amine-reactive surfaces comprising N-hydroxysuccinimide ester groups as well as much more unusual Fischer alkoxymetallocarbene groups were generated on gold-coated surfaces via self-assembled monolayers of carboxy- and azido-terminated thiolates, respectively. These functions were further used to immobilize homothymine peptide nucleic acid (PNA) decamer in a covalent fashion involving the primary amine located at its N-terminus. These stepwise processes were monitored by polarization modulation reflection - absorption infrared spectroscopy (PM-RAIRS) that gave useful information on the molecular composition of the organic layers. PNA grafting and hybridization with complementary DNA strand were successfully transduced by quartz crystal microbalance (QCM) measurements. Unfortunately, attempts to transduce the hybridization optically by IR in a label-free fashion were inconclusive. Therefore we undertook to introduce an IR reporter group, namely a transition metalcarbonyl (TMC) entity at the 5\u2032 terminus of complementary DNA. Evidence for the formation of PNA-DNA heteroduplex was brought by the presence of \u3bd(C 61O) bands in the 2000 cm-1 region of the IR spectrum of the gold surface owing to the metalcarbonyl label

Pseudo-single crystal electrochemistry on polycrystalline electrodes : visualizing activity at grains and grain boundaries on platinum for the Fe2+/Fe3+ redox reaction

The influence of electrode surface structure on electrochemical reaction rates and mechanisms is a major theme in electrochemical research, especially as electrodes with inherent structural heterogeneities are used ubiquitously. Yet, probing local electrochemistry and surface structure at complex surfaces is challenging. In this paper, high spatial resolution scanning electrochemical cell microscopy (SECCM) complemented with electron backscatter diffraction (EBSD) is demonstrated as a means of performing ‘pseudo-single-crystal’ electrochemical measurements at individual grains of a polycrystalline platinum electrode, while also allowing grain boundaries to be probed. Using the Fe2+/3+ couple as an illustrative case, a strong correlation is found between local surface structure and electrochemical activity. Variations in electrochemical activity for individual high index grains, visualized in a weakly adsorbing perchlorate medium, show that there is higher activity on grains with a significant (101) orientation contribution, compared to those with (001) and (111) contribution, consistent with findings on single-crystal electrodes. Interestingly, for Fe2+ oxidation in a sulfate medium a different pattern of activity emerges. Here, SECCM reveals only minor variations in activity between individual grains, again consistent with single-crystal studies, with a greatly enhanced activity at grain boundaries. This suggests that these sites may contribute significantly to the overall electrochemical behavior measured on the macroscale

Double-layer effects and distance dependence of electron transfer in reduction of nitro aromatic radical anions

The first example of the effect of an electric double layer on the reduction of electrochemically generated radical species is reported. The anion radical of methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate (pesticide bifenox) is electrochemically reduced in acetonitrile to a phenylhydroxylamine derivative in a process involving three electrons. This heterogeneous reaction is strongly influenced by the concentration and nature of the cation of the indifferent electrolyte. Depending on the type of tetraalkylammonium cation, the redox potential changes by 0.45 V. The kinetic parameters were obtained for five tetraalkylammonium hexafluorophosphate salts. The Frumkin correction, which assumes that the outer Helmholtz plane coincides with the reaction site, was applied to kinetic data of the radical anion reduction. The correction of the apparent rate accounted for the observed effect only in the case of tetramethylammonium salt. The presence of higher tetraalkylammonium homologues causes deviations from the predicted dependence of the electron-transfer rate on the æ2 potential of the outer Helmholtz plane. Hence, the nature of the cation of the electrolyte exerts a further effect extending beyond the electrostatic repulsion only. The corrected rate of electron transfer decreases exponentially with increasing size of the alkyl chain of the indifferent electrolyte cation in the order methyl > ethyl > propyl > butyl > hexyl. The rate decay is characterized by an exponent â ) 0.83. This confirms that the reaction plane for the reduction of the bifenox radical anion is different for each electrolyte. Due to this fact the Frumkin correction cannot fully account for the observed dependence of the heterogeneous rate on the solution composition. The observed effect is not specific to the bifenox radical. A similar influence of the concentration and nature of the cation of the indifferent electrolyte was observed for other nitro compounds, namely, nitrobenzene, nitrobenzoate, and nitrofen

Decomposition reactions of bifenox anion radical involving intramolecular electron transfer

The nitropesticide bifenox (methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate) is reduced in aprotic media in two reduction steps. The first heterogeneous electron transfer involves the acceptance of one electron leading to the formation of a relatively stable nitro anion radical that was characterized by EPR spectroscopy. The electrochemically generated radical anion eventually decomposes to two stable products. Intramolecular electron transfer coupled with chemical steps leads in the first case to the cleavage of the carboxymethyl group and the formation of the nitropesticide nitrofen ((2,4-dichlorophenyl)(4-nitrophenyl) ether) and in the second case to a trans-esterification process, i.e., an exchange of the methyl group for an alkyl of the supporting electrolyte. Both these final products retain pesticidal activity, which explains the high efficiency of bifenox as a pesticide. The electrochemical characteristics show that the radical anion undergoes a further two-electron reduction at more negative potentials. The second reduction step is strongly influenced by the nature and concentration of the tetraalkylammonium cation of the supporting electrolyte. The influence of the electrolyte concentration was accounted for by the effect of the electrode double-layer structure on the electron transfer kinetics (Frumkin effect) due to repulsion of the radical anion from a negatively charged electrode|electrolyte interface

Models of pesticides inside cavities of molecular dimensions. A role of the guest inclusion in the dechlorination process

The reduction mechanism of the pesticide vinclozoline (3-(3,5-dichlorophenyl)-5-methyl-5-vinyl-1,3-oxazolidine-2,4-dione) was studied in nonaqueous solvents in the confined environment of a cyclodextrin (CD) cavity. The effect of the cavity dimensions on the mechanism of the redox process was evaluated using glucose as a reference and using three cyclodextrin molecules of different cavity sizes, namely, RCD, âCD, and çCD. In the absence of CD the main reduction product of vinclozoline in the first reduction step is dichloroaniline. An addition of glucose leads to a quantitative change of mechanism with 10 products in total. Addition of CD, however, leads exclusively to dechlorination of the phenyl ring. The degree of dechlorination depends strongly on the choice of cyclodextrin molecule. The importance of the complex formation equilibria in the change of the mechanism is supported by a series of semiempirical AM1 quantum-mechanical calculations. Very good correlation (correlation coefficient 0.995) was obtained between the complex stabilization energy of the inclusion complex and the degree of pesticide dechlorination. Additionally, we were able to show that the complexes are stabilized by the formation of intermolecular hydrogen bonds between the host and guest species. CD molecules do not simply act as proton donors in a nonaqueous environment, but also protect parts of the molecule included within the cavity and steer the degradation process toward fewer products

Neutron reflection from triethylene glycol monododecyl ether adsorbed at the air-liquid interface: the variation of the hydrocarbon chain distribution with surface

The structure of layers of triethylene glycol monododecyl ether (C12E3) adsorbed at the air/water interface at four concentrations from the critical micelle concentration (cmc = 5.5 × 10-5 M) down to 1 × 10-6 M have been determined using neutron specular reflection together with isotopic substitution. The distribution of any part of the layer is described in terms of the full width of the distribution at half height, σ; i.e., σ is a measure of the thickness of that part of the layer. The width, σ, of the alkyl chain part of the layer varied from 14 ± 1 A at an area per molecule (A) of 36 A2 down to 10 ± 1.5 A at an A of 89 A2. The corresponding widths of the ethylene glycol region varied from 13 ± 1 to 5 ± 2 A, and the separation of the two chain regions changed from 8 ± 0.5 to 4.5 ± 1 A. The separation of the alkyl chains from the midpoint of the water distribution was found to be 2 A greater than from the ethylene glycol chains at the cmc but exactly the same at 1 × 10-6 M. Isotopic labeling of the two halves of the alkyl chain was used to establish that the alkyl chain is substantially tilted away from the surface normal at all concentrations and that at least one gauche conformation occurs on average in the alkyl chain at the highest surface concentration. The shape of the overall chain distribution is better described by a Gaussian distribution than by a uniform layer. The static displacement of the surfactant molecules in the direction normal to the surface is shown to be strongly disordered at the cmc, the mean displacement being about 9 A, which is of the order of half the projected length of the molecule in the vertical direction. The ethylene glycol chain is oriented close to the normal direction when A = 36 A2 but seems to lie nearly flat in the surface plane when A = 89 A2

Electrochemical evidence of host-guest interaction. Change of redox mechanism of fungicides iprodione and procymidone in the nano-cavity of cyclodextrins

The reduction mechanism of two pesticides, iprodione w3-(3,5-dichlorophenyl)-N-(1-methylethyl)-2,4-dioxo-1-imidazolidinecarboxamidex and procymidone w3-(3,5-dichlorophenyl)-1,5-dimethyl-3-azabicyclow3.1.0xhexane-2,4- dionex, was studied in the absence and presence of b-cyclodextrin in a dimethylsulfoxide solvent. Distribution of the reduction products changed in the presence of b-cyclodextrin. The complex formation led to protection of the imidazolidine ring of iprodione against cleavage, with a subsequent decrease in aniline-type products. In the case of procymidone, the mechanism exclusively led to the formation of monochloro derivatives of the original pesticide. Host–guest-type interactions of these pesticides with cyclodextrin nano-cavities was unambiguously demonstrated by voltammetric measurements that were more sensitive than conventional spectroscopic methods, such as UV-VIS and NMR

The Electrochemistry of Halogenated Benzonitriles

Autoprotonation in reduction mechanism of 3,5-dihalogeno-4-hydroxy-benzonitriles was studied in dimethylsulfoxide by electrochemical methods. The overall one electron reduction process was found. Aryl radical formed by the cleavage of halogenide undergoes further electron transfer in EC type reaction. The anion is protonated by parent molecule resulting in total consumption of two electrons per two starting molecules. Addition of stronger proton donor increases the height of reduction wave up to two electrons per molecule. The addition of potassium hydroxide causes the decrease of the reduction wave. The reduction products were identified by GC/MS analysis in the absence and presence of strong proton donor

THE INFLUENCE OF THE HOST-GUEST INTERACTION ON THE OXIDATION OF NATURAL FLAVONOID DYES

The influence of the molecular cavity protection on degradation processes of bioorganic compounds quercetin and luteolin used as the original dyes in old tapestries was studied. The degradation processes were studied by electrochemical methods in aqueous media. The products of the exhaustive electrolysis were separated and identified by GC-MS analysis. Cyclic voltammetry characteristics indicate that the inclusion complex is formed. The inclusion affects the redox potentials of both oxidation waves related to the different dissociation forms of the flavonoid molecule. It was shown that decomposition products formed by the oxidation of quercetin are stabilized in the cavity of β-cyclodextrin, including the main oxidation product 2(3′,4′-dihydroxybenzoyl)-2,4,6-trihydroxybenzofuran-3(2H)-one. The formation of the 1:1 inclusion complex of luteolin with β-cyclodextrin is supported by the enhancement of fluorescence intensity. In the case of quercetin, a decrease of fluorescence intensity occurs when 1:1 inclusion complex with β-cyclodextrin is formed