Acid In Homogeneous Solution Research Articles

Electrocatalytic Production of Hydrogen Gas by a Cobalt Formamidinate Complex

ABSTRACT. A molecular cobalt complex, Co(DippF)2 (where DippF is the anion of N,N’-bis[2,6-diisopropylphenyl]-formamidine), (1),is able to electrochemically produce hydrogen gas from the reduction of organic acids in homogeneous solutions. Compound 1 has a distorted square planar structure as evidenced through X-ray crystallography studies, and an effective magnetic moment of 4.13, obtained by the Evans method, that corresponds to three unpaired electrons. Compound 1 shows an irreversible cathodic peak at –1.59 V vs Fc/Fc+ which is assigned to the reduction of CoII to CoI. In the presence of organic acids the onset of catalytic current is observed at –1.2 V, –1.45 V and –1.89 V vs. Fc/Fc+ with p-toluenesulfonic acid, benzoic acid and phenol as the proton source, respectively, in MeCN as the solvent. Detection of hydrogen gas was obtained by GC-MS with Faradaic efficiencies ranging from 85% to 100%. Kinetic studies using foot-of-the-wave analysis (FOWA) reveal a linear dependence of the observed rate constant, kobs, against acid concentration in the range of 0.065 to 10.02 s-1.

Simulating the XOR Gates Based on the Induced Hairpin Formation

The hairpin structure of DNA molecules have been widely employed for a variety of biosensors and nanoscale molecular assembly applications. For example, the commonly known molecular beacons can report the presence of specific nucleic acids in homogeneous solutions with high accuracy. Recently, Smith et al. proposed to induce hairpin formation through the sequence-specific binding of a small-molecule ligand G-G mismatch. Not only did this make the control of the hairpin formation flexible, but more important the induced hairpin still keeps a high sensitivity to specific hybridization. In this paper, we simulate the working process of logical XOR gates based on induced hairpin formation. Keywords: DNA computing, Induced hairpin formation, Logical XOR gates

Quantitating the Dynamics of NBD Hexanoic Acid in Homogeneous Solution and in Solutions Containing Unilamellar Vesicles

We report here on the motional and fluorescence lifetime dynamics of the chromophore NBDHA (6-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)hexanoic acid) in neat solvents and in aqueous solutions containing unilamellar vesicles of varying composition. We measure the transient response of this chromophore by time-correlated single-photon counting, using one- and two-photon excitation to resolve the Cartesian components of the rotational diffusion constant, D. Our experimental data for NBDHA in selected solvents of varying viscosity demonstrate that one- and two-photon excitation probe different components of the rotational diffusion constant and that this chromophore reorients as a prolate rotor with an aspect ratio of approximately 2. For NBDHA in aqueous solutions containing unilamellar vesicles of varying composition, we recover the same reorientation behavior regardless of vesicle composition. Fluorescence lifetime and steady-state fluorescence data show the chromophore to reside in a polar environment that is different from neat water. We understand these data in the context of the chromophore residing in close proximity to the unilamellar vesicle polar headgroups in all cases.

A new DNA computing model for the NAND gate based on induced hairpin formation

Hairpin structure of DNA molecules has been widely employed in a variety of biosensors and in nanoscale molecular assembly applications. For example, the well known molecular beacons can report the presence of specific nucleic acids in homogeneous solutions with high accuracy. Recently, Smith et al. proposed the induction of hairpin formation through sequence-specific binding of a small-molecule ligand to a G–G mismatch. Not only did this method offer great flexibility in controlling hairpin formation, more importantly the induced hairpin maintains high degree of sensitivity toward specific hybridization. In this contribution, we present a theoretical model for the logical NAND gate based on induced hairpin formation.

Light-Up Probes: Thiazole Orange-Conjugated Peptide Nucleic Acid for Detection of Target Nucleic Acid in Homogeneous Solution

We have constructed light-up probes for nucleic acid detection. The light-up probe is a peptide nucleic acid (PNA) oligonucleotide to which the asymmetric cyanine dye thiazole orange (TO) is tethered. It combines the excellent hybridization properties of PNA and the large fluorescence enhancement of TO upon binding to DNA. When the PNA hybridizes to target DNA, the dye binds and becomes fluorescent. Free probes have low fluorescence, which may increase almost 50-fold upon hybridization to complementary nucleic acid. This makes the light-up probes particularly suitable for homogeneous hybridization assays, where separation of the bound and free probe is not necessary. We find that the fluorescence enhancement upon hybridization varies among different probes, which is mainly due to variations in free probe fluorescence. For eight probes studied the fluorescence quantum yield at 25°C in the unbound state ranged from 0.0015 to 0.08 and seemed to depend mainly on the PNA sequence. The binding of the light-up probes to target DNA is highly sequence specific and a single mismatch in a 10-mer target sequence was readily identified.

Kinetics and mechanism of the primary steps of degradation of carotenoids by acid in homogeneous solution.

The kinetics of reaction between trifluoroacetic acid as an acid of medium strength and the carotenoids beta-carotene, zeaxanthin, canthaxanthin, and astaxanthin has been examined in detail including the effects of dioxygen, acid concentration, and carotenoid structure. Reaction between acid and carotenoid leads to species absorbing in the red and near-infrared (NIR) spectral regions, intermediates that subsequently disappear. ESR experiments clearly show that these species are not carotenoid radicals, although their NIR absorption is similar to the absorption of carotenoid radical cations. Under most reaction conditions, the disappearance of carotenoids follows pseudo-zero-order kinetics, whereas the reaction order is >1 with respect to acid, and the long-lived (hours) intermediates are suggested to be mono- (700 nm) and diprotonated carotenoid ( approximately 950 nm). Acid induces cis/trans-isomerization via the protonated intermediates, which also decay to nonradical species with shorter conjugated systems-most probably carotenoid esters. Slow protonization of the methine carbon is the primary step in the degradation, but dioxygen increases the rate as a result of formation of a charge-transfer complex with the carotenoids as indicated by a red-shift of the NIR absorption bands. Carotenoids with carbonyl groups (astaxanthin and canthaxanthin) have slower rates of degradation than beta-carotene and zeaxanthin, indicating preferential nondegradative protonation of the carbonyl groups.

Accelerated photo-oxidation of concentrated p-coumaric acid in homogeneous solution. Mechanistic studies, intermediates and precursors formed in the dark

This study shows that p-coumaric acid undergoes accelerated degradation via photo-assisted Fenton reactions. Simulated solar radiation (90 mW/cm 2) promotes the full mineralization of a concentrated solution of p-coumaric acid (420 mg C/l) in less than 15 min when suitable times for the oxidant addition were used in the presence of very low Fe-ion concentration (4 mg Fe-ion/l). The mineralization of p-coumaric acid has been studied as a function of the common variables as pH, concentration of reagents and of the substrate used. The reduction of the total organic compound (TOC) was observed to be a function of the surface-to-volume ( s/ v) ratio on the reaction vessel due to the formation of light induced radicals in the solution surface. Decoloration of these concentrated solutions were observed in <1 min by stopped-flow techniques. The most important reaction intermediates were determined by HPLC, GC–MS and FTIR suggesting a reaction network for the degradation process. The Fenton reagent is shown to form precursor intermediates which are susceptible to degradation under light. Degradation performed via alternate dark and light cycles showed the importance of the dark precursor species. Implications of this observation as an energy saving device during the mineralization process is accounted for in this study.

Thiyl radical-induced cis/trans -isomerization of methyl linoleate in methanol and of linoleic acid residues in liposomes

Purpose: To investigate the role of a thiol-containing biologically active compound in lipid peroxidation of membranes. Materials and methods: Thiyl radicals were generated from 3-(2-mercaptoethyl)quinazoline-2,4(1H,3H)-dione (MECH) using pulse radiolysis and gamma -radiolysis in aqueous and alcoholic solutions saturated with N2O. The products were analysed by 1H NMR and by HPLC. Results: The thiyl radicals abstract bisallylic hydrogens from {cis-9,cis-12}-methyl linoleate, yielding a pentadienyl radical. In the absence of oxygen, a thiyl radical-induced cis/trans-isomerization leads to linoleic-type isomers. These chain-type isomerization reactions can occur with the long living pentadienyl radical, followed by a 'repair' reaction of the attached thiol, and with the thiyl radical adduct with a double bond of the fatty acid residue. Conclusions: The results show that the mechanism of cis/trans isomerization is an integral part of the thiyl radical attack on polyunsaturated fatty acids in homogeneous solutions and in bilayers.

A selective catalytic voltammetric determination of vitamin C in pharmaceutical preparations and complex matrices of fresh fruit juices

A simple, selective and precise voltammetric method for the determination of ascorbic acid in pharmaceutical preparations and fresh fruit juices—complex matrices containing various reducing compounds—is described. The method is based on the electrocatalytic oxidation of ascorbic acid in homogeneous solution using electrogenerated ferriciniumcarboxylic acid as mediator. The pH and mediator concentration affecting the performance of the electrocatalytic oxidation of the analyte were optimized. The method was applied to determine vitamin C in deeply coloured, viscous and turbid fruit juice samples with ascorbic acid contents ranging from 15–45 mg per 100 ml, without further dilution, concentration or other pre-treatment of the samples. The amount of mediator used varied depending on the ascorbic acid concentration in the samples. The method was also used for pharmaceutical analysis using a calibration graph. For fruit juice samples the standard addition technique was adopted to prevent the matrix affecting the accuracy of the determination. The relative standard deviation for the analysis of vitamin C in fruit juices ranged from 1.5–5%. The reliability of the method was established by parallel determination against the official methods.

Molecular beacons: probes that fluoresce upon hybridization.

We have developed novel nucleic acid probes that recognize and report the presence of specific nucleic acids in homogeneous solutions. These probes undergo a spontaneous fluorogenic conformational change when they hybridize to their targets. Only perfectly complementary targets elicit this response, as hybridization does not occur when the target contains a mismatched nucleotide or a deletion. The probes are particularly suited for monitoring the synthesis of specific nucleic acids in real time. When used in nucleic acid amplification assays, gene detection is homogeneous and sensitive, and can be carried out in a sealed tube. When introduced into living cells, these probes should enable the origin, movement, and fate of specific mRNAs to be traced.

Antioxidant Activity of the Fused Heterocyclic Compounds, 9H-Xanthene-2, 7-diols

9H-Xanthene-2, 7-diols were shown to function well as hydrogen donor and suppress the oxidation of linoleic acid in homogeneous solution. In contrast to most other phenolic antioxidants these 9H-xanthene-2, 7-diols react with 26 peroxyl radicals per molecule and their antioxidant activities were close to that of α-tocopherol. The relationship of the antioxidant activity and electronic effects on stabilization of derived phenoxyl radicals were discussed.