One electron oxidation induced dimerization of 5-hydroxytryptophol: role of 5-hydroxy substitution

Reaction of one-electron oxidant (Br(2)(*-)) with tryptophol (TP) and 5-hydroxytryptophol (HTP) have been studied in aqueous solution in the pH range from 3 to 10, employing nanosecond pulse radiolysis technique and the transients detected by kinetic spectrophotometry. One-electron oxidation of TP has produced an indolyl radical that absorbs in the 300-600 nm region with radical pK(a) = 4.9 +/- 0.2, while the reaction with HTP has produced an indoloxyl radical with lambda(max) at 420 nm and radical pK(a) < 3. Hydroxyl radicals ((*)OH) react with these two compounds producing (*)OH radical adducts that undergo water elimination to give one-electron-oxidized indolyl and indoloxyl radical species, respectively. The indoloxyl radicals react with the parent compound to form dimer radicals with an average association constant of (6.7 +/- 0.4) x 10(4) M(-1). No such dimerization is observed with indolyl radical, indicating that the presence of the 5-hydroxy group markedly alters its ability to form a dimer. A possible explanation behind such a difference in reactivity has been supported with ab initio quantum chemical calculations.     

Designs of multi-bit sigma delta modulator

This paper presents new design variants of third order multi-bit sigma delta modulator (SDM): low distortion SDM and cascaded SDM. The proposed modulator based on the conventional SDMsuch L-0MASH(Multi-stAge noise SHaping) and interstage feedback topology. TheMASHSDM is not a single loop system. One of the drawback is that performance is limited by uncancelled noise from the first modulator and interstage feedback topology only cancels nonlinear errors introducing by multi-bitDACin the final stage, but the rest stage still containsDACnonlinearity errors without any noise shaping which still degrade overall system performance. An improved version of cascaded multi-bit SDM is proposed to overcome these problems mentioned above. In addition a third order low distortion SDM is also proposed. Simulation results verify the superiority of the both proposed modulator.     

Different approaches in thin-layer chromatography for enantioresolution of acebutolol using colistin sulfate as chiral selector

JPC – Journal of Planar Chromatography – Modern TLC - This work reports the enantioresolution of (RS)-acebutolol (ACB) by a thin-layer chromatographic method involving colistin sulfate...     

Nuclear matrix elements in the 7/2−–9/2+ beta decay of175Yb

The angular correlation of the \({7 \mathord{\left/ {\vphantom {7 2}} \right. \kern-0em} 2}^ - \xrightarrow[{353keV}]{\beta }{9 \mathord{\left/ {\vphantom {9 2}} \right. \kern-0em} 2}^ + \xrightarrow[{114keV}]{\gamma }{7 \mathord{\left/ {\vphantom {7 2}} \right. \kern-0em} 2}^ + \) is measured atE _β =300 keV and found to beA ₂(β,γ)=0.108 ± 0.028. The result is combined with the data on longitudinal polarisation nuclear orientation to obtain the nuclear matrix elements which would give a simultaneous fit to all experimental data. The matrix elements thus obtained have predicted theβ spectrum shape correction factor. The sizes of the matrix elements indicate a cancellation effect in vector matrix elements which explains deviation from ξ-approximation. From the ratio of higher order matrix elements,λ, the deviation from theCVC ratio due to Fujita is found and thus the importance of off-diagonal elements inH _c is noted. The experimental matrix elements are compared with the model-predicted ones.     

Redox reactions of 2-hydroxy-3-methoxybenzaldehyde (o-vanillin) in aqueous solution

Pulse radiolysis technique has been employed to study the reactions of oxidizing (^√OH, N₃^√) and reducing radicals (e⁻_aq, CO₂^√−, acetone ketyl radical) with 2-hydroxy-3-methoxybenzaldehyde (o-vanillin) at different pH. Hydroxyl radicals react mostly by addition reaction forming radical adducts (λ_max=420 nm) and the oxidation is only a minor process even in the alkaline region. The reaction with azide radicals produced phenoxyl radicals (λ_max=340 nm), which are formed on fast deprotonation of solute radical cation. Using PMZ^√+/PMZ and ABTS^√−/ABTS²⁻ as the reference couple, different methods are employed to determine the one-electron reduction potential of o-vanillin and the average value is estimated to be 1.076±0.004 V vs. NHE at pH 6. The phenoxyl radicals of o-vanillin were able to oxidize ABTS²⁻ quantitatively. The e_aq⁻ is observed to react with o-vanillin with rate constant value of 2×10¹⁰ dm³ mol⁻¹ s⁻¹. CO₂^√− and acetone ketyl radical are also observed to react with o-vanillin by electron transfer mechanism and showed the formation of transient absorption bands with λ_max at 350 and 390 nm at pH 4.5 and 9.7, respectively. The pK_a of the one-electron reduced species was determined to be 8.1. The results indicate that the aldehydic group is the most preferred site for electron addition.     

Formation of Dimer Radical Cations of Selenourea on Oxidation: Pulse Radiolysis Studies

Abstract

Reactions of oxidizing radicals like hydroxyl (·OH) radical, specific electron transfer agents like N ₃ ·, and I ₂ ^?. radicals were studied with selenourea (SeU) and compared with thiourea (ThU) using pulse radiolysis technique in microsecond time scales. Both the compounds efficiently react with ·OH radicals, however, SeU undergoes easier oxidation by secondary oxidants as compared to ThU. The results were supported by cyclic voltammetry studies. The radical cations of both SeU and ThU formed on oxidation undergo dimerization with the parent molecule to form two-centered three-electron-hemi bonded radical cations absorbing at 410 and 400 nm respectively with the stabilization energies of 21.1 and 20.5 kcal/mol for SeU and ThU, respectively. Preliminary studies indicated that at low concentration of SeU, the dimerization is prevented and the oxidation reaction produced metallic Se nanoparticles.     

Emission characteristics of laser dyes C1 and Rh6G in cross-linked polyvinyl alcohol solutions

The lifetimes and the relative quantum yields for fluorescence of two laser dyes Coumarin 1 and Rhodamine 6G have been determined in cross-linked polyvinyl alcohol matrix. The cross-linking has been achieved using gamma radiation. The relative fluorescence quantum yields of the dyes increased with increasing cross-linking of the polymer, but the fluorescence lifetimes remained unchanged within experimental error. The results have been attributed to a reduction in the loss of the excitation energy via the internal conversion of the excited singlet state due to increase in the viscosity of the medium brought about by the increased cross-linking of the polymer.