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

Transient species of esculetin produced in pulse radiolysis: experimental and quantum chemical investigations

Radiation chemical studies of esculetin (E), a dihydroxycoumarin derivative, were performed using a pulse radiolysis technique employing kinetic spectrometer and quantum chemical calculations. Both the oxidizing radicals, hydroxyl (˙OH) and azide (N₃˙) radicals, and the reducing radical hydrated electron (e_aq^-) and hydrogen atom (H˙) reactions of E were used for the present study. The reaction of ˙OH and N₃˙ radicals with E produced transients that absorbed at 410 nm; additionally, another broad band at 510 nm was observed for the ˙OH radical reaction. The reaction of ˙OH radicals with E formed the phenoxyl radical and ˙OH-adducts. It was revealed that 32% of the ˙OH radical reaction products of E were oxidizing in nature and 47% were reducing in nature. The carbonyl group of E was reduced by e_aq^- and subsequently converted to a neutral radical adduct upon protonation. Similarly, the H˙ atom reaction with E yielded a neutral adduct along with H˙ atom addition products. The transient product absorbed at 380 nm when E was reduced by e_aq^- and the H˙ atom; additionally, the H˙ atom addition product absorbed at 500 nm. In the case of E, the oxidizing radicals were reactive towards the aromatic ring and the phenolic OH group, whereas the reducing radicals were reactive towards the carbonyl group of E. Quantum chemical calculations using DFT and TD-DFT methods have supported the experimental observation. There was good agreement between the experimental and theoretical data on a number of occasions. Based on the energetics of the transients, it was suggested that the addition products were exothermic in nature. In the addition reaction with the ˙OH radical, there was a slight increase in the C-C bond length adjacent to the addition site compared to the remaining bonds. During the reduction process through the carbonyl group, the [double bond splayed left]C[double bond, length as m-dash]O bond length was increased from 1.221 Å to 1.358 Å. There was an excellent correlation between the calculated and experimentally observed absorption maximum for the oxidized product of E. Overall, these redox studies may find application in developing hydroxycoumarin derivatives as an antioxidant or as an electron transporting agent in biochemical processes. In addition, this information will be helpful for understanding the mechanism of removing pollutant dyes by advanced oxidation processes.

Formation of two centre three electron bond by hydroxyl radical induced reaction of thiocoumarin: evidence from experimental and theoretical studies

Radiation chemical studies of thioesculetin (1), a thioketone derivative of coumarin, were performed by both pulse radiolysis technique and DFT calculations. Hydroxyl (^•OH) radical reaction with 1 resulted transients absorbing at 320, 360 and 500 nm. To identify the nature of the transients, the reaction was studied with specific one-electron oxidant (N₃^•) radical, where 360 nm band was absent. The transient absorption at 500 nm was concentration-dependent. The overall impression for ^•OH radical reaction was that the transient absorbing at 320, 360 and 500 nm was due to sulphur centred monomer radical, hydroxysulfuranyl and dimer radical of 1 respectively. The equilibrium constant between the monomer to dimer radical was 3.75 × 10⁴ M^-1. From the transients' redox nature, it was observed that 57 and 24% of ^•OH radical yielded to oxidising and reducing products respectively. Further, the product analysis by HPLC suggested that the dimer radical disproportionate to esculetin and thioesculetin. DFT energy calculation for all the possible transients revealed that dimer radical has the lowest energy. The HOMO of 1 and its monomer radical suggested that the electron density was localised on the sulphur atom. The bond length between the two sulphur atoms in dimer radical was 2.88 Å which was less than the van der Waals distance. Bond order between the two sulphur atoms was 0.55, suggesting that the bond was two centre three electron (2c-3e). From TD-DFT calculation, the electronic transition of dimer radical was at 479 nm which was in close agreement with the experimental value. The nature of the electronic transition was σ → σ* from a 2c - 3e bond.

Crystal structure of 4-[(3-meth-oxy-2-oxidobenzyl-idene)azaniumyl]-benzoic acid methanol monosolvate

In the crystal of the title compound, C15H13NO4·CH3OH, the Schiff base mol-ecule exists in the zwitterionic form; an intra-molecular N-H⋯O hydrogen bond stabilizes the mol-ecular structure. The benzene rings are nearly co-planar, subtending a dihedral angle of 5.34 (2)°. In the crystal, classical O-H⋯O and weak C-H⋯O hydrogen bonds link the Schiff base mol-ecules and methanol solvent mol-ecules into a three-dimensional supra-molecular architecture. The crystal studied was refined as an inversion twin.

Spectroscopic analyses on interaction of o-Vanillin-D-Phenylalanine, o-Vanillin-L-Tyrosine and o-Vanillin-L-Levodopa Schiff Bases with bovine serum albumin (BSA)

In this work, three o-Vanillin Schiff Bases (o-VSB: o-Vanillin-D-Phenylalanine (o-VDP), o-Vanillin-L-Tyrosine (o-VLT) and o-Vanillin-L-Levodopa (o-VLL)) with alanine constituent were synthesized by direct reflux method in ethanol solution, and then were used to study the interaction to bovine serum albumin (BSA) molecules by fluorescence spectroscopy. Based on the fluorescence quenching calculation, the bimolecular quenching constant (K(q)), apparent quenching constant (K(sv)), effective binding constant (K(A)) and corresponding dissociation constant (K(D)) as well as binding site number (n) were obtained. In addition, the binding distance (r) was also calculated according to Foster's non-radioactive energy transfer theory. The results show that these three o-VSB can efficiently bind to BSA molecules, but the binding array order is o-VDP-BSA>o-VLT-BSA>o-VLL-BSA. Synchronous fluorescence spectroscopy indicates that the o-VDP is more accessibility to tryptophan (Trp) residues of BSA molecules than to tyrosine (Tyr) residues. Nevertheless, the o-VLT and o-VLL are more accessibility to Tyr residues than to Trp residues.

Novel reactions of one-electron oxidized radicals of selenomethionine in comparison with methionine

Pulse radiolysis studies on hydroxyl (*OH) radical reactions of selenomethionine (SeM), a selenium analogue of methionine, were carried out, and the resultant transient radical cations and their subsequent reactions have been reported. At pH<3, the >Se*-OH radical adducts produced on reaction of SeM with *OH radical were converted to selenium centered radical cations (Se*+M), which react with another molecule of SeM to form dimer radical cation M(Se therefore Se)M+. At pH 7, the >Se*-OH radical adducts were converted to a monomer radical of the type (Se therefore N)M+ that acquires intramolecular stability through interaction with the lone pair of the N atom and this radical is denoted as SeM*+. SeM*+ decayed by first order kinetics, and the reduction potential of the couple SeM*+/SeM was determined to be 1.21+/-0.05 V vs NHE at pH 7. SeM*+ oxidized ABTS2- and TMPD with rate constants of (2.5+/-0.1)x10(8) and (6.1+/-0.2)x10(8) M(-1) s(-1), respectively, and reacted with hydroxide ion with a rate constant of (3.8+/-0.9)x10(5) M(-1) s(-1). SeM*+ reacts with molecular oxygen, and the rate constant for this reaction was determined to be (4.3+/-0.2)x10(8) M(-1) s(-1); similar reaction with methionine could not be observed experimentally. Like methionine radical cations, SeM*+ undergoes decarboxylation, although with lesser yield, to produce reducing 3-methyl-selenopropyl amino radicals (referred as alpha-amino radicals). The formation of these radicals was confirmed both by the estimation of the liberated CO2 and by one-electron reduction of MV2+, thionine, and PNAP. These results have been supported by quantum chemical calculations. Implications of these results in the biological role of SeM have also been briefly discussed.