Triplet state and phenoxyl radical formation of 3, 4‐methylenedioxy phenol: a combined laser flash photolysis and pulse radiolysis study

Formations of triplet state, molecular cation radical, and phenoxyl radical of 3,4‐methylenedioxy phenol (sesamol, SOH) in organic solvents have been investigated by laser photolysis as well as pulse radiolysis techniques. Photolysis of SOH in cyclohexane has been found to produce both triplet state (λ_max ～ 480 nm) and phenoxyl radical (425–430 nm) of SOH by mono‐photonic processes. However, radical cation (λ_max = 450 nm) and phenoxyl radical of SOH have been observed on radiolysis in cyclohexane. Further, radiolysis of SOH in benzene has been found to produce phenoxyl radical only. Mechanism of phenoxyl radical formation by photo‐excitation of SOH has been studied and triplet energy level of SOH is estimated to lie between 1.85 and 2.64 eV. Copyright © 2007 John Wiley & Sons, Ltd.     

Dimer radical cation of 4‐thiouracil: a pulse radiolysis and theoretical study

Pulse radiolysis with optical absorption detection has been used to study the reactions of hydroxyl radical (OH^?) with 4‐thiouracil (4TU) in aqueous medium. The transient absorption spectrum for the reaction of OH^? with 4TU is characterized by λ_max 460 nm at pH 7. A second‐order rate constant k_(4TU+OH) of 1.7 × 10¹⁰ M^?1 s^?1 is determined via competition kinetics method. The transient is envisaged as a dimer radical cation [4TU]₂^?+, formed via the reaction of an initially formed radical cation [4TU]^?+ with another 4TU. The formation constant of [4TU]₂^?+ is 1.8 × 10⁴ M^?1. The reactions of dibromine radical ion (Br₂^??) at pH 7, dichlorine radical ion (Cl₂^??) at pH 1, and azide radical (N₃^?) at pH 7 with 4TU have also produced transient with λ_max 460 nm. Density functional theory (DFT) studies at BHandHLYP/6–311 + G(d,p) level in aqueous phase showed that [4TU]₂^?+ is characterized by a two‐centerthree electron (2c‐3e) [?S∴S?] bond. The interaction energy of [?S∴S?] bond in [4TU]₂^?+ is ?13.01 kcal mol^?1. The predicted λ_max 457 nm by using the time‐dependent DFT method for [4TU]₂^?+ is in agreement with experimental λ_max. Theoretical calculations also predicted that compared with [4TU]₂^?+, 4‐thiouridine dimer is more stable, whereas 4‐thiothymine dimer is less stable. Copyright © 2013 John Wiley & Sons, Ltd.     

Charge transfer reactions from tryptophan and tyrosine to sulfur‐centered dimer radical cation in aqueous –sulfuric acid medium: a pulse radiolysis study

Kinetics and mechanism of one‐electron oxidation of N‐acetyl methionine (NAM), tryptophan (TrpH), tyrosine (TyrOH), and phenylalanine (Phe) have been studied in 33% v/v H₂SO₄ solution. The solvent radical (SO₄^?¯) oxidized NAM, TrpH, TyrOH, and Phe to produce NAM₂^?+ (480 nm), TrpH^?+ (330, 580 nm), TyrO^? (350, 410 nm), and Phe(?H)^? (320 nm), with rate constants (10⁹ M^?1 s^?1) 0.6, 2.7, 3.9, 1.6, respectively. Time resolved radical transformation from NAM₂^?+ to TrpH^?+ and TyrO^? have been observed to occur with k(10⁸ M^?1 s^?1) = 3.60 and 0.35, respectively. However, NAM₂^?+ to Phe(?H)^? and TrpH^?+ to TyrO^? radical transformations have not been observed in this medium. The study shows the kinetics and mechanism of oxidation of some amino acids in strong acidic solutions. To the best of our knowledge, radical cations of amino acids and electron transfer reactions between them could be studied in strong acidic solutions for the first time. Copyright © 2016 John Wiley & Sons, Ltd.     

The equilibrium and kinetics of intermolecular hydrogen bonding in nitroarene anion radical–alcohol system: 1,3‐dinitro‐benzene anion radical – R–OH (R═CH3–, C2H5–, (CH3)2 CH–)

To explore the possibility of hydrogen bonding of a stable anion radical with DNA – component sugar, hormones, steroid, and so on (through hydroxyl group), as a first step, the possibility of hydrogen bonding of 1,3‐dinitrobenzene anion radical (1,3‐DNB^??) with aliphatic alcohols was studied. It was found that 1,3‐DNB^?? anion radical undergoes hydrogen bonding with alcohols: methanol, ethanol, and 2‐proponal. The hydrogen‐bonding equilibrium constant K_eq and the (hydrogen‐bonding) rate constants k₂ were evaluated through the use of linear scan and cyclic voltammetry theory and techniques. The K_eq was found to be in the range of 1.4–6.0 m ^?1, whereas the rate constants k₂ were found to be in the range of 1.5–3.6 m ^?1 s^?1, depending upon the hydrogen‐bonding agent and the equation used for the calculation of the rate constants. The hydrogen‐bonding number n was found to be around 0.5 or 1.0. The implication of this study in, for example, the replication of DNA, the prevention of the formation of super oxide, and so on is discussed. Copyright © 2012 John Wiley & Sons, Ltd.