Pulse radiolysis studies of 2‐ and 3‐hydroxybenzyl alcohols: inhibition of dehydration of ·OH‐(hydroxybenzyl alcohols) adducts by H2PO4− ions

Reactions of ·OH/O ^.? radicals and H‐atoms as well as specific oxidants such as Cl₂^.? and N₃· radicals have been studied with 2‐ and 3‐hydroxybenzyl alcohols (2‐ and 3‐HBA) at various pH using pulse radiolysis technique. At pH 6.8, ·OH radicals were found to react quite fast with both the HBAs (k = 7.8 × 10⁹ dm³ mol^?1 s^?1 with 2‐HBA and 2 × 10⁹ dm³ mol^?1 s^?1 with 3‐HBA) mainly by adduct formation and to a minor extent by H‐abstraction from ? CH₂OH groups. ·OH‐(HBA) adduct were found to undergo decay to give phenoxyl type radicals in a pH dependent way and it was also very much dependent on buffer‐ion concentrations. It was seen that ·OH‐(2‐HBA) and ·OH‐(3‐HBA) adducts react with HPO₄^2? ions (k = 2.1 × 10⁷ and 2.8 × 10⁷ dm³ mol^?1 s^?1 at pH 6.8, respectively) giving the phenoxyl type radicals of HBAs. At the same time, this reaction is very much hindered in the presence of H₂PO ions indicating the role of phosphate ion concentration in determining the reaction pathway of ·OH adduct decay to final stable product. In the acidic region adducts were found to react with H⁺ ions. At pH 1, reaction of ·OH radicals with HBAs gave exclusively phenoxyl type radicals. Proportion of the reducing radicals formed by H‐abstraction pathway in ·OH/O ^.? reactions with HBAs was determined following electron transfer to methyl viologen. H‐atom abstraction is the major pathway in O ^.? reaction with HBAs compared to ·OH radical reaction. H‐atom reaction with 2‐ and 3‐HBA gave transient species which were found to transfer electron to methyl viologen quantitatively. Copyright © 2010 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.     

4‐Phenyl‐1,2,4‐triazoline‐3,5‐dione in the ene reactions with cyclohexene, 1‐hexene and 2,3‐dimethyl‐2‐butene. The heat of reaction and the influence of temperature and pressure on the reaction rate

The values of the enthalpy (53.3; 51.3; 20.0 kJ mol^?1), entropy (?106; ?122; ?144 J mol^?1K^?1), and volume of activation (?29.1; ?31.0; ?cm³ mol^?1), the reaction volume (?25.0; ?26.6; ?cm³ mol^?1) and reaction enthalpy (?155.9; ?158.2; ?150.2 kJ mol^?1) have been obtained for the first time for the ene reactions of 4‐phenyl‐1,2,4‐triazoline‐3,5‐dione 1 , with cyclohexene 4 , 1‐hexene 6 , and with 2,3‐dimethyl‐2‐butene 8 , respectively. The ratio of the values of the activation volume to the reaction volume (?V^≠_corr/ΔV_{r ? n}) in the ene reactions under study, 1 + 4 → 5 and 1 + 6 → 7 , appeared to be the same, namely 1.16. The large negative values of the entropy and the volume of activation of studied reactions 1 + 4 → 5 and 1 + 6 → 7 better correspond to the cyclic structure of the activated complex at the stage determining the reaction rate. The equilibrium constants of these ene reactions can be estimated as exceeding 10¹⁸ L mol^?1, and these reactions can be considered irreversible. Copyright © 2014 John Wiley & Sons, Ltd.     

Role of gemini surfactants (m‐s‐m type; m = 16, s = 4–6) on the reaction of [Zn(II)‐Gly‐Phe]+ with ninhydrin

In the present paper, reaction of zinc‐glycylphenylalanine ([Zn(II)‐Gly‐Phe]⁺) with ninhydrin has been investigated in gemini (m‐s‐m type; m = 16, s = 4–6) surfactants at temperature (70 °C) and pH (5.0). Monitoring the appearance of product at 400 nm was used to follow the kinetics, spectrophotometrically. The order of the reaction with respect to [Zn(II)‐Gly‐Phe]⁺ was unity while with respect to [ninhydrin] was fractional. The reaction constants involved in the mechanism were obtained. In addition to the rate constant (k_Ψ) increase and leveling‐off regions are observed with the geminis, just like as seen with conventional surfactant hexadecyltrimethylammonium bromide (CTAB), the former produced a third region of increasing k_Ψ at higher concentrations. A close agreement between observed and calculated rate constants was found under varying experimental conditions. A suitable mechanism consistent with the experimental findings has been proposed. Copyright © 2014 John Wiley & Sons, Ltd.