Pulse radiolysis of 3-pyridine methanol and 3-pyridine carboxaldehyde in aqueous solutions

Reactions of e_aq^-, H-atom and OH radicals with 3-pyridine methanol (3-PM) and 3-pyridine carboxaldehyde (3-PCA) have been studied at various pHs using pulse radiolysis technique. e_aq^- was found to be highly reactive with both 3-PM and 3-PCA (k approx. 10¹⁰ dm³ mol¹ s^-1). Semi-reduced species formed in both cases were strongly reducing in nature. In the case of 3-PM, electron addition leads to the formation of pyridinyl radicals whereas in the case of 3-PCA, PyCHOH type radicals are formed. At pH 6.8, H-atom reaction with 3-PCA also gives semi-reduced species (PyCHOH), whereas at pH 1, H-atoms add to the ring. (CH₃)₂˙COH radicals were found to transfer electron to 3-PCA at all the pH values tested and by making use of changes in the absorption spectra, pK_a values of the semi-reduced species were determined to be 4.5 and 10.6. OH radicals were found to undergo addition reaction with 3-PCA, whereas in the case of 3-PM they reacted by H-abstraction as well as addition reaction. By following the yield of methylviologen radical cation formed by electron transfer reaction, it was estimated that approx. 50% of OH radicals react with 3-PM by H-atom abstraction at pH 6.8, giving reducing radicals, whereas at pH 3.2, where 3-PM is in the protonated form, the same is only about 10%. At pH 13, O^-˙ radical anions were found to react exclusively by H-atom abstraction. Reaction of SO₄^-˙ radicals with 3-PCA was found to give a species identical to the one formed by one electron reduction of nicotinic acid at acidic pH values.     

功能高分子膜离子选择电极研究VI.压片PVC膜氯离子选择电极的暂态极化行为

Both fast triangle-wave potential sweep method and fast square-wave potential sweep method have been applied to study transient behavior of the functional polymer membrane chloride electrode. On the basis of current-time curve and potential-time curve, the electrochem. parameters of the electrode were calculated successfully under various conditions. The characterization of the level of ion transfer has been made by means of electrochem. polarization resistance Rr which is related not only to the membrane composition and its shape (area etc.), but also to concentration of the chloride ions in solution The Rr values calculated from both methods are the same. Cl- in the concentration range of 10-4-10-1 mol/L showed linear relation between i and log c, where c is the concentration The slope is proportional to the membrane area.     

云团中顺磁性粒子对三重态正电子素寿命的影响

Ferrell很早就提出了正电子素o-Ps的猝灭理论,指出介质中的顺磁性粒子是影响o-Ps寿命的重要因素.Lee等测出O_2猝灭o-Ps的速率常数为10~(10)～10~(11)mol·L~(-1)·s~(-1),得出了顺磁猝灭反应是扩散控制的结论.但前人没有对云团中瞬态活性粒子对o-Ps的影响加以探讨.本文通过o-Ps寿命与氧分压的关系来研究云团中瞬态活性粒子对o-Ps猝灭的影响.     

Pulse-radiolysis studies of nimesulide in aqueous solution: effect of microheterogeneous media

Nimesulide, a non-steroidal anti-inflammatory drug, forms semi-oxidized species (_max = 350 nm) on reaction with N·₃ and CCl₃OO· in aqueous solution. Their oxidizing nature is confirmed by their ability to undergo an electron-transfer reaction with ABTS^2-, the rate constant for the reaction k = 4.7 × 10⁹ dm³ mol^-1 s^-1. ·OH-adduct constitutes about 94% of the species formed on reaction with ·OH radicals, the remaining 5-6% species are oxidizing in nature. The rate constant for the formation of ·OH-adduct, i.e. k(·OH + Nim-NO₂) = 2.8 × 10¹⁰ dm³ mol^-1 s^-1. Oxygen adds to both e^- _aq and ·OH-adducts of nimesulide with rate constants of 9.5 × 10⁶ dm³ mol^-1 s^-1 and 1.4 × 10⁷ dm³ mol^-1 s^-1, respectively. In the presence of cyclodextrins the nature of the transient species formed is much the same. Binding constants of the drug with CDs are generally quite low in comparison to BSA and range between 37 and 390 dm³ mol^-1. Hydrated electrons add on to nimesulide at the nitro group forming a semi-reduced species with _max = 320 and 500 nm. The rate constant k for this reaction is 1.4 × 10¹⁰ dm³ mol^-1 s^-1. The transient species formed on reaction of e^- _aq or (CH₃)·₂COH radicals with nimesulide seem to be identical, as is seen from their decay rates. The reduction potential of nimesulide for the couple (Nim-NO₂/Nim-NO·₂ ^-) is found to be -0.52 V vs. NHE at pH 7, by cyclic voltammetric and pulse radiolysis techniques.     

Investigations on the nature of the transient species formed on pulse radiolysis of a aqueous solution of 4-(methylthio)benzoic acid

The transient absorption spectrum (max = 320, 400 and 550 nm) obtained on reaction of OH radicals with 4-(methylthio)benzoic acid is assigned to a solute radical cation with a positive charge on the benzene ring. The reaction with specific one-electron oxidants also produced similar spectrum and the oxidation potential for the formation of solute radical cation is estimated to be between 1.4 and 1.6 V vs NHE. The reaction of e_aq ^- with the solute showed the formation of a transient absorption band at 320 nm and is assigned to solute radical anion with reduction potential more negative than-1.5 V.     

Scavenging of reactive oxygen radicals by resveratrol: antioxidant effect

Pulse radiolysis of resveratrol was carried out in aqueous solutions at pH ranging from 6.5 to 10.5. The one-electron oxidized species formed by the N^•₃ radicals at pH 6.5 and 10.5 were essentially the same with λ_max at 420 nm and rate constant varying marginally (k = (5−6.5) × 10⁹ dm³ mol⁻¹ s⁻¹). The nature of the transients formed by NO^•₂, NO^• radical reaction at pH 10.5 was the same as that with N^•₃, due to the similarity in decay rates and the absorption maximum. Reaction of ^•OH radical with resveratrol at pH 7 gives an absorption maximum at 380 nm, attributed to the formation of carbon centered radical. The repair rates for the thymidine and guanosine radicals by resveratrol were approx. 1 × 10⁹ dm³ mol⁻¹ s⁻¹, while the repair rate for tryptophan was lower by nearly an order of magnitude (k = 2 × 10⁸ dm³ mol⁻¹ s⁻¹). The superoxide radical anion was scavenged by resveratrol, as well as by the Cu–resveratrol complex with k = 2 × 10⁷ and 1.5 × 10⁹ dm³ mol⁻¹ s⁻¹, respectively. Its reduction potential was also measured by cyclic voltammetry.