A pulse radiolysis study of the OH radical induced autoxidation of methanesulfinic acid

Phenylamine has been oxidized by radiolytically generated hydroxyl and sulfate radicals, the ensuing intermediates and their reactions have been studied by pulse radiolysis and product analysis in the absence and presence of oxidants such as Fe(CN)63- and O2. Upon OH radical attack, hydroxycyclohexadienyl-type radicals are mainly formed while Η-abstraction reactions can be neglected. In the presence of Fe(CN)63- these radicals are for the most part oxidized to the corresponding tyrosines (80%), except for the ipso-OH-adduct radicals (≈ 20%). It is concluded that ˙OH-addition is almost random, but with a slight avoidance of the metaposition relative to the ortho-, para- and ipso-positions. Oxygen adds reversibly to the OH-adduct radicals (kf = 1.8 × 108 dm3 mol-1 s-1, kr = 5.4 × 104 s-1). In this case, tyrosine formation occurs by HO2˙-elimination. However, due to side reactions, tyrosine formation only reaches 52% of the OH radical yield. The tyrosine yield drops to 10% in the absence of an oxidant.Upon SO4˙⁻-attack, decarboxylation becomes a major process (33% of SO4˙⁻) alongside the production of tyrosines (43%). Here, with Fe(CN)63- as the oxidant the formation of p-Tyr (18.5%) and m-Tyr (16.5%) is preferred over o-Tyr formation (8.5%). It is believed that in analogy to other systems a radical cation is formed immediately upon SO4˙⁻-attack which either reacts with water under the formation of hydroxycyclohexadienyl-type (“OH-adduct”) radicals, or decarboxylates after intramolecular electron transfer. The radical cation can also arise indirectly through H+-catalysed water elimination from the ˙OH-adduct radicals. At pH 2 and a dose rate of 0.0046 Gy s-1 CO2 formation matches the OH radical yield when ˙OH is the attacking radical. Below pH 2, G(CO2) decreases with falling pH. This indicates the occurrence of another, unimolecular, pathway under these conditions competing effectively with decarboxylation. This appears to be a relatively slow deprotonation reaction of the carboxylprotonated phenylalanine radical cation which gives rise to the benzyl-type radical.

Download Full-text

Reactions of OH Radicals with Acetylacetone in Aqueous Solution. A Pulse Radiolysis and Electron Spin Resonance Study

Zeitschrift für Naturforschung B ◽

10.1515/znb-1982-0318 ◽

1982 ◽

Vol 37 (3) ◽

pp. 368-375 ◽

Cited By ~ 11

Author(s):

R. K. Broszkiewicz ◽

T. Söylemez ◽

D. Schulte-Frohlinde

Keyword(s):

Double Bond ◽

Pulse Radiolysis ◽

Visible Region ◽

Alkaline Condition ◽

Main Reaction ◽

Electron Spin Resonance Study ◽

Oh Radicals ◽

Oh Radical ◽

Keto Form ◽

Optical Absorbance

Abstract Pulse radiolysis experiments monitoring optical absorbance as well as conductivity and in-situ ESR radiolysis studies show that the OH radical reacts with the enol (k=8.6 x 109 M-1 s-1) and the enolate (k = 7.4 X 109 M-1 s-1) forms of acetylacetone by addition to the C = C double bond in aqueous N2O saturated solution. The OH reaction with enol leads to equal amounts of two radicals, CH3COCHOHCOHCH3 (2) and CH3COCHC(OH)2CH3 (4), as determined by scavenger reactions. At pH less than 1 the radical CH3COCHCOCH3 (1) is observed by ESR spectroscopy showing that radical 2 and/or 4 eliminate water by proton catalyzed reactions. Under alkaline condition the OH adducts to the enolate eliminate OH -with rate constants larger than 105 s-1 leading to radical 1. G(OH-) is determined to be 5.6 showing that addition is the main reaction of OH with enolate. To a much smaller degree the OH radical is proposed to abstract an H atom from that CH3 group which is attached to the C -C double bond in enol and enolate, producing substituted allyl radicals which absorb in the visible region. The reaction of OH with the keto form has not been observed indicating that the rate constant of this reaction is significantly smaller than those with enol and enolate.

Download Full-text

Different channels of ·OH radical reaction with tryptophanol in aqueous solutions: A pulse radiolysis study

Research on Chemical Intermediates ◽

10.1163/156856703322539672 ◽

2003 ◽

Vol 29 (6) ◽

pp. 641-648 ◽

Cited By ~ 2

Author(s):

G. H. Naik ◽

K. Indira Priyadarsini ◽

Hari Mohan

Keyword(s):

Aqueous Solutions ◽

Pulse Radiolysis ◽

Radical Reaction ◽

Oh Radical

Download Full-text

The Reactions of Hydroxyl Radicals with 1,4-and 1,3-Cyclohexadiene in Aqueous Solution

Zeitschrift für Naturforschung B ◽

10.1515/znb-1988-0919 ◽

1988 ◽

Vol 43 (9) ◽

pp. 1201-1205 ◽

Cited By ~ 12

Author(s):

Xian-Ming Pan ◽

Eugenie Bastian ◽

Clemens von Sonntag

Keyword(s):

Hydroxyl Radicals ◽

Pulse Radiolysis ◽

Hydrogen Abstraction ◽

Material Balance ◽

Oh Radicals ◽

Oh Radical ◽

Product Analysis ◽

Γ Irradiation ◽

Abstraction Reaction ◽

G Value

Abstract The reactions of radiolytically generated hydroxyl radicals and H atoms with 1,4- and 1,3-cyclohexadiene were studied by pulse radiolysis and product analysis. Hydrogen abstraction from these substrates by the OH radical yields the cyclohexadienyl radical (ε (310 nm) = 4400 dm3 mol-1 cm-1 from the reaction of the H atom with benzene) with an efficiency of 50% (0.29 ,μmol J-1) in the case of 1,4-cyclohexadiene and 25% (0.15 ,μmol J-1) in the case of 1,3-cyclohexadiene as determined by pulse radiolysis. The remaining OH radicals add to the olefin. In 1.4-cyclohexa- diene the yield of the resulting adduct radicals has been determined in a steady-state 60Co-γ-irradiation experiment by reducing it with added 1.4-dithiothreitol (DTT) to 4-hydroxycyc- lohexene. There are two sites of OH radical attack in the case of 1.3-cyclohexadiene, and only the alkyl radical is reduced quantitatively by DTT (G(3-hydroxycyclohexene) = 0.15 ,μmol J-1). From material balance considerations it is concluded that the allylic radical must be formed with a G value of 0.28 ,μmol J-1 but largelv escapes reduction by DTT (G(4-hvdroxycyclohexene) = 0.03 ,μmol J-1). H atoms add preferentially to the double bonds of 1,4- and 1,3-cyclohexadiene (78% and 93%, respectively), while the O.- radical (the basic form of the OH radical) undergoes mainly H- abstraction (92% and 83%, respectively). The radicals formed in these systems decay bimolecularly (2k = 2.8 x 109 dm3 mol-1 s-1). In their combination reactions the cyclohexadienyl radicals form the four possible dimers in proportions such that the dienyl radical moiety shows a 2:1 preference to react from its central (1a) rather than from a terminal carbon atom (1b). Cyclohexadienyl radicals and the OH- and H-adduct radicals also cross-tcrminate by disproportionation and dimerization. Material balance has been obtained for the 1,4-cyclohexadiene system in N2O-Saturated solution (10-2 mol dm-3) at a dose rate of 0.14 Gy s-1, the products (G values in ,μmol J-1) being: benzene (0.085), 4-hydroxycyclohexene (0.25), cyclohexadienyl-dimers (0.144). cvclohexadienyl-OH-adduct- dimers (0.02), OH-adduct-dimers (0.02). Some of the 4-hydroxycyclohcxene is formed in an H-abstraction reaction by the OH-adduct radical from 1,4-cyclohexadiene.

Download Full-text

Radiolysis of Cytosine, 5-Methyl Cytosine and 2'-Deoxycytidine in Deoxygenated Aqueous Solution A Pulse Spectroscopic and Pulse Conductometric Study of the OH Adduct

Zeitschrift für Naturforschung B ◽

10.1515/znb-1978-0314 ◽

1978 ◽

Vol 33 (3) ◽

pp. 321-328 ◽

Cited By ~ 22

Author(s):

Alfred Hissung ◽

Clemens von Sonntag

Keyword(s):

Pulse Radiolysis ◽

Radical Reactions ◽

Oh Radical ◽

Conductivity Measurements ◽

Free Radical Reactions ◽

Product Mixture ◽

Conductometric Study ◽

Methyl Cytosine ◽

G Value ◽

Induced Reaction

Abstract Using the pulse radiolysis technique the OH adducts of cytosine, 5-methyl cytosine and 2'-deoxycytidine were investigated in alkaline N2O saturated aqueous solutions. Ab-sorption spectra were recorded and their change with time was correlated with the change in conductivity. An OH -induced reaction (1.4·108)·mol-1·s-1) was observed with OH adducts of cytosine and 5-methyl cytosine leading to radicals (pK=10.3), the G-values of which were 3.2 and 1.6 respectively (conductivity measurements). These intermediates were assigned to the C-5 OH radical adducts. After completion of the free radical reactions (2k = 6 · 108)·mol-1·s-1) the resulting product mixture showed pK-values of 8.3 (cytosine, G=0.6), 10.5 (5-methyl cytosine, G=0.8), 10.7 (cytosine, G=1.5; 2′-deoxycytidine, G=1.5) and 12.4 (cytosine, no G-value, determinable with optical detection). Ammonia, which is a product measured after γ-radiolysis of all the three compounds investigated (G(NH3) = 0.6 ± 0.1), is not released within 15 ms after the pulse.

Download Full-text