Ortwin Brede

Free electron transfer from several phenols to radical cations of non-polar solvents

Electron-transfer reactions from phenols to parent radical cations of solvents were studied using pulse radiolysis. Phenols bearing electron-withdrawing, electron-donating and bulky substituents were investigated in non-polar solvents such as cyclohexane, n-dodecane, n-butyl chloride and 1,2-dichloroethane. The experiments revealed the direct, synchronous formation of phenoxyl radicals and phenol radical cations in all cases and in nearly the same relative amounts. This was explained by two competing electron-transfer channels which depend on the geometry of encounter between the parent solvent radical cations and the solute phenol molecules. The mechanism is analysed at a microscopic level, treating diffusion as a slow process and the local electron transfer as an extremely rapid event. Furthermore, the effect of various phenol substituents and solvent types on the electron-transfer mechanism and on the decay kinetics of the solute phenol radical cations was analysed. The results were further substantiated using a quantum chemical approach.

Radiation-induced fragmentation of cardiolipin in a model membrane.

Purpose: To obtain evidence for the possibility of free‐radical fragmentation of cardiolipin under the action of ionizing radiation as measured by its aqueous dispersion from liposomes. Materials and methods: Liposomes of tetramyristoylcardiolipin (TMCL) were exposed to γ‐rays from 60Co or 137Cs sources at doses between 1 and 24 kGy. Fragmentation products were identified using thin‐layer chromatography and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS). Results: Using MALDI‐TOF MS and thin‐layer chromatography, it was shown that γ‐irradiation of liposomes consisting of TMCL was accompanied by free‐radical fragmentation of the lipid to form dimiristoylphosphatidic acid and dimiristoylphosphatidyl hydroxyacetone. The yields of dimiristoylphosphatidic acid were greater than those of dimiristoylphosphatidyl hydroxyacetone, and formation of the named compounds was inhibited by dissolved oxygen. Conclusion: It is shown for the first time that on γ‐irradiation, cardiolipin can undergo free‐radical fragmentation in its polar component.

Elementary reactions of the antioxidant action of trans-stilbene derivatives: resveratrol, pinosylvin and 4-hydroxystilbene

Using pulse radiolysis, elementary reactions of oxidizing radicals such as OH˙, N3˙ and Tl2+ with three trans-stilbene derivatives (4-hydroxy-, 3,5-dihydroxy- and 3,5,4′-trihydroxystilbene) were studied in detail. Depending on the reaction conditions (pH, oxidizing radical), hydroxycyclohexadienyl and phenoxyl radicals of all three polyphenols could beidentified from the transient absorption spectra. The hydroxycyclohexadienyl radicals decay by water- or OH−-elimination forming phenoxyl radicals in a delayed manner. For trans-3,5-dihydroxystilbene it was found that its phenoxyl radicals were mesomeric with keto C-centered radical forms which could explain their sensitivity towards oxygen. From comparison of the spectral and kinetic properties of the transients derived from trans-resveratrol (trans-3,5,4′-trihydroxystilbene) and its analogues, it could be concluded that in the neutral and acidic solution the para-hydroxy group of trans-resveratrol is more reactive than its meta-hydroxy groups, whereas the reactivity of meta-hydroxy groups increases in alkaline solution. This occurrence enables trans-resveratrol to effectively scavenge free radicals in the pH range 2–12.

Thiyl radicals in biosystems: effects on lipid structures and metabolisms.

Abstract.Thiyl radicals are intermediates of enzyme- and radical-driven biochemical processes, and their potential as reactive species in the biological environment has been somehow underestimated. From organic chemistry, however, it is known that thiyl radicals isomerize the double bonds of unsaturated fatty acids to a mixture with very dominating trans isomers. Recently, this reaction has been particularly studied for biosystems, focusing on the effect of thiyl radicals on the natural all-cis double bonds of unsaturated phospholipids, which undergo a conversion to the unnatural trans form. In this paper we report briefly the role of thiyl radicals in biosystems, describe the main features of the radical-induced cis-trans isomerization process under both in vitro and in vivo conditions, and reflect on some consequences for membrane structures, lipid metabolism and enzymatic reactions.

Thiol radical cations and thiyl radicals as direct products of the free electron transfer from aromatic thiols to n-butyl chloride radical cations

Radical and ionic reactions were observed in the pulse radiolysis of thiophenols (ArSH=thiophenol, o-, m- and p-thiocresol or 2-thionaphthol) in n-butyl chloride solution. The main source of aromatic thiyl radicals is the reaction of butyl radicals with the thiols, which proceeds at 1.0–5.6×108 dm3 mol−1 s−1. This radical generation path is completely quenched in the presence of oxygen. Under these conditions, only the electron transfer reaction between n-butyl chloride parent ions and the thiophenols remains and could be well analyzed. It takes place at a rate constant of about 1.5×1010 dm3 mol−1 s−1 and takes two parallel paths–common electron transfer yielding thiophenol radical cations and a more complex ionic reaction resulting directly in thiyl radicals. The latter is thought to proceed via an encounter complex geometry, ArSH···ClBu•+, in which electron transfer is directly followed by immediate deprotonation. The thiyl radicals and the thiol radical cations are characterized by their optical absorption spectra and their kinetic properties. Quantum chemical calculations underpin our mechanistic interpretation and provide information about the charge distribution and reactivity of the thiol radical cations.

Picosecond fluorescence of nucleic acid bases

Abstract After excitation with UV femtosecond laser pulses, the dynamics of the first excited singlet states of nucleic acid bases in polar solutions were directly determined by fluorescence detection with a sufficient time resolution using a sensitive streak camera. The single-shot detection followed by a numerical synchronization allowed the study of emission lifetimes of singlet states, even with low quantum yields of the order of 10−4. The measured fluorescence decays in a few picoseconds. The typical values for the S1-state lifetimes in water are between 6 and 9 ps for the purine bases and between 1 and 2 ps for the pyrimidines. In the nucleosides and nucleotides of purine bases, the lifetimes decrease towards 1 ps. Studying less polar solvents, such as ethanol and acetonitrile, especially for adenine dissolved in ethanol, a remarkable lifetime of 16 ps was found.

Primary processes of the radiation-induced cationic polymerization of aromatic olefins studied by pulse radiolysis

Abstract By pulse radiolysis of solutions of aromatic olefins (styrene, 1-methylstyrene, 1,1-diphenylethylene) in non-polar solvents (cyclohexane, carbon tetrachloride, n-butylchloride) the mechanism and kinetics of primary processes of radiation-induced cationic polymerization were investigated. In cyclohexane, radical cations of the olefins are generated by charge transfer from solvent cations (k about 1011 dm3 mol-1 s1). These cations dimerize in a diffusion-controlled reaction (k ≈ 1010 dm3 mol-1 s1). The next step of chain-growth is slower by 3 to 4 orders of magnitude. In carbon tetrachloride and in n-butyl chloride growing olefin cations are produced by a reaction of radical cations with solvent as well as by addition of solvent carbonium ions to the monomer. In strongly acidic aqueous solution of olefins radical cations produced indirectly from hydroxycyclo-hexadienyl radicals dimerize and react in a subsequent step by deprotonation forming non-saturated dimer radicals. The reaction mechanism established that in the case of radiation-induceders of magnitude. In cationic polymerization it is not possible to define a uniform first step of the chain reaction.

Mechanism of thiyl radical-catalyzed isomerization of unsaturated fatty acid residues in homogeneous solution and in liposomes.

NMR spectroscopy and gas chromatography were used on methanolic solutions of fatty acid methyl esters and on small bilayer liposomes to study the radical-induced denaturation of the fatty acid residues from the natural cis-configuration into trans-isomers. To analyze the mechanism of the thiyl radical-catalyzed lipid isomerization, we compared the effects of thiols on oleic and linoleic fatty acid residues using pulse radiolysis, gamma-radiolysis and chemolysis (AAPH) to generate thiyl radicals. The isomerization step takes place within the adduct of the thiyl radical to an olefinic group of unsaturated fatty acids, but not within the pentadienyl radical. The stability of the adduct can be described by an equilibrium constant of (12+/-5) mol(-1) dm(3). The isomerization rate depends on the structure of the thiol. However, the resulting isomeric equilibrium (trans-fraction: 81%) does not depend on the structure of the thiyl radical or the organization of the lipids. Quantum chemical calculations were performed to estimate the barriers for rotation, the geometry and the enthalpy difference between cis- and trans-thiyl radical adducts.

Stability of phenol and thiophenol radical cations – interpretation by comparative quantum chemical approaches

Abstract The deprotonation kinetics of phenol-type radical cations, formed via a very efficient electron transfer in the pulse radiolysis of non-polar solutions, for example n -chlorobutane, is governed mainly by electronic effects due to the nature of the phenol substituents, whereas steric effects are of minor importance; thiophenols, which are sulphur analogues of phenols, exhibit a similar behavior. Comparative quantum chemical calculations show that the calculated spin densities at the hetero atoms correlate well with the experimentally determined radical cation lifetimes. Not only the Density Functional Theory (DTF) B3LYP but also the semiempirical quantum chemical model PM3 can be applied for the open shell systems mentioned.

Fast formation of aromatic cations in cyclohexane observed by pulse radiolysis

Abstract The fast formation of cations of benzophenone, biphenyl, naphthalene, anthracene and phenanthrene in liquid cyclohexane was studied by nanosecond pulse radiolysis. Rate constants between 2.3 × 10 10 M −1 s −1 (for naphthalene) and not below 2.5 × 10 12 M −1 s −1 (for benzophenone) were derived and compared with the corresponding rate constants for anion formation.