Radiolytic Techniques Research Articles

Radiolytic yields of solvated electrons in ionic liquid and its solvation dynamics at low temperature

We present an investigation of the solvated electron in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (P14NTf2) using pulse radiolytic techniques. Temperature-dependent studies reveal that the yield of the solvated electron decreases with decreasing temperature. The lower initial yield measured indicates that we have a loss of some electrons before they become fully solvated. There may be a high probability that the excess dry electrons (pre-solvated electron) react before the electron solvation is completed because the solvation dynamics is slowing down with decreasing temperature.

On the mechanism of reduction of maleate and fumarate by NiI(1,4,8,11-tetraazacyclotetradecane)+ in aqueous solutions

Ni(I)(1,4,8,11-tetraazacyclotetradecane)(+), Ni(I)L(+), was produced in neutral aqueous solutions by radiolytic techniques. The mechanism and kinetics of the reaction of Ni(I)L(+) with maleate and fumarate were studied applying pulse-radiolysis and analysis of the final products. Surprisingly the mechanisms of reduction of maleate and fumarate differ considerably. Based on the results it was also shown that Ni(II)L(2+) is an electrocatalyst for the reduction of alkenes in aqueous solutions.

Fast Repair Activities Towards dGMP Hydroxyl Radical Adducts by Silybin and its Analogues

The repair activities of silybin (SLB) and its analogues towards the oxidizing deoxyguanosine monophosphate (dGMP) hydroxyl radical adducts are investigated by pulse radiolytic techniques. On pulse irradiation of nitrous oxide saturated 2.0 mM dGMP aqueous solution containing 0.1 mM silybin at neutral pH, the transient absorption spectrum of the dGMP hydroxyl radical adducts decreases with the formation of the phenoxyl radical of silybin within tens of microseconds, indicating that there is a repair reaction between the dGMP hydroxyl radical adduct and silybin. The rate constant of the repair reaction is calculated to be 1.0 x 10(9) M(-1)s(-1) for silybin. The repair activity of hesperetin (HESP), naringin (NAN) and naringenin (NAR) towards hydroxyl radical adducts of dGMP are also studied.

1,4-benzothiazines as key intermediates in the biosynthesis of red hair pigment pheomelanins.

Following the discovery of cysteinyldopas as the early intermediates in the biogenesis of pheomelanins, the typical red hair pigments, the reactivity of the biosynthetic precursors under biomimetic conditions was extensively investigated. As a result, the early stages of pheomelanogenesis were envisaged as involving oxidative cyclization of cysteinyldopas, mainly the 5-S-isomer, to 1,4-benzothiazine (BTZ) intermediates which undergo oxidative polymerization leading eventually to the pigments. In the last decade, several aspects of the chemistry and biosynthesis of pheomelanins were re-examined. In particular, (i) transient BTZ intermediates were identified by pulse radiolytic techniques and NMR analysis; (ii) the effect of reaction conditions and additives on the rearrangement vs. redox exchange reaction paths of such intermediates were investigated in detail; (iii) the mechanism of the oxidative polymerization of BTZs was characterized by the first isolation of oligomer species, and (iv) the pigment eventually resulting from oxidation of 5-S-cysteinyldopa (CD) was directly analyzed by spectroscopic and chemical methodologies in comparison with pheomelanins isolated from human hair. These advances led eventually to an integrated picture of the biogenetic route highlighting the intervention of various chemical and enzymatic factors which affect the kinetics of the different steps and the nature of the key benzothiazine precursors. A likely biogenetic route was also postulated for the delta2,2'-bi(2H-1,4-benzothiazine) pigments, termed trichochromes, whose origin had remained an open issue since their first isolation from red human hair and avian feathers. Finally, a more detailed description of the structure of pheomelanin pigments in terms of the monomer units, their mode of linking, and postsynthetic modifications was gained.

MECHANISTIC ENZYMOLOGY OF OXYGEN ACTIVATION BY THE CYTOCHROMES P450

The P450 cytochromes represent a universal class of heme-monooxygenases. The detailed mechanistic understanding of their oxidative prowess is a critical theme in the studies of metabolism of a wide range of organic compounds including xenobiotics. Integral to the O2 bond cleavage mechanism by P450 is the enzyme's concerted use of protein and solvent-mediated proton transfer events to transform reduced dioxygen to a species capable of oxidative chemistry. To this end, a wide range of kinetic, structural, and mutagenesis data has been accrued. A critical role of conserved acid-alcohol residues in the P450 distal pocket, as well as stabilized waters, enables the enzyme to catalyze effective monooxygenation chemistry. In this review, we discuss the detailed mechanism of P450 dioxygen scission utilizing the CYP101 hydroxylation of camphor as a model system. The application of low-temperature radiolytic techniques has enabled a structural and spectroscopic analysis of the nature of critical intermediate states in the reaction.

Generation and identification of the transient intermediates of allophycocyanin by laser photolytic and pulse radiolytic techniques

Purpose : To explore the photophysical and photochemical properties of allophycocyanin (APC) and contribute to the understanding of the molecular mechanism of APC photosensitization. Materials and methods : Laser-flash photolytic and pulse radiolytic techniques were used for the first time to characterize the transient intermediates involved in APC photochemistry and photophysics. The excited triplet state and radical cation of APC were identified by acetone sensitization and one-electron oxidation. Results : The 248-nm laser-flash photolysis of APC in N 2 -saturated aqueous solution (pH 7.0) yields the triplet state and radical cation of APC. The APC radical cations were generated by ionization via a monophotonic process, with a quantum yield of 0.17. Conclusions : APC can undergo both photo-excitation and photo-ionization under the present experimental conditions. These new findings suggest that APC has the potential to act as both a type I and type II photosensitizer.

Photophysics of Nanometer Sized Metal Particles: Electron−Phonon Coupling and Coherent Excitation of Breathing Vibrational Modes

The wide variety of applications of metal nanoparticles has motivated many studies of their properties. Some important practical issues are how the size, composition and structure of these materials affect their catalytic and optical properties. In this article we review our recent work on the photophysics of metal nanoparticles. The systems that have been investigated include Au particles with sizes ranging from 2 nm diameter (several hundred atoms) to 120 nm diameter, and bimetallic core−shell particles composed of Au, Ag, Pt and/or Pb. These particles, which have a rather narrow size distribution, are prepared by radiolytic techniques. By performing time-resolved laser measurements we have been able to investigate the coupling between the electrons and phonons in the particles, and their low frequency “breathing” modes. These experiments show that for Au the time scale for electron−phonon coupling does not depend on size, in contrast to metals such as Ga and Ag. On the other hand, the frequency of the acoustic breathing modes strongly depends on the size of the particles, as well as their composition. These modes are impulsively excited by the rapid lattice heating that accompanies ultrafast laser excitation. The subsequent coherent nuclear motion modulates the transmitted probe laser intensity, giving a “beat” signal in our experiments. Unlike quantum-beats in molecules or semiconductors, this signal can be completely understood by classical mechanics.

Fast interconversion of C4H7+ cations in the gas phase and in a gaseous microsolvated environment

The equilibration of cyclobutyl 1 and 1' and the cyclopropylmethyl cation (2) has been studied in the gas phase by utilizing FT-ICR mass spectrometry and high-pressure radiolytic techniques. A suitable gaseous nucleophile, C6X6 (X=H,D), was used to sample the equilibration of C4H7+ ions, produced from both cyclobutanol and cyclopropylmethanol. These are either dispersed in the bulk gas or confined within a C4H7+/C6X6 complex that contains a molecule of solvent (H2O). The analysis of the products shows that, irrespective of their source and of the intermolecular or intracomplex nature of the process, the C4H7+ ions undergo equilibration before they are trapped. The equilibrium (1+1')/2 ratio is very close to unity at 300 K, and the results from the intracomplex trapping experiment show that equilibration occurs within a time interval < or =10(-10) s.

Fast Interconversion of C4H7+ Cations in the Gas Phase and in a Gaseous Microsolvated Environment

The equilibration of cyclobutyl 1 and 1′ and the cyclopropylmethyl cation (2) has been studied in the gas phase by utilizing FT-ICR mass spectrometry and high-pressure radiolytic techniques. A suitable gaseous nucleophile, C6X6 (X=H,D), was used to sample the equilibration of C4H7+ ions, produced from both cyclobutanol and cyclopropylmethanol. These are either dispersed in the bulk gas or confined within a C4H7+/C6X6 complex that contains a molecule of solvent (H2O). The analysis of the products shows that, irrespective of their source and of the intermolecular or intracomplex nature of the process, the C4H7+ ions undergo equilibration before they are trapped. The equilibrium (1+1′)/2 ratio is very close to unity at 300 K, and the results from the intracomplex trapping experiment show that equilibration occurs within a time interval ≤10−10 s.

Anion and electron solvation in alcohols

Pulse radiolytic techniques have been used to measure the solvation of anions and electrons in alcohols. The results have shown the importance of the microscopic liquid structure in the solvation process. The activation energy for benzophenone solvation is equal to the hydrogen-bond energy for the liquids, which shows that the solvent reorganization requires the breaking of the hydrogen bonds between solvent molecules. For electron solvation, primary alcohols have a lower activation energy because the initial hydrogen-bonded structure of the liquid is amenable to solvation. The solvation of an electron in a secondary alcohol requires require hydrogen-bond breakage.

Fast repair of oxidizing OH adducts of DNA by hydroxycinnamic acid derivatives. A pulse radiolytic study

Using pulse radiolytic techniques, it has been demonstrated that the interactions of oxidizing OH adducts of DNA (ssDNA and dsDNA), polyA and polyG with hydroxycinnamic acid derivatives proceed via an electron transfer process ( k =5–30×10 8 dm 3 mol −1 s −1 ). In addition, the rates for fast repair of OH adducts of dAMP, polyA and DNA (ssDNA and dsDNA) are slower than the corresponding rates for the rest OH adducts of DNA constituents. The slower rates for repair of oxidizing OH adducts of dAMP may be the rate determining step during the interaction of hydroxycinnamic acid derivatives with OH adducts of DNA containing the varieties of OH adducts of DNA constituents.

Radiation-Induced Reactions of 2,4,6-Trinitrotoluene in Aqueous Solution

Radiolysis of aqueous solutions of TNT was examined to provide fundamental information concerning the reactions of TNT with radical species in water. γ-Radiation was used in conjunction with radical scavengers to compare yields for radiation-induced TNT transformation under oxidizing and reducing conditions and in the presence and absence of oxygen. Pulse radiolytic techniques were employed to determine rate constants and absorption spectra for the reactions of TNT with the hydroxyl radical and the aqueous electron. TNT was rapidly transformed under both reducing (1% tert-butyl alcohol, N 2 sparged) and oxidizing (N 2 O sparged) conditions although rates under reducing conditions were greater. The initial yield for transformation of a 350 μmol L -1 TNT solution under reducing conditions was 0.14 μmol/J as opposed to 0.10 μmol/J measured in oxidizing conditions. The reactions of TNT with reduced oxygen species were found to be highly inefficient in aqueous solution. Although TNT is transformed by both oxidizing and reducing radicals, TNT degradation yields in the absence of a radical scavenger were low, indicating that under these conditions there were significant secondary reactions in which the species resulting from reactions between TNT and the primary radicals further reacted to reform the parent compound. The bimolecular rate constant for the reaction between TNT and - OH was determined to be 4.3 x 10 8 mol -1 s -1 . Byproduct analyses from γ-radiolysis suggest that hydroxyl radical abstraction of a methyl hydrogen to form the trinitrotoluyl radical is an initial oxidative reaction. The bimolecular rate constant for the reaction between TNT and e aq - was measured as 3.5 x 10 10 mol -1 s -1 . These results provide quantitative and qualitative insight into the reactions between TNT and the various aqueous radicals produced in many remediation processes.

Pulse and gamma radiolytic studies on the formation of Cd, Pd and Cu clusters in aqueous solutions

Investigations on the formation of Cd, Pd and Cu sols using radiolytic techniques have been carried out. The reducing properties of the Cd particles were confirmed by dissolving MV 2+, N 2O and O 2. By using high intensity electron pulses we observed the formation of the highly stable (approx. 8 days) Cd particles that showed absorption at both 270 and 450 nm. In the case of Cu, the stability of the sol depends on the type of reducing radical used, such as formate and isopropanol. It has been shown for Pd and Cd sols that the presence of both the stabilizer and agglomerated particles play an important role in stabilizing the small clusters.

Electron Transfer Studies in C78 (C2v‘), C76 (D2), C70 (D5h), and C60 (Ih) Surfactant Aqueous Solutions

C60 (Ih) and several higher fullerenes, namely, C70 (D5h), C76 (D2), and C78 (C2v‘), have been solubilized via capping with suitable surfactants. Excited states and reduced states of these surfactant systems were probed by photolytic and radiolytic techniques. Flash photolytic irradiation at 355 nm of surfactant solutions of C60, C70, C76, and C78 revealed the rapid grow-in of transient (*S1 → *Sn) absorptions with λmax at 920, 660, 650, and 890 nm, respectively. The associated intersystem crossing rates to the energetically lower lying excited triplet states are typically on the order of 9.0 × 108 s-1. Reductive quenching of the excited triplet states by diazabicyclooctane (DABCO) occurs with (0.8−7.4) × 107 M-1 s-1 and yields the fullerene π-radical anions. Direct reduction of surfactant capped fullerene monomers have been studied by means of time-resolved pulse radiolysis with measurements being conducted in the near-IR region. Radiolytic reduction of (C76)surfactant and (C78)surfactant resulted in the...

PH dependence of the stability constants of copper(I) complexes with fumaric and maleic acids in aqueous solutions

The stability constants of the copper(I) π-complexes with the different acidic forms of fumaric and maleic acids (H 2L, HL − and L 2−) were derived as (0.23, 1.2 and 2.8)×10 4 M −1 (maleic acid) and (0.73, 1.1 and 1.5)×10 4 M −1 (fumaric acid), respectively, by either electrochemical or pulse radiolytic techniques. The influence of electron-donating/withdrawing substituents on the π-ligands are discussed.

Electron transfer to buckminster-fullerenes and functionalized fullerene derivatives in aqueous and protic media, as studied by radiolytic techniques

The primary radical products, namely C60 *− and C70 *− which were formed by reactions with either the solvated electrons or (CH3)2 *C(OH) radicals exhibit distinct absorption bands in the near-IR. Reaction of a water-soluble C60/γ-cyclodextrin complex with α-hydroxyalkyl radicals and hydrated electrons also involves electron transfer, as indicated by the dependence of the rate constants on the redox potential of the reducing species. Pulse radiolysis of micellar C60 solutions in BRIJ 35 and Triton X-100, on the other, exhibited electron transfer from various reducing radicals to the fullerence core. Water soluble fullerence mono-derivatives, e.g. C60[C(COO− 2]2 (1) and C60(C9H11O2)(COO−) (2) did not show any noticeable reactivity towards strongly reducing species which can be ascribed to the formation of clusters in which the hydrophobic fullerence core is shielded by a surrounding layer of negatively charged carboxylate functions. Upon incorporation into γ-cyclodextrin the reduction of 1 and 2 occurs rapidly as indicated by both an accelerated decay of the hydrated electron absorption and the formation of the characteristic near-IR absorption due to (C60 *−[C(COO−)/γ-CD and (C60 *−) (C9H11O2)(COO−)/γ-CD at 1030 nm. The all-equatorial bis- and tris-adducts, e.g. equatorial-C60[C(COO−)2]2 and equatorial-C60[C(COO−)2]3, did not show any evidence with respect to the occurrence of aggregation phenomena and yielded the respective radical anions equatorial-(C60 *−) [C(COO−)2]n in high yields.

Capped Fullerenes: Stabilization of Water-Soluble Fullerene Monomers As Studied by Flash Photolysis and Pulse Radiolysis

Functionalization of C60 with hydrophilic groups promotes the water solubility of the fullerene core which leads in aqueous solution to the irreversible formation of clusters. Capping the surface of water-soluble fullerene derivatives with surfactants (cetyltrimethylammonium chloride or Triton X-100) was found to exclude formation of fullerene clusters and, in turn, to stabilize fullerene monomers. Flash photolytic and pulse radiolytic techniques were employed to generated excited and reduced states of fullerene monomers and fullerene clusters 1−3. Fullerene aggregation was found to have no significant effects on the physicochemical properties related to the generation and lifetime of the excited singlet state. Nanosecond studies of the processes following photoexcitation revealed noticeable differences between monomeric and colloidal derivatives 1−3 for the absorption maximum of the corresponding *T1 → *Tn absorptions. Clustering was found to invoke also remarkable changes for the decay of the fullerene's excited triplet state. Similarly, λmax of the π-radical anions 1−3 is very sensitive to the environmental parameters of the accommodating assembly. In surfactant media, the maxima ([(C60•-)(C4H10N+)]surfactant 1010 nm) were generally blue-shifted relative to the analogous γ-CD complexes ([(C60•-)(C4H10N+)]/γ-CD 1030 nm). On the basis of comparison with fullerene clusters and γ-CD-incorporated complexes, evidence was found that suggests the presence of fullerene monomers in a core(fullerene)−shell(surfactant) type structure.

Pulse Radiolysis in Imaging Sciences: Silver, Silver Halides, and Other Clusters

Pulse radiolysis has been utilized over the last three decades to study a variety of physical and chemical systems, including those relevant to imaging processes. In this overview, we outline the similarities between photolysis and radiolysis and highlight the differences. In particular, we focus on the time-resolved variants of the two disciplines: pulse radiolysis versus flash photolysis. The strength (and weakness) of the radiolytic techniques is their nonspecificity; the energy is always absorbed by the majority medium, the solvent and not the solute. Therefore, once thermalization occurs (&#x226A; 1 ps), the primary reactive intermediates are the same regardless of the solute. From this time on, the chemistry that follows is the chemistry of radicals, radical ions, excited states, metal ions at unstable oxidation state, and other reactive molecular products such as metallic and semiconductor clusters. Thus, radiation chemistry principles that were developed for one discipline are easily transportable to another. The pulse radiolysis technique with a wide arsenal of detection methodologies is currently used to identify short-lived intermediates and to determine their kinetic and thermodynamic properties. Together these studies provide mechanistic insight into the behavior of many chemical and physical systems. We demonstrate the utility of the approach in several areas of interest to imaging sciences, in particular, clustering of silver atoms, growth of silver halides, and medium effects on these systems. Other systems of relevance to imaging sciences include reactivity and redox potentials of quinone and one-electron reduced/oxidized dyes.

Radiolytic Reduction of Tetrapropylporphycene and Its Iron, Cobalt, Nickel, Copper, and Tin Complexes

One-electron and multielectron reductions of 2,7,12,17-tetrapropylporphycene (H2TPrPc) and its Fe, Co, Ni, Cu, and Sn complexes in 2-PrOH solutions have been studied by radiolytic techniques. Formation and decay of intermediates formed upon one-electron reduction have been followed by kinetic spectrophotometric pulse radiolysis, and the absorption spectra of stable reduction products have been recorded following γ-radiolysis. H2TPrPc and SnIVTPrPc are reduced to the π-radical anions and then to the dianions, which are stable in alkaline 2-PrOH. In neutral or acidic solutions, the π-radical anions undergo proton-enhanced disproportionation and the dianions undergo protonation. With the transition metal complexes, redox reactions of FeIII/FeII and CoIII/CoII have been observed. The porphycenes of FeII, CoII, NiII, and CuII are reduced in most cases to form the π-radical anions, MIIPc•-. The exception is the case of the CoIIPc, which forms CoIIPc•- in alkaline 2-PrOH but yields a short-lived CoIPc in neutral 2-PrOH. CuIIPc•- has some CuI character, resulting in rapid demetallation in acidic solutions. The radical anions MIIPc•- also undergo proton-enhanced disproportionation to yield MIIPcH2.

One-Electron Oxidation of Metalloporphycenes As Studied by Radiolytic Methods

One-electron and two-electron oxidations of 2,7,12,17-tetrapropylporphycene (H2TPrPc) and its Fe, Co, Ni, Cu, and Sn complexes in CH2Cl2, CCl4, and 2-PrOH solutions have been studied by radiolytic techniques. Formation and decay of intermediates formed upon one-electron oxidation have been followed by kinetic spectrophotometric pulse radiolysis, and the absorption spectra of stable oxidation products have been recorded following γ-radiolysis. H2TPrPc is oxidized to the π-radical cation and then to the dication, which is stable in aprotic solvents but is transformed to a different product in 2-PrOH. Similar oxidation to the π-radical cation and then to the dication was observed for the CuII, SnIV, CoIII, and FeIII porphycenes. CoII and NiII porphycenes underwent radiolytic oxidation to form stable CoIII and NiIII products. The stability of the latter is in contrast with previous electrochemical observations, and the difference is ascribed to the effect of the axial ligand.