Pulse Radiolysis and Laser Flash Photolysis

Since the heroic early mechanistic investigations, there have been two developments of major significance in radical chemistry. The first was the advent of electron spin resonance (ESR) spectroscopy (and the associated technique of chemically induced dynamic nuclear polarisation, CIDNP) [24], which provided structural as well as kinetic information the second is the more recent development of a wide range of synthetically useful radical reactions [20]. Another recent development, the combination of the pulse radiolysis and laser-flash photolysis techniques, is enormously powerful for the study of radicals but beyond the scope of this book. 

Reactions of OH, 0 and SO " radicals with cresols were studied in detail by pulse radiolysis and laser flash photolysis techniques combined with product analysis.The rate constants of the OH reaction with cresols are very high (fe 1 x 10 ° dm mol s ) whereas 0 was found to be less reactive k 2.4 x 10 dm mol s ). The second-order rate constants of the reaction for SO " reaction with cresols are in the range of (3 to 6) x 10 dm mol s" The transient absorption spectra obtained in the reaction of OH with isomers of cresols have peaks in the region 295-325 nm. Merga et al and Choure et carried out a detailed product analysis by HPLC on radiolysis of chlorotoluenes and cresols. Table 2 lists the products obtained in deoxygenated and oxygenated solutions of these systems. 

The most important recent results obtained by means of time-resolved techniques (pulse radiolysis and laser flash photolysis) on model peptides containing single or multiple Met-residues and in selected naturally occurring peptides (Met-enkephalin and amyloid peptide) and proteins (thioredoxin and calmodulin) have been recently reviewed. ... 

Nauser T, Casi G, Koppenol W, Schoneich C. (2008) Reversible intramolecular hydrogen transfer between cysteine thiyl radicals and glycine and alanine in model peptides Absolute rate constants derived from pulse radiolysis and laser flash photolysis. JPhys Chem B 112 15034-15044. 

Biological Aspects.—The lowest excited triplet states of all-rran.s- -carotene produced by pulse radiolysis has been studied by time-resolved resonance Raman spectroscopy.Six transient Raman bands at 965, 1009, 1125, 1188, 1236, and 1496cm were observed and assigned to the triplet state of ) -carotene. The authors conclude that the molecule may be substantially twisted, presumably at the 15,15 band in the triplet state. Further work has also been carried out by the same workers on the triplet state of all-rran -retinal. The results indicate increased 7r-electron delocalization in the triplet state and propose that the relaxed excited triplet-state exists in either sAX-trans or 9-cis conformation. Das and Becker" have also employed pulse radiolysis and laser flash photolysis to study several photophysical properties of the triplet states of the series of polyenals (29)—(33) related to retinal (31) as homologues (Table 35). Results are presented... 

Using fast time-resolved pulse radiolysis and laser flash photolysis methods, the authors have been able to directly probe the dynamics of electron and triplet exciton transport along the conjugated segments. The results show that transport of both carriers along these organometallic molecular wires is relatively rapid, occurring on timescales less than 200 ps over a... 

Even though the experimental apparatus for pulse radiolysis and laser flash photolysis are very similar their initial effects on the samples are very different. In pulse radiolysis, unlike laser flash photolysis where it is the solute which is excited, the energy from the ionising radiation is absorbed by the most abundant species, which in dilute solutions is the solvent. Upon absorption of the radiation the solvent-derived intermediates can interact with the solute thus forming solute transient intermediates. Hence, in pulse radiolysis the choice of solvent is extremely important in determining the type of species formed. 

A large range of free radicals and other reactive oxygen species (ROS) can be produced biologically and in vivo and a variety of antioxidant species quench these ROS. Pulse radiolysis and laser flash photolysis are useful techniques for producing these radicals and ROS and for studying their reaction mechanisms. 

Table 1, Absorption Maxima of Aryl and DIarylalkene Radical Cations Generated by Pulse Radiolysis or Laser Flash Photolysis in Solution...

The fates of the G(-H) radicals in DNA are mostly determined by reactions with other substrates. Here, we consider the reactions of the G(-H) radicals with types of free radicals that are generated in vivo under conditions of oxidative stress. One of these radicals is the nitrogen dioxide radical, NO2. This radical can be generated in vivo by the oxidation of nitrite, N02, a process that can be mediated by myeloperoxidase [111, 112] as well as by other cellular oxidants [113, 114]. An alternative pathway of the generation of NO2 is the homolysis of peroxynitrite [102, 115] or nitrosoperoxycarbonate formed by the reaction of peroxynitrite with carbon dioxide [99-101]. The redox potential, E°( NO2/NO2")=1.04 V vs NHE [116] is less than that of guanine, E7[G(-H)7G] = 1.29 V vs NHE [8]. Pulse radiolysis [117] and laser flash photolysis [109] experiments have shown that, in agreement with these redox potentials, N02 radicals do not react with intact DNA. However, N02 radicals can oxidize 8-oxo-dG that has a lower redox potential ( 7=0.74 vs NHE [56]) than any of the normal nucleobases [109]. 

Recent flash photolysis spectra for 4,4 -DPE reveal two transients in water-containing alcohols, one species with lmax = 460nm and one with 500 nm the former is assigned to a radical of the type H-DPE, the latter to its protonated form, H2-DPE + [484-486], An example of the absorption spectra of the two radicals of 4,4 -DPE is shown in Figure 19. Previous ESR [487] and recent pulse radiolysis studies [488] confirm this assignment. Formation of radicals by pulse radiolysis and laser photolysis (e.g., photoionization in polar solvents) are useful for the identification of intermediates in photoprocesses bypassing isomerization [172, 489-491]. 

Many experimental methods may be distinguished by whether and how they achieve time resolution—directly or indirectly. Indirect methods avoid the requirement for fast detection methods, either by detemiining relative rates from product yields or by transfonuing from the time axis to another coordinate, for example the distance or flow rate in flow tubes. Direct methods include (laser-) flash photolysis [27], pulse radiolysis [28]... 

Photolytic methods are used to generate atoms, radicals, or other highly reactive molecules and ions for the purpose of studying their chemical reactivity. Along with pulse radiolysis, described in the next section, laser flash photolysis is capable of generating electronically excited molecules in an instant, although there are of course a few chemical reactions that do so at ordinary rates. To illustrate but a fraction of the capabilities, consider the following photochemical processes ... 

Pulse Radiolysis and Pulse Radiolysis-Laser Flash Photolysis... 

Janata E (1992b) Instrumentation of kinetic spectroscopy. 10. A modular data acquisition system for laser flash photolysis and pulse radiolysis experiments. Radiat Phys Chem 40 437-443 Janata E, Lilie J, Martin M (1993) Instrumentation of kinetic spectroscopy. 11. An apparatus for AC-conductivity measurements in laser flash photolysis and pulse radiolysis experiments. Radiat Phys Chem 43 353-356... 

Nasr, C. Vinodgopal, K. Hotchandani, S. Kamat, P. V. Excited state and reduced forms of a textile diazo dye, Naphthol blue black. Spectral characterization using laser flash photolysis and pulse radiolysis. Radiat. Phys. Chem. 1997, 49, 159. 

Tanielian C, Duffy K, Jones A. Kinetic and mechanistic aspects of photocatalysis by polyoxotungstates a laser flash photolysis, pulse radiolysis, and continuous photolysis study. J Phys Chem B 1997 101 4276-82. 

Triplet excited states of four derivatives of ubiquinone-6 (256), in which various ring substituents are progressively altered, have been studied by laser flash photolysis (265 nm) and pulse radiolysis (9—12MeV electrons). The triplet absorption spectra, extinction coefficients, lifetimes, energy levels, and quantum efficiencies of formation were determined.117... 

The initial effects of high energy radiation on condensed systems are discussed. Evidence of short lived intermediates, i.e. radical ions and excited states is illustrated by fast pico-second and nanosecond pulse radiolysis A discusion of the nature of early events leading to excited states is discussed at length, and in particular comparisons are made to corresponding experiments at low photon energies, i.e. via laser flash photolysis. 

Janata E. (1992) Instrumentation of kinetic spectroscopy-7. A precision integrator for measuring the excitation in laser flash photolysis and pulse radiolysis experiments. Radiat Phys Chem 39 315-317. 

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