Method flash photolysis

The kinetics of reactions of NO with ferri- and ferro-heme proteins and models under ambient conditions have been studied by time-resolved spectroscopic techniques. Representative results are summarized in Table I (22-28). Equilibrium constants determined for the formation of nitrosyl complexes of met-myoglobin (metMb), ferri-cytochrome-c (Cyt111) and catalase (Cat) are in reasonable agreement when measured both by flash photolysis techniques (K= konlkQff) and by spectroscopic titration in aqueous media (22). Table I summarizes the several orders of magnitude range of kon and kQs values obtained for ferri- and ferro-heme proteins. Many k0f[ values were too small to determine by flash photolysis methods and were determined by other means. The small values of kQ result in very large equilibrium constants K for the... 

A flash photolysis method has been developed that prepares the MoVI-Fe11 state and thus allows the rate constants k3 and k 3 to be measured. Solutions containing 5-deazariboflavin, semicarbazide, and sulfite oxidase are subjected to 555 nm flash photolysis. The deazariboflavin is excited to a triplet state, which is then reduced by semicarbazide to form the 5-deazariboflavin semiquinone radical. This radical is then rapidly oxidized back to its parent species through the one-electron reduction of sulfite oxidase. 

R. J. Cherry, Measurement of protein rotational diffusion in membranes by flash photolysis, Methods Enzymol. L1X, 47-61 (1978). 

The direct detection of the S <- Sj absorption in organic compounds has so far been achieved by a nanosecond or picosecond laser flash photolysis method. The general features of transient absorption spectra of metalloporphyrins actually suggest the presence of strong absorption bands in visible or ultraviolet region (38-40). However, as the transient absorption of the state often overlaps with that of ground state depletion, it is usually difficult to evaluate the absolute absorption cross sections for the transition by... 

Laser flash photolysis methods have also been applied to the study of nitrenium ion trapping rates and hfetimes. This method relies on short laser pulses to create a high transient concentration of the nitrenium ion, and fast detection technology to characterize its spectrum and lifetime The most frequently used detection method is fast UV-vis spectroscopy. This method has the advantage of high sensitivity, but provides very little specific information about the structure of the species being detected. More recently, time-resolved infrared (TRIR) and Raman spectroscopies have been used in conjunction with flash photolysis methods. These provide very detailed structural information, but suffer from lower detection sensitivity. 

The first nitrenium ion to be examined using any flash photolysis method was the 4-dimethylaminophenylnitrenium ion (131). However, the workers who carried out these experiments apparently did not consider this species to be a nitrenium ion. It was generated by pulsed Xe lamp photolysis from the corresponding azide (132, Fig. 13.64), and was detected through its absorption at 325 nm. As the resonance structure 131 implies, this nitrenium ion (or quinonediimine) is especially stable. In fact, its lifetime exceeds 100 ms at neutral pH, and it decays through hydrolysis of the C=N bond to give iminoquinone (133). 

Using Norrish s flash photolysis method [54], Thrush [55] examined the decomposition of hydrogen azide in the presence of an excess of inert gas. The absorption spectra characteristic of the radicals NH and NH2 were observed. He therefore suggested an alternative scheme for the decomposition of hydrazoic acid, different from that proposed by Beckmann and Dickinson [56] ... 

The flash photolysis method was first described by Norrish and Porter3 and independently developed by at least two other groups... 

Using the standard flash photolysis method to produce (NH3)5Ru(II) (His 48)Mb(Fe(III)), both forward and reverse electron transfer was observed in the system... 

The pulse radiolysis method has been described in detail in some of the early papers (22, 22), in a brief review of the subject (23), and in a current comprehensive review (14). It is, in brief, a fast reaction method in which the external perturbation applied to the system is a microsecond pulse of electrons. The current is sufficiently high to produce an instantaneous concentration of transient species high enough to be observed by fast measurement of the optical absorption. Spectra may be recorded either photographically or spectrophotometrically. The kinetics are studied by fast spectrophotometry. Since a perturbing pulse as short as 0.4 /xsec. has been used, the time resolution has approached 10-7 sec. The flash photolysis method used in some of the other studies (27, 15) has been reviewed in detail (24). 

The eventual products in reaction (1) have been identified as SO and MSA from experiments involving the steady photolysis of mixtures of DMS and a photolytic precursor of OH (4-91 Absolute measurements of lq have been obtained using the discharge-flow method with resonance fluorescence or electron paramagnetic resonance (EPR) detection of OH (10-141. and the flash photolysis method with resonance fluorescence or laser induced fluorescence (LIF) detection of OH (14-181. Competitive rate techniques where Iq is measured relative to the known rate constant for a reaction between OH and a reference organic compound (18-211 have also been employed to determine k at atmospheric pressure of air. 

Using /ra .r-l,l,3,3-tetraphenyl-2,4-dineopentyl-l,3-disilacyclobutane 35a as the precursor, the reactivity of the transient l,l-diphenyl-2-neopentylsilene has been studied in various solvents by laser flash photolysis methods. These experiments verified that the isomeric compounds m-35b and ( )-36 and the expected adduct of MeOH to silene 37 are the only primary products of photolysis of 35a at low (ca. 10%) conversion (Equation 9). Similar experiments with m-35b showed it to be ca. 4 times more photoreactive than the trans-isomer, producing tram-35a, 37, and -36 with initial yields of 69%, 11%, and 19%, respectively <20050M2309>. 

Fig. 8.4. Photochemical reaction scheme of AppA. If the reaction is monitored by the flash photolysis method, the spectrally red-shifted product AppABLUP directly returns to the ground state (broken line). However, using the diffusion detection method, the dimerization reaction takes place after the AppABLUP formation...

The processes of triplet state quenching of dyes K1-K3 and K4 by nitroxyl radical in liquid solutions and in complexes with DNA (cdna = 2.5 x 10 mol l ) were studied by the flash photolysis method. 

Polyamide (6) was irradiated with a single 20 ns flash (530 nm) in A. iV-dimethyl-ac tamide. The kinetics of the cis to trans isomerization of backbone azobenzene residues was followed by time resolved optical absorption, and the subsequent conformation change of the total polymer chain by time resolved light scattering. Before each laser e eriment, the polymer was brought to a compact conformation by eontiimous ultraviolet irradiation, and then the unfolding proce was traced by the laser flash photolysis method. 

The nanosecond-picosecond two-color two-laser flash photolysis method is also useful to study the excited state of radicals, that is, the D, state. We applied nanosecond-picosecond two-color two-laser flash photolysis to detect the absorption and fluorescence spectra of the ketyl radical of benzophenone and its derivatives (BPH and BPDH ) in the excited state in the UV-visible region [111], since BPH and BPDH are well investigated radicals in various fields. Since BPH and BPDH are generated from irreversible ways, such as photoionization, we employed a streak camera to realize the single-shot detection of these intermediates. 

Laser flash photolysis is one of the most efficient methods for the direct spectroscopic observation of free radicals and for monitoring the kinetics of formation and decay in real-time. This method is an extension of conventional flash photolysis method [26] that was invented by Norrish and Porter in 1949, and who were awarded by the Nobel Prize in 1967. We have used this approach to investigate the generation and reactions of free radicals with DNA. In this technique, a laser light pulse is used to produce short-lived intermediates in solution contained in an optical cuvette, and the kinetics of their formation and decay are monitored by transient absorption spectroscopy. The apparatus we used is shown in Figure 4.1. 

In Section 2.11, we discussed fluxional processes in relation to the timescales of NMR and IR spectroscopies. A range of techniques are now available to probe electron-transfer reactions, and the recent development of femtosecond (fs) and picosecond (ps) flash photolysis methods now allow... 

The situation is considerably different for the iron(III) analog (TPPS)Fe "(H20)2 which is hexacoordinate in aqueous solution. In this case the reaction with NO is considerably slower (kon = 4.5 x 10 s ) and kojr (500 s ) is sufficiently large to be measurable by the flash photolysis method as well [92]. Temperature and hydrostatic pressure effects were probed and AH, AS and AV values of 69 + 3 kj mol , 95 10 J mol K and +9+1 cm mol were determined for the "orf reaction and 76 + 6 kJ mol , 60 + 11 ) mol and 18 + 2 cm mol respectively, for the off reaction [93j. The activation parameters for the on reaction compare very favorably with those measured for exchange between coordinated and solvent water for aqueous solutions of (TPPS)Fe "(H20)2 [94] and indicate that kinetics for the reaction of NO with this complex are dominated by the lability of the coordinated water. Furthermore, the large and positive values of AVj and ASj point to a dissociative substitution mechanism as described in Eq. (6.43). 

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