Pulse radiolysis strengths

Rate constants for azurin Ru(NH3)5-modified at His83 have been determined by two methods in which the fully oxidized Ru(III)Cu(II) protein is reduced to Ru(II)Cu(II) by (a) flash photolysis using [Ru(bipy)3] as re-ductant [128], and (b) pulse radiolysis with CO2 as reductant [50]. Intramolecular rate constants of 1.9 + 1.4s (22°C) and 2.5+0.8 s (four runs at 17 °C), respectively are in reasonably satisfactory agreement. Both studies were in 0.10 M phosphate, but solutions for pulse radiolysis also have in addition 0.10 M sodium formate present. Ionic strengths were therefore 0.22 and 0.32 M (the value of 0.31 M in Refs. [50] and [129] is closer to and should be adjusted to the latter value in Ref. [130] an ionic strength of 0.32 M applies to the and O2... 

The hydrated electron is characterized by its strong absorption at 720 nm (e = 1.9 x 104 dm3 mol-1 cm-1 (Hug 1981) the majority of the oscillator strength is derived from optical transitions from the equilibrated s state to the p-like excited state (cf. Kimura et al. 1994 Assel et al. 2000). The 720-nm absorption is used for the determination of its reaction rate constants by pulse radiolysis (for the dynamics of solvation see, e.g Silva et al. 1998 for its energetics see, e.g Zhan et al. 2003). IP only absorbs in the UV (Hug 1981), and rate constants have largely been determined by EPR (Neta et al. 1971 Neta and Schuler 1972 Mezyk and Bartels 1995) and competition techniques (for a compilation, see Buxton et al. 1988). In many aspects, H and eaq behave very similarly, which made their distinction and the identification of eaq" difficult (for early reviews, see Hart 1964 Eiben 1970 Hart and Anbar 1970), and final proof of the existence of the... 

Even if pulse radiolysis has been used [13], most studies of the initial electron entry into cytochrome oxidase are based on the finding that at low ionic strength cytochrome c forms a strong electrostatic complex with the oxidase [14]. Cytochrome c itself cannot be photoactivated, but in the complex with the enzyme it can be reduced by photochemically generated radicals, such as that of 5-diazariboflavin [15]. Another approach has been to employ a photoinduced uroporphyrin/NADH reduction system [16]. 

Using the methylviologen cation radical (MV +) formed by pulse radiolysis, monophasic kinetics of cytochrome reduction are observed with a rate constant of 4.5 X 108 M 1/s (1.1 X 108 M 1/s on a per heme basis) at pH 8.0 with the Hildenborough cytochrome (36). This very fast second-order process approaches the diffusion controlled limit. Moreover, the reverse reaction can be estimated to be 7.8 X 104 M-1/s, which suggests that the reaction takes place primarily with the highest potential heme (the A E 0 between heme I and MV + is 190 mV, consistent with an equilibrium constant of approximately 103). Interestingly, the kinetics with MV + are ionic strength dependent, which is consistent with a plus-plus interaction,... 

Fig. 16. Effect of ionic strength on dismutation rate of O2 by superoxide dismutase as measured experimentally by pulse radiolysis at pH 8. Ionic strengths were adjusted using NaCl. O, Bovine SOD , human SOD expressed in yeast , mutant Cys-6Ala, Cys-lllSer of human SOD expressed in yeast , Thr-137Ile SOD A, Arg-143Lys.

The kinetics of electron transfer reactions between spinach plastocyanin and [Fe(CN)6] ", [Co(phen)3] , and Fe(II) cytochrome c have been studied as a function of ionic strength. Applications of the equations of Van Leeuwen support the proposal of two sites of electron transfer, with [Co(phen)3] binding near residues 42-45 and the interaction of [Fe(CN)6] at a hydrophobic region near the copper ion. Pulse radiolysis has been employed to measure the rates of electron transfer from Ru(II) to Cu(II) in plastocyanins from Anabaena variabilis and Scenedesmus obliquus which have been modified at His-59 by [Ru(NH3)5] . The small intramolecular rates (<0.082 and <0.26 s , respectively) over a donor-acceptor distance of 12 A indicate that electron transfer from the His-59 site to the Cu center is not a preferred pathway. A more favorable route, via the acidic (residues 42-44) patch ( 14 A to Cu), is supported by the rate of >5 x 10 s for the reduction of PCu(II) by unattached [Ru(NH3)5im] . The intramolecular electron transfer from Fe(II) in horse cytochrome c to Cu(II) in French bean plastocyanin ( 12 A from heme edge to Cys-84 S), in a carbodiimide cross-linked covalent complex, proceeds with a rate of 1.05 x 10 s . The presence of the... 

Hydrolysis constants of stable reducible metal ions can be measured by pulse radiolysis combined with conductivity measurements, and this method yields data for low concentrations and ionic strengths. The change in conductance accompanying reactions (13) is largely due to protons formed with elq during the radiation pulse. If M + is partially hydrolysed. 

Thiyl radicals easily add oxygen in glasses even at 77 K, forming thiyl peroxyl radicals, the species, first detected by their optical absorption in pulse radiolysis studies. These species are discussed in more detail along with other sulfoxyls in Chapter 8. Note here that thiol peroxyl radicals are fairly weakly bound species with a S-OO bond energy of only about 7-13 kcal mol" [99-103]. Thus any complexation of comparable strength, as with thiolate anions and neutral thiols... 

The redox interactions of [Os(bipy)3] and [Os(bipy)3] with II and SCNl formed by pulse radiolysis have been studied and the reactions are likely to be outer-sphere in nature since no intermediate adducts were detected. This allows calculation of the one-electron reduction potentials for I2,12, and (SCN)J of 1.063, 0.172, and 1.331 V, respectively, at 22 C and 0.1 M ionic strength. 

The species and their associated stability constants are consistent with those of uranium(VI) and are retained in the present review. Schmidt et al. (1980) used a pulse radiolysis transient conductivity technique to study the hydrolysis of neptu-nium(VI) in a self-medium (very low ionic strength). They could only determine a stability constant for Np020H and obtained a stability constant of between log Pi = -5. 4 to —5.5. This value is considered consistent with that obtained by Lemire et al. (2001) at zero ionic strength from the earlier data of Cassol et al. (1972a). 

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