Selective generation and reactivity of 5'-adenosinyl and 2'-adenosinyl radicals

The reaction of hydrated electrons (e(-)(aq) with 8-bromoadenosine 7 has been investigated by radiolytic methods coupled with product studies. Pulse radiolysis revealed that one-electron reductive cleavage of the C-Br bond gives the C8 radical 8 followed by a fast radical translocation to the sugar moiety. The reaction is partitioned between C5' and C2' positions in a 60:40 ratio leading to 5'-adenosinyl radical 9 and 2'-adenosinyl radical 11. This radical translocation from C8 to different sites of the sugar moiety has also been addressed computationally by means of DFT B3LYP calculations. In addition, ketone 21 was prepared and photolyzed providing an independent generation of C2' radical 11. Both C5' and C2' radicals undergo unimolecular reactions. Radical 9 attacks adenine with a rate constant of 1.0 x 10(4) s(-1) and gives the aromatic aminyl radical 10, whereas C2' radical 11 liberates adenine with a rate constant of 1.1 x 10(5) s(-1).     

The fate of C5' radicals of purine nucleosides under oxidative conditions

The factors that influence the reactivity of C5' radicals in purine moieties under aerobic conditions are unknown not only in DNA, but also in simple nucleosides. 5',8-Cyclopurine lesions are the result of a rapid C5' radical attack to the purine moieties before the reaction with oxygen. These well-known lesions among the DNA modifications were suppressed by the presence of molecular oxygen in solution. Here we elucidate the chemistry of three purine-substituted C5' radicals (i.e., 2'-deoxyadenosin-5'-yl, 2'-deoxyinosin-5'-yl, and 2'-deoxyguanosin-5'-yl) under oxidative conditions using gamma-radiolysis coupled with product studies. 2'-Deoxyadenosin-5'-yl and 2'-deoxyinosin-5'-yl radicals were selectively generated by the reaction of hydrated electrons (e(aq)(-)) with 8-bromo-2'-deoxyadenosine and 8-bromo-2'-deoxyinosine followed by a rapid radical translocation from the C8 to the C5' position. Trapping these two C5' radicals with Fe(CN)6(3-) gave corresponding hydrated 5'-aldehydes in good yields that were isolated and fully characterized. When an oxygen concentration in the range of 13-266 microM (typical oxygenated tissues) is used, the hydrated 5'-aldehyde is accompanied by the 5',8-cyclopurine nucleoside. The formation of 5',8-cyclopurines is relevant in all experiments, and the yields increased with decreasing O2 concentration. The reaction of HO(*) radicals with 2'-deoxyadenosine and 2'-deoxyguanosine under normoxic conditions was also investigated. The minor path of C5' radicals formation was found to be ca. 10% by quantifying the hydrated 5'-aldehyde in both experiments. Rate constants for the reactions of the 2'-deoxyadenosin-5'-yl with cysteine and glutathione in water were determined by pulse radiolysis to be (2.1 +/- 0.5) x 10(7) and (4.9 +/- 0.6) x 10(7) M(-1) s(-1) at 22 degrees C, respectively.     

Pulse radiolysis study on free radical scavenger edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one)

The reactions between edaravone and various one-electron oxidants such as (*)OH, N(3)(*), Br(2)(-), and SO(4)(-), have been studied by pulse radiolysis techniques. The transient species produced by the reaction of edaravone with (*)OH radical shows an absorption band with lambda(max)=320 nm, while the oxidation by N(3)(*), Br(2)(-), SO(4)(-) and CCl(3)OO(*) results in an absorption band with lambda(max)=345 nm. Different from the previous reports, the main transient species by the reaction of edaravone with (*)OH radical in the absence of O(2) is attributed to OH-adducts. At neutral condition (pH 7), the rate constants of edaravone reacting with (*)OH, N(3)(*), SO(4)(-), CCl(3)OO(*), and e(aq)(-) are estimated to be 8.5x10(9), 5.8x10(9), 6x10(8), 5.0x10(8) and 2.4x10(9)dm(3)mol(-1)s(-1), respectively. From the pH dependence on the formation of electron adducts and on the rate constant of edaravone with hydrated electron, the pK(a) of edaravone is estimated to be 6.9+/-0.1.     

Chemical radiation studies of 8-bromo-2'-deoxyinosine and 8-bromoinosine in aqueous solutions

The reactions of hydrated electrons (e(aq) (-)) with 8-bromo-2'-deoxyinosine (8) and 8-bromoinosine (12) have been investigated by radiolytic methods coupled with product studies and have been addressed computationally by means of BB1K-HMDFT calculations. Pulse radiolysis revealed that one-electron reductive cleavage of the C--Br bond gives the C8 radical 9 or 13 followed by a fast radical translocation to the sugar moiety. Selective generation of a C5' radical occurs in the 2'-deoxyribo derivative, whereas in the ribo analogue the reaction is partitioned between the C5' and C2' positions with similar rates. Both C5' radicals undergo cyclizations, 10-->11 and 14-->15, with rate constants of 1.4 x 10(5) and of 1.3 x 10(4) s(-1), respectively. The redox properties of radicals 10 and 11 have also been investigated. A synthetically useful photoreaction has also been developed as a one-pot procedure that allows the conversion of 8 to 5',8-cyclo-2'-deoxyinosine in a high yield and a diastereoisomeric ratio (5'R)/(5'S) of 4:1. The present results are compared with data previously obtained for 8-bromoadenine and 8-bromoguanine nucleosides. Theory suggests that the behavior of 8-bromopurine derivatives with respect to solvated electrons can be attributed to differences in the energy gap between the pi*- and sigma*-radical anions.     

Cyclic voltammetry of the electron transfer reaction between bis(cyclopentadienyl)iron in 1,2-dichloroethane and hexacyanoferrate in water.

The electron transfer (ET) reaction between bis(cyclopentadienyl)iron(II) ([Fe(II)(C(5)H(5))2]) in 1,2-dichloroethane (1,2-DCE) and hexacyanoferrate redox couple ([Fe(II/III)(CN)6](4-/3-)) in water (W) at the interface has been studied by using cyclic voltammetry. The voltammetric results can be explained well by a theoretical equation for the so-called IT-mechanism, in which a homogeneous ET reaction between [Fe(C(5)H(5))2] (partially distributed from 1,2-DCE) and [Fe(CN)6](3-) takes place in the W phase and the resultant [Fe(C(5)H(5))2]+ ion is responsible for current passage across the interface. The forward rate constant of the homogeneous ET reaction, [Fe(C(5)H(5))2] + [Fe(CN)6](3-) = [Fe(C(5)H(5))2]+ + [Fe(CN)6](4-) in W phase, k(f)(IT), was determined to be (2.9 +/- 2.2)x 10(10) M(-1) s(-1), which was in good agreement with k(f)(IT) = (3.2 +/- 2.0)x 10(10) M(-1) s(-1), which had been determined by using normal-pulse voltammetry.     

Reduction of octacyanomolybdate(V) by thioglycolic acid in aqueous media

In aqueous media at 25 degrees C [Mo(CN)(8)](3-) is reduced by thioglycolic acid (HSCH(2)COOH, TGA), and the reaction is strongly accelerated by the presence of trace amounts of copper ions. Dipicolinic acid (dipic) is an effective inhibitor of the copper catalysis. Both with and without dipic the reaction has the stoichiometry 2[Mo(CN)(8)](3-) + 2TGA --> 2[Mo(CN)(8)](4-) + RSSR, where RSSR is the disulfide derived from formal oxidative dimerization of TGA. In the presence of dipic, PBN (N-tert-butyl-alpha-phenyl-nitrone), and with a large excess of TGA the rate law for consumption of [Mo(CN)(8)](3-) is first order in both [TGA] and [Mo(CN)(8)(3-)]. The complex pH dependence is consistent with (-)SCH(2)CO(2)(-) being highly reactive (k = 1.8 x 10(4) M(-1) s(-1)), the monoanion being less reactive, and HSCH(2)CO(2)H being unreactive. A mechanism is proposed in which the dianion undergoes electron transfer to [Mo(CN)(8)](3-), thus generating the thiyl radical. Analysis of the electron-transfer rate constant in terms of Marcus theory yields an effective self-exchange rate constant for the thiolate/thiyl redox couple that is in reasonable agreement with the value derived previously from the reaction of TGA with [IrCl(6)](2-). When copper catalysis is inhibited, the two reactions differ substantially in that the yield of (-)O(3)SCH(2)CO(2)(-) is significant for [IrCl(6)](2-) but undetectable for [Mo(CN)(8)](3-).     

Reaction of HO* with guanine derivatives in aqueous solution: formation of two different redox-active OH-adduct radicals and their unimolecular transformation reactions. Properties of G(-H)*

The reaction of *OH with 2'-deoxyguanosine yields two transient species, both identified as OH adducts (G*-OH), with strongly different reactivity towards O2, or other oxidants, or to reductants. One of these, identified as the OH adduct at the C-8 position (yield 17% relative to *OH), reacts with oxygen with k=4 x 10(9)M(-1)s(-1); in the absence of oxygen it undergoes a rapid ring-opening reaction (k = 2 x 10(5) s(-1) at pH4-9), visible as an increase of absorbance at 300-310 nm. This OH adduct and its ring-opened successor are one-electron reductants towards, for example, methylviologen or [Fe(III)(CN)6]3-. The second adduct, identified as the OH adduct at the 4-position (yield of 60-70% relative to *OH), has oxidizing properties (towards N,N,N',N'-tetra-methyl-p-phenylenediamine, promethazine, or [Fe(II)(CN)6]4-). This OH adduct undergoes a slower transformation reaction (k = 6 x 10(3) s(-1) in neutral, unbuffered solution) to produce the even more strongly oxidizing (deprotonated, depending on pH) 2'-deoxyguanosine radical cation, and it practically does not react with oxygen (k< or = 10(6)M(-1)s(-1)). The (deprotonated) radical cation, in dilute aqueous solution, does not give rise to 8-oxoguanosine as a product. However, it is able to react with ribose with k< or =4 x 10(3)M(-1)S(-1).     

Formation of spectral intermediate G-C and A-T anion complex in duplex DNA studied by pulse radiolysis

The dynamics of electron adducts of 2'-deoxynucleotides and oligonucelotides (ODNs) were measured spectroscopically by nanosecond pulse radiolysis. The radical anions of the nucleotides were produced within 10 ns by the reaction of hydrated electrons (e(aq)(-)) and were protonated to form the corresponding neutral radicals. At pH 7.0, the radical anion of deoxythymidine (dT(*-)) was protonated to form the neutral radical dT(H)(*) in the time range of microseconds. The rate constant for the protonation was determined as 1.8 x 10(10) M(-1) s(-1). In contrast, the neutral radical of dC(H)(*) was formed immediately after the pulse, suggesting that the protonation occurs within 10 ns. The transient spectra of excess electrons of the double-stranded ODNs 5'-TAATTTAATAT-3' (AT) and 5'-CGGCCCGGCGC-3' (GC) differed from those of pyrimidine radicals (C and T) and their composite. In contrast, the spectra of the electron adducts of the single-stranded ODNs GC and AT exhibited characteristics of C and T, respectively. These results suggest that, in duplex ODNs, the spectral intermediates of G-C and A-T anions complex were formed. On the microsecond time scale, the subsequent changes in absorbance of the ODN AT had a first-order rate constant of 4 x 10(4) s(-1), reflecting the protonation of T.     

Cation-independent electron transfer between ferricyanide and ferrocyanide ions in aqueous solution

Electron transfer between Fe(CN)(6)(3-) and Fe(CN)(6)(4-) in homogeneous aqueous solution with K(+) as the counterion normally proceeds almost exclusively by a K(+)-catalyzed pathway, but this can be suppressed, and the direct Fe(CN)(6)(3)(-)-Fe(CN)(6)(4-) electron transfer path exposed, by complexing the K(+) with crypt-2.2.2 or 18-crown-6. Fe((13)CN)(6)(4-)-NMR line broadening measurements using either crypt-2.2.2 or (with extrapolation to zero uncomplexed [K(+)]) 18-crown-6 gave consistent values for the rate constant and activation volume (k(0) = (2.4 +/- 0.1) x 10(2) L mol(-1) s(-1) and Delta V(0) = -11.3 +/- 0.3 cm(3) mol(-1), respectively, at 25 degrees C and ionic strength I = 0.2 mol L(-1)) for the uncatalyzed electron transfer path. These values conform well to predictions based on Marcus theory. When [K(+)] was controlled with 18-crown-6, the observed rate constant k(ex) was a linear function of uncomplexed [K(+)], giving k(K) = (4.3 +/- 0.1) x 10(4) L(2) mol(-2) s(-1) at 25 degrees C and I = 0.26 mol L(-1) for the K(+)-catalyzed pathway. When no complexing agent was present, k(ex) was roughly proportional to [K(+)](total), but the corresponding rate constant k(K)' (=k(ex)/[K(+)](total)) was about 60% larger than k(K), evidently because ion pairing by hydrated K(+) lowered the anion-anion repulsions. Ionic strength as such had only a small effect on k(0), k(K), and k(K)'. The rate constants commonly cited in the literature for the Fe(CN)(6)(3-/4-) self-exchange reaction are in fact k(K)'[K(+)](total) values for typical experimental [K(+)](total) levels.     

Kinetics and mechanism of the interaction of nitric oxide with pentacyanoferrate(II). Formation and dissociation of [Fe(CN)5NO]3 -

The interaction of NO with [Fe(CN)(5)H(2)O](3)(-) (generated by aquation of the corresponding ammine complex) to produce [Fe(CN)(5)NO](3)(-) was studied by UV-vis spectrophotometry. The reaction product is the well characterized nitrosyl complex, described as a low-spin Fe(II) bound to the NO radical. The experiments were performed in the pH range 4-10, at different concentrations of NO, temperatures and pressures. The rate law was first-order in each of the reactants, with the specific complex-formation rate constant, k(f)( )()= 250 +/- 10 M(-)(1) s(-)(1) (25.4 degrees C, I = 0.1 M, pH 7.0), DeltaH(f)() = 70 +/- 1 kJ mol(-)(1), DeltaS(f)() = +34 +/- 4 J K(-)(1) mol(-)(1), and DeltaV(f)() = +17.4 +/- 0.3 cm(3) mol(-)(1). These values support a dissociative mechanism, with rate-controlling dissociation of coordinated water, and subsequent fast coordination of NO. The complex-formation process depends on pH, indicating that the initial product [Fe(CN)(5)NO](3)(-) is unstable, with a faster decomposition rate at lower pH. The decomposition process is associated with release of cyanide, further reaction of NO with [Fe(CN)(4)NO](2)(-), and formation of nitroprusside and other unknown products. The decomposition can be prevented by addition of free cyanide to the solutions, enabling a study of the dissociation process of NO from [Fe(CN)(5)NO](3)(-). Cyanide also acts as a scavenger for the [Fe(CN)(5)](3)(-) intermediate, giving [Fe(CN)(6)](4)(-) as a final product. From the first-order behavior, the dissociation rate constant was obtained as k(d) = (1.58 +/- 0.06) x 10(-)(5) s(-)(1) at 25.0 degrees C, I = 0.1 M, and pH 10.2. Activation parameters were found to be DeltaH(d)() = 106.4 +/- 0.8 kJ mol(-)(1), DeltaS(d)() = +20 +/- 2 J K(-)(1) mol(-)(1), and DeltaV(d)() = +7.1 +/- 0.2 cm(3) mol(-)(1), which are all in line with a dissociative mechanism. The low value of k(d) as compared to values for the release of other ligands L from [Fe(II)(CN)(5)L](n)()(-) suggests a moderate to strong sigma-pi interaction of NO with the iron(II) center. It is concluded that the release of NO from nitroprusside in biological media does not originate from [Fe(CN)(5)NO](3)(-) produced on reduction of nitroprusside but probably proceeds through the release of cyanide and further reactions of the [Fe(CN)(4)NO](2)(-) ion.     

(NEt(4))(2)[Fe(CN)(2)(CO)('S(3)')]: an iron thiolate complex modeling the [Fe(CN)(2)(CO)(S-Cys)(2)] site of [NiFe] hydrogenase centers

In the search for complexes modeling the [Fe(CN)(2)(CO)(cysteinate)(2)] cores of the active centers of [NiFe] hydrogenases, the complex (NEt(4))(2)[Fe(CN)(2)(CO)('S(3)')] (4) was found ('S(3)'(2-)=bis(2-mercaptophenyl)sulfide(2-)). Starting complex for the synthesis of 4 was [Fe(CO)(2)('S(3)')](2) (1). Complex 1 formed from [Fe(CO)(3)(PhCH=CHCOMe)] and neutral 'S(3)'-H(2). Reactions of 1 with PCy(3) or DPPE (1,2-bis(diphenylphosphino)ethane) yielded diastereoselectively [Fe(CO)(2)(PCy(3))('S(3)')] (2) and [Fe(CO)(dppe)('S(3)')] (3). The diastereoselective formation of 2 and 3 is rationalized by the trans influence of the 'S(3)'(2-) thiolate and thioether S atoms which act as pi donors and pi acceptors, respectively. The trans influence of the 'S(3)'(2-) sulfur donors also rationalizes the diastereoselective formation of the C(1) symmetrical anion of 4, when 1 is treated with four equivalents of NEt(4)CN. The molecular structures of 1, 3 x 0.5 C(7)H(8), and (AsPh(4))(2)[Fe(CN)(2)(CO)('S(3)')] x acetone (4 a x C(3)H(6)O) were determined by X-ray structure analyses. Complex 4 is the first complex that models the unusual 2:1 cyano/carbonyl and dithiolate coordination of the [NiFe] hydrogenase iron site. Complex 4 can be reversibly oxidized electrochemically; chemical oxidation of 4 by [Fe(Cp)(2)PF(6)], however, led to loss of the CO ligand and yielded only products, which could not be characterized. When dissolved in solvents of increasing proton activity (from CH(3)CN to buffered H(2)O), complex 4 exhibits drastic nu(CO) blue shifts of up to 44 cm(-1), and relatively small nu(CN) red shifts of approximately 10 cm(-1). The nu(CO) frequency of 4 in H(2)O (1973 cm(-1)) is higher than that of any hydrogenase state (1952 cm(-1)). In addition, the nu(CO) frequency shift of 4 in various solvents is larger than that of [NiFe] hydrogenase in its most reduced or oxidized state. These results demonstrate that complexes modeling properly the nu(CO) frequencies of [NiFe] hydrogenase probably need a [Ni(thiolate)(2)] unit. The results also demonstrate that the nu(CO) frequency of [Fe(CN)(2)(CO)(thiolate)(2)] complexes is more significantly shifted by changing the solvent than the nu(CO) frequency of [NiFe] hydrogenases by coupled-proton and electron-transfer reactions. The "iron-wheel" complex [Fe(6)[Fe('S(3)')(2)](6)] (6) resulting as a minor by-product from the recrystallization of 2 in boiling toluene could be characterized by X-ray structure analysis.     

The reactions of cytidine and 2'-deoxycytidine with SO4.- revisited. Pulse radiolysis and product studies

The reactions of SO4.- with 2'-deoxycytidine 1a and cytidine 1b lead to very different intermediates (base radicals with 1a, sugar radicals with 1b). The present study provides spectral and kinetic data for the various intermediates by pulse radiolysis as well as information on final product yields (free cytosine). Taking these and literature data into account allows us to substantiate but also modify in essential aspects the current mechanistic concept (H. Catterall, M. J. Davies and B. C. Gilbert, J. Chem. Soc., Perkin Trans. 2, 1992, 1379). SO4.- radicals have been generated radiolytically in the reaction of peroxodisulfate with the hydrated electron (and the H. atom). In the reaction of SO4.- with 1a (k = 1.6 x 10(9) dm3 mol-1 s-1), a transient (lambda max = 400 nm, shifted to 450 nm at pH 3) is observed. This absorption is due to two intermediates. The major component (lambda max approximately 385 nm) does not react with O2 and has been attributed to an N-centered radical 4a formed upon sulfate release and deprotonation at nitrogen. The minor component, rapidly wiped out by O2, must be due to C-centered OH-adduct radical(s) 6a and/or 7a suggested to be formed by a water-induced nucleophilic replacement. These radicals decay by second-order kinetics. Free cytosine is only formed in low yields (G = 0.14 x 10(-7) mol J-1 upon electron-beam irradiation). In contrast, 1b gives rise to an intermediate absorbing at lambda max = 530 nm (shifted to 600 nm in acid solution) which rapidly decays (k = 6 x 10(4) s-1). In the presence of O2, the decay is much faster (k approximately 1.3 x 10(9) dm3 mol-1 s-1) indicating that this species must be a C-centered radical. This has been attributed to the C(5)-yl radical 8 formed upon the reaction of the C(2')-OH group with the cytidine SO4(.-)-adduct radical 2b. This reaction competes very effectively with the corresponding reaction of water and the release of sulfate and a proton generating the N-centered radical. Upon the decay of 8, sugar radical 11 is formed with the release of cytosine. The latter is formed with a G value of 2.8 x 10(-7) mol J-1 (85% of primary SO4.-) at high dose rates (electron beam irradiation). At low dose rates (gamma-radiolysis) its yield is increased to 7 x 10(-7) mol J-1 due to a chain reaction involving peroxodisulfate and reducing free radicals. Phosphate buffer prevents the formation of the sugar radical at the SO4(.-)-adduct stage by enhancing the rate of sulfate release by deprotonation of 2b and also by speeding up the decay of the C(5)-yl radical into another (base) radical. Cytosine release in cytidine is mechanistically related to strand breakage in poly(C). Literature data on the effect of dioxygen on strand breakage yields in poly(C) induced by SO4.- (suppressed) and upon photoionisation (unaltered) lead us to conclude that in poly(C) and also in the present system free radical cations are not involved to a major extent. This conclusion modifies an essential aspect of the current mechanistic concept.     

Kinetics of electron attachment to SF3CN, SF3C6F5, and SF3 and mutual neutralization of Ar+ with CN- and C6F5(-)

The additions of two sulfur fluoride derivatives (SF(3)C(6)F(5) and SF(3)CN) to a flowing afterglow were studied by variable electron and neutral density mass spectrometry. Data collection and analysis were complicated by the high reactivity of the neutral species. Both species readily dissociatively attach thermal electrons at 300 K to yield SF(3) + X(-) (X = C(6)F(5), CN). Attachment to SF(3)C(6)F(5) also results in SF(3)(-) + C(6)F(5) as a minor product channel. The determined electron attachment rate constants were 1(-0.6) (+1) × 10(-7) cm(3) s(-1) for SF(3)C(6)F(5), a lower limit of 1 × 10(-8) cm(3) s(-1) for SF(3)CN, and 4 ± 3 × 10(-9) cm(3) s(-1) for SF(3). Mutual neutralization rate constants of C(6)F(5)(-) and CN(-) with Ar(+) at 300 K were determined to be 5.5(-1.6) (+1.0) × 10(-8) and 3.0 ± 1 × 10(-8) cm(3) s(-1), respectively.     

Product and pulse radiolysis studies on radical-ion splitting of N(1)-C(5')-linked dimer hydrates of 5-substituted uracils by one-electron reduction in anoxic aqueous solution

Steady-state gamma-radiolysis, pulse radiolysis, and cyclic voltammetry have been performed to identify the mechanism by which N(1)-C(5')-linked homodimer hydrates [1-(6'-hydroxy-5',6'-dihydrothymin-5'-yl)thymine (2a) and [1-(5'-fluoro-6'-hydroxy-5',6'-dihydrouracil-5'-yl)-5-fluorouracil (2b)], N(1)-C(6')-linked dimer hydrate [1-(5'-hydroxy-5',6'-dihydrothymin-6'-yl)thymine (3a)], and N(1)-C(5')-linked heterodimer hydrate [1-(6'-hydroxy-5',6'-dihydrothymin-5'-yl)-5-fluorouracil (2ba)] undergo radiolytic reductive splitting to regenerate the parent monomers in anoxic aqueous solution. Radiolytic reductions of the thymine homodimer hydrates 2a and 3a by hydrated electrons (e(aq)-) regenerated the parent thymine (1a) almost quantitatively, while the 5-fluorouracil homodimer hydrates cis-2b and trans-2b afforded 1-(uracil-5'-yl)-5-fluorouracil efficiently along with a small amount of the parent 5-fluorouracil (1b). In contrast to 2b, the heterodimer hydrate analogue 2ba with noneliminating 5'-methyl substituent releases 5-fluorouracil 1b almost quantitatively in the radiolytic reduction. The pulse radiolysis studies suggested that the electron adducts are produced primarily at the thymine and 5-fluorouracil structural unit in the dimer hydrates 2a,b, respectively, in which the resulting dimer hydrate radical anion of 2b (2b*-) was more stable than that of 2a (2a*-). As characterized by pulse radiolysis and cyclic voltammetry, the 5-fluorouracil homodimer hydrate 2b bearing F-substituent at C(5') undergoes one-electron reduction to eliminate exclusively fluoride ion along with the formation of dimer hydrate C(5') radical (2b(-F)*) with oxidizing property. The formation of a possible dimer hydrate radical intermediate 2b(-F)* was also supported by the effect of amines as the reducing additives on the yields of 1b and 4b in the radiolytic reduction of 2b.     

Synthesis, structures, and reactivity of weakly coordinating anions with delocalized borate structure: the assessment of anion effects in metallocene polymerization catalysts.

The formation of adducts of tris(pentafluorophenyl)borane with strongly coordinating anions such as CN(-) and [M(CN)(4)](2)(-) (M = Ni, Pd) is a synthetically facile route to the bulky, very weakly coordinating anions [CN[B(C(6)F(5))(3)](2)](-) and [M[CNB(C(6)F(5))(3)](4)](2-) which are isolated as stable NHMe(2)Ph(+) and CPh(3)(+) salts. The crystal structures of [CPh(3)][CN[B(C(6)F(5))(3)](2)] (1), [CPh(3)][ClB(C(6)F(5))(3)] (2), [NHMe(2)Ph](2)[Ni[CNB(C(6)F(5))(3)](4)].2Me(2)CO (4b.2Me(2)CO), [CPh(3)](2)[Ni[CNB(C(6)F(5))(3)](4)].2CH(2)Cl(2) (4c.2CH(2)Cl(2)), and [CPh(3)](2)[Pd[CNB(C(6)F(5))(3)](4)].2CH(2)Cl(2) (5c.2CH(2)Cl(2)) are reported. The CN stretching frequencies in 4 and 5 are shifted by approximately 110 cm(-1) to higher wavenumbers compared to the parent tetracyano complexes in aqueous solution, although the M-C and C-N distances show no significant change on B(C(6)F(5))(3) coordination. Zirconocene dimethyl complexes L(2)ZrMe(2) [L(2) = Cp(2), SBI = rac-Me(2)Si(Ind)(2)] react with 1, 4c or 5c in benzene solution at 20 degrees C to give the salts of binuclear methyl-bridged cations, [(L(2)ZrMe)(2)(mu-Me)][CN[B(C(6)F(5))(3)](2)] and [(L(2)ZrMe)(2)(mu-Me)](2)[M[CNB(C(6)F(5))(3)](4)]. The reactivity of these species in solution was studied in comparison with the known [[(SBI)ZrMe](2)(mu-Me)][B(C(6)F(5))(4)]. While the latter reacts with excess [CPh(3)][B(C(6)F(5))(4)] in benzene to give the mononuclear ion pair [(SBI)ZrMe(+).B(C(6)F(5))(4)(-)] in a pseudo-first-order reaction, k = 3 x 10(-4) s(-1), [(L(2)ZrMe)(2)(mu-Me)][CN[B(C(6)F(5))(3)](2)] reacts to give a mixture of L(2)ZrMe(mu-Me)B(C(6)F(5))(3) and L(2)ZrMe(mu-NC)B(C(6)F(5))(3). Recrystallization of [Cp' '(2)Zr(mu-Me)(2)AlMe(2)][CN[B(C(6)F(5))(3)](2)] affords Cp' '(2)ZrMe(mu-NC)B(C(6)F(5))(3) 6, the X-ray structure of which is reported. The stability of [(L(2)ZrMe)(2)(mu-Me)](+)X(-) decreases in the order X = [B(C(6)F(5))(4)] > [M[CNB(C(6)F(5))(3)](4)] > [CN[B(C(6)F(5))(3)](2)] and increases strongly with the steric bulk of L(2) = Cp(2) < SBI. Activation of (SBI)ZrMe(2) by 1 in the presence of AlBu(i)(3) gives extremely active ethene polymerization catalysts. Polymerization studies at 1-7 bar monomer pressure suggest that these, and by implication most other highly active ethene polymerization catalysts, are strongly mass-transport limited. By contrast, monitoring propene polymerization activities with the systems (SBI)ZrMe(2)/1/AlBu(i)(3) and CGCTiMe(2)/1/AlBu(i)(3) at 20 degrees C as a function of catalyst concentration demonstrates that in these cases mass-transport limitation is absent up to [metal] approximately 2 x 10(-5) mol L(-1). Propene polymerization activities decrease in the order [CN[B(C(6)F(5))(3)](2)](-) > [B(C(6)F(5))(4)](-) > [M[CNB(C(6)F(5))(3)](4)](2-) > [MeB(C(6)F(5))(3)](-), with differences in activation barriers relative to [CN[B(C(6)F(5))(3)](2)](-) of DeltaDeltaG = 1.1 (B(C(6)F(5))(4)(-)), 4.1 (Ni[CNB(C(6)F(5))(3)](4)(2-)) and 10.7-12.8 kJ mol(-)(1) (MeB(C(6)F(5))(3)(-)). The data suggest that even in the case of very bulky anions with delocalized negative charge the displacement of the anion by the monomer must be involved in the rate-limiting step.     

Potentiometric flow injection determination of amylase activity by using hexacyanoferrate(III)-hexacyanoferrate(II) potential buffer

A highly sensitive potentiometric flow injection determination of amylase activity was carried out, utilizing a redox reaction of hexacyanoferrate(III) in alkaline media with reducing sugar as product of the enzymatic hydrolysis reaction of starch with amylase. The analytical method is based on the potential change detection of a flow-through type redox electrode detector due to the composition change of a [Fe(CN)(6)](3-)-[Fe(CN)(6)](4-) potential buffer solution, which is caused by the redox reaction with the product of the enzymatic reaction. A linear relationship exists between the potential change (peak height) and the activity of amylase. Amylase of a wide activity range from 2.5x10(-2) to 1.2x10(-4) U ml(-1) can be determined by the changing the concentrations of the [Fe(CN)(6)](3-)-[Fe(CN)(6)](4-) potential buffer from 10(-3) to 10(-5) M. The lower detection limit of amylase activity is 6.0x10(-5) U ml(-1). The sampling rate and relative standard deviation are 15 h(-1) and 0.9% (n=5) for 3.8x10(-3) U ml(-1) of amylase. The present method was successfully applied to determine amylase activity in real samples (commercial digestive medicines) with an accuracy of 4% compared with analytical results obtained using the present method with those achieved using the conventional titration method.     

Mechanistic control of product selectivity. Reactions between cis-/trans cis-/trans-[OsVI(tpy)(Cl)2(N)]+ and triphenylphosphine sulfide

Reactions between the Os(VI)-nitrido complexes cis- and trans-[Os(VI)(tpy)(Cl)2(N)]+ (tpy is 2,2':6',2"-terpyridine) and triphenylphosphine sulfide, SPPh3, give the corresponding Os(IV)-phosphoraniminato, [Os(IV)(tpy)(Cl)2(NPPh3)]+, and Os(II)-thionitrosyl, [Os(II)(tpy)(Cl)2(NS)]+, complexes as products. The Os-N bond length and Os-N-P angle in cis-[Os(IV)(tpy)(Cl)2(NPPh3)](PF6) are 2.077(6) A and 138.4(4) degrees. The rate law for formation of cis- and trans-[Os(IV)(tpy)(Cl)2(NPPh3)]+ is first order in both [Os(VI)(tpy)(Cl)2(N)]+ and SPPh3 with ktrans(25 degrees C, CH3CN) = 24.6 +/- 0.6 M(-1) s(-1) and kcis(25 degrees C, CH3CN) = 0.84 +/- 0.09 M(-1) s(-1). As found earlier for [Os(II)(tpm)(Cl)2(NS)]+, both cis- and trans-[Os(II)(tpy)(Cl)2(NS)]+ react with PPh3 to give [Os(IV)(tpy)(Cl)2(NPPh3)]+ and SPPh3. For both complexes, the reaction is first order in each reagent with ktrans(25 degrees C, CH3CN) = (6.79 +/- 0.08) x 10(2) M(-1) s(-1) and kcis(25 degrees C, CH3CN) = (2.30 +/- 0.07) x 10(2) M(-1) s(-1). The fact that both reactions occur rules out mechanisms involving S atom transfer. These results can be explained by invoking a common intermediate, [Os(IV)(tpy)(Cl)2(NSPPh3)]+, which undergoes further reaction with PPh3 to give [Os(IV)(tpy)(Cl)2(NPPh3)]+ and SPPh3 or with [Os(VI)(tpy)(Cl)2(N)]+ to give [Os(IV)(tpy)(Cl)2(NPPh3)]+ and [Os(II)(tpy)(Cl)2(NS)]+.     

Independent generation of C5'-nucleosidyl radicals in thymidine and 2'-deoxyguanosine

The synthesis of the C5' tert-butyl ketone of thymidine 1a and 2'-deoxyguanosine 2 is achieved by reaction of 5'-C-cyano derivatives with tert-butyl lithium followed by acid hydrolysis. The 5'R configuration is assigned by X-ray crystal structure determination of an opportunely protected derivative of 1a. The (5'S)-isomers of both nucleosides are not stable, and a complete decomposition occurs in the reaction medium. The photochemistry of 1a and 2 effectively produced the thymidin-5'-yl radical and the 2'-deoxyguanosin-5'-yl radical, respectively. In the thymidine system, the C5' radical is fully quenched in the presence of a physiological concentration of thiols. In the 2'-deoxyguanosine system, the C5' radical undergoes intramolecular attack onto the C8-N7 double bond of guanine leading ultimately to the 5',8-cyclo-2'-deoxyguanosine derivative. The cyclization of the 2'-deoxyguanosin-5'-yl radical occurs with a rate constant of ca. 1x10(6) s-1 and is highly stereoselective affording only the (5'S)-diastereomer.     

Photoinduced energy transfer coupled to charge separation in a Ru(II)-Ru(II)-acceptor triad

The bichromophoric system Ru-Ru(C)-PI ([(bpy)3Ru-Ph-Ru(dpb)(Metpy-PI)][PF6]3, where bpy is 2,2'-bipyridine, Hdpb is 1,3-di(2-pyridyl)-benzene, Metpy is 4'-methyl-2,2':6',2' '-terpyridine and PI is pyromellitimide) containing two Ru(II) polypyridyl chromophores with a N6 and a N5C ligand set, respectively, was synthesized and characterized. Its photophysical properties were investigated and compared to those of the monochromophoric cyclometalated complexes Ru(C)-PI ([Ru(dpb)(Metpy-PI)][PF6]), Ru(C)-phi-PI ([Ru(dpb)(ttpy-PI)][PF6], ttpy is 4'-p-tolyl-2,2':6',2' '-terpyridine), Ru(C)-phi ([Ru(dpb)(ttpy)][PF6]), and Ru(C) ([Ru(dpb)(Metpy)][PF6]). Excitation of the Ru(C) unit in the dyads leads to oxidative quenching, forming the Ru(C)(III)-phi-PI*- and Ru(C)(III)-Pl.- charge-separated (CS) states with k(f)(ET) = 7.7 x 10(7) s(-1) (CH3CN, 298 K) in the tolyl-linked Ru(C)-phi-PI and k(f)(ET) = 4.4 x 10(9) s(-1) (CH2Cl2, 298 K) in the methylene-linked Ru(C)-PI. In the Ru-Ru(C)-PI triad, excitation of the Ru(C) chromophore leads to dynamics similar to those in the Ru(C)-PI dyad, generating the Ru(II)-Ru(C)(III)-PI*- CS state, whereas excitation of the Ru unit results in an initial energy transfer (k(EnT) = 4.7 x 10(11) s(-1)) to the cyclometalated Ru(C) unit. Subsequent electron transfer to the PI acceptor results in the formation of the same Ru(II)-Ru(C)(III)-PI*- CS state with k(f)(ET) = 5.6 x 10(9) s(-1) that undergoes rapid recombination with k(b)(ET) = 1 x 10(10) s(-1) (CH2Cl2, 298 K). The fate of the Ru(II)-Ru(C)(III)-PI*- CS state upon a second photoexcitation was studied by pump-pump-probe experiments in an attempt to detect the fully charge-separated Ru(III)-Ru(C)(II)-PI*- state.     

Substituent effect on formation of heterometallic molecular wheels: synthesis, crystal structure, and magnetic properties

The reaction of manganese(III) Schiff bases of the type salen(2-) (N,N'-ethylenebis(salicylideneaminato)) with X-substituted (X = CH(3), Cl) pyridinecarboxamide dicyanoferrite(III) [Fe(X-bpb)(CN)(2)](-) gave rise to a series of cyanide-bridged Mn(6)Fe(6) molecular wheels, [Mn(III)(salen)](6)[Fe(III)(bpmb)(CN)(2)](6) x 7H(2)O (1), [Mn(salen)](6)[Fe(bpClb)(CN)(2)](6) x 4H(2)O x 2CH(3)OH (2), [Mn(salen)](6)[Fe(bpdmb)(CN)(2)](6) x 10H(2)O x 5CH(3)OH (3), [Mn(5-Br(salpn))](6)[Fe(bpmb)(CN)(2)](6) x 24H(2)O x 8CH(3)CN (4), and [Mn(5-Cl(salpn))](6)[Fe(bpmb)(CN)(2)](6) x 25H(2)O x 5CH(3)CN (5). Compared with [Fe(bpb)(CN)(2)](-), which always gives rise to 1D or polynuclear species when reacting with Mn(III) Schiff bases, the introduction of substituents (X) to the bpb(2-) ligand has a driving force in formation of the novel wheel structure. Magnetic studies reveal that high-spin ground state S = 15 is present in the wheel compounds originated from the ferromagnetic Mn(III)-Fe(III) coupling. For the first time, the quantum Monte Carlo study has been used to modulate the magnetic susceptibility of the huge Mn(6)Fe(6) metallomacrocycles, showing that the magnetic coupling constants J range from 3.0 to 8.0 K on the basis of the spin Hamiltonian [Formula: see text]. Hysteresis loops for 1 have been observed below 0.8 K, indicative of a single-molecule magnet with a blocking temperature (TB) of 0.8 K. Molecular wheels 2-5 exhibit frequency dependence of alternating-current magnetic susceptibility under zero direct-current magnetic field, signifying the slow magnetization relaxation similar to that of 1. Significantly, an unprecedented archlike Mn(2)Fe(2) cluster, [Mn(5-Cl(salpn))](2)[Fe(bpmb)(CN)(2)](2) x 3H(2)O x CH(3)CN (6), has been isolated as an intermediate of the Mn(6)Fe(6) wheel 5. Ferromagnetic Mn(III)-Fe(III) coupling results in a high-spin S = 5 ground state. Combination of the high-spin state and a negative magnetic anisotropy (D) results in the observation of slow magnetization relaxation in 6.