Structure formation in heavy metal oxyhydrates at low rates of gel formation

Oxyhydrate gels have a hydrophilic surface, due to which they undergo destruction and secondary polymerization in aqueous media. Prolonged storage in aqueous solution gives rise to regions with selfsimilar helical ordering in gels. Structuring of this kind is also observed when synthesis is conducted under conditions that provide low gelation rates. Electromagnetic UV and visible radiation is another means to change the gel structure; it makes the oligomer species pass into the excited state, due to which one of the directions of structuring becomes dominant. This work summarizes the results of computer simulation of gel agglomerates. For oxyhydrate systems, helical ordering was found to be one of the local energy minima. The units of a macrohelix can lie at various angles relative to one another, and they can change, after absorption of energy, the helix pitch and the order of elements in the helix.     

Rate and driving force for protonation of aryl radical anions in ethanol

Aryl radical anions created in liquid alcohols decay on the microsecond time scale by transfer of protons from the solvent. This paper reports a 4.5 decade range of rate constants for proton transfer from a single weak acid, ethanol, to a series of unsubstituted aryl radical anions, Ar-*. The rate constants correlate with free energy change, DeltaG(o), despite wide variations in the two factors that contribute to DeltaG(o): (a) the reduction potentials of the aryls and (b) the Ar-H* bond strengths in the product radicals. For aryl radical anions containing CH2OH substituents, such as 2,2'-biphenyldimethanol*- which is protonated with a rate constant of 3x10(9) s(-1), the faster rates do not fit well in the free energy correlation, suggesting a change in mechanism.     

Hydroxyl radical formation by O-O bond homolysis in peroxynitrous acid

Peroxynitrite decay in weakly alkaline media occurs by two concurrent sets of pathways which are distinguished by their reaction products. One set leads to net isomerization to NO(3)(-) and the other set to net decomposition to O(2) plus NO(2)(-). At sufficiently high peroxynitrite concentrations, the decay half-time becomes concentration-independent and approaches a limiting value predicted by a mechanism in which reaction is initiated by unimolecular homolysis of the peroxo O-O bond, i.e., the following reaction: ONOOH --> (*)OH + (*)NO(2). This dynamical behavior excludes alternative postulated mechanisms that ascribe decomposition to bond rearrangement within bimolecular adducts. Nitrate and nitrite product distributions measured at very low peroxynitrite concentrations also correspond to predictions of the homolysis model, contrary to a recent report from another laboratory. Additionally, (1) the rate constant for the reaction ONOO(-) --> (*)NO + (*)O(2)(-), which is critical to the kinetic model, has been confirmed, (2) the apparent volume of activation for ONOOH decay (DeltaV() = 9.7 +/- 1.4 cm(3)/mol) has been shown to be independent of the concentration of added nitrite and identical to most other reported values, and (3) complex patterns of inhibition of O(2) formation by radical scavengers, which are impossible to rationalize by alternative proposed reaction schemes, are shown to be quantitatively in accord with the homolysis model. These observations resolve major disputes over experimental data existing in the literature; despite extensive investigation of these reactions, no verifiable experimental evidence has been advanced that contradicts the homolysis model.     

Gold nanoparticle size controlled by polymeric Au(I) thiolate precursor size

We developed a method in preparing size-controllable gold nanoparticles (Au NPs, 2-6 nm) capped with glutathione by varying the pH (between 5.5 and 8.0) of the solution before reduction. This method is based on the formation of polymeric nanoparticle precursors, Au(I)-glutathione polymers, which change size and density depending on the pH. Dynamic light scattering, size exclusion chromatography, and UV-vis spectroscopy results suggest that lower pH values favor larger and denser polymeric precursors and higher pH values favor smaller and less dense precursors. Consequently, the larger precursors led to the formation of larger Au NPs, whereas smaller precursors led to the formation of smaller Au NPs. Using this strategy, Au NPs functionalized with nickel(II) nitriloacetate (Ni-NTA) group were prepared by a mixed-ligand approach. These Ni-NTA functionalized Au NPs exhibited specific binding to 6x-histidine-tagged Adenovirus serotype 12 knob proteins, demonstrating their utility in biomolecular labeling applications.     

Disproportionation pathways of aqueous hyponitrite radicals (HN2O2(*)/N2O2(*-))

Pulse radiolysis and flash photolysis are used to generate the hyponitrite radicals (HN2O2(*)/N2O2(*-)) by one-electron oxidation of the hyponitrite in aqueous solution. Although the radical decay conforms to simple second-order kinetics, its mechanism is complex, comprising a short chain of NO release-consumption steps. In the first, rate-determining step, two N2O2(*-) radicals disproportionate with the rate constant 2k = (8.2 +/- 0.5) x 10(7) M(-1) s(-1) (at zero ionic strength) effectively in a redox reaction regenerating N2O2(2-) and releasing two NO. This occurs either by electron transfer or, more likely, through radical recombination-dissociation. Each NO so-produced rapidly adds to another N2O2(*-), yielding the N3O3(-) ion, which slowly decomposes at 300 s(-1) to the final N2O + NO2(-) products. The N2O2(*-) radical protonates with pKa = 5.6 +/- 0.3. The neutral HN2O2(*) radical decays by an analogous mechanism but much more rapidly with the apparent second-order rate constant 2k = (1.1 +/- 0.1) x 10(9) M(-1) s(-1). The N2O2(*-) radical shows surprisingly low reactivity toward O2 and O2(*-), with the corresponding rate constants below 1 x 10(6) and 5 x 10(7) M(-1) s(-1). The previously reported rapid dissociation of N2O2(*-) into N2O and O(*-) does not occur. The thermochemistry of HN2O2(*)/N2O2(*-) is discussed in the context of these new kinetic and mechanistic results.     

Photocatalytic electron transfer across the membranes of lipid vesicles mediated by diglycylamide of mesoporphyrin-IX N-metylglycomate

A peculiarity of the photocatalytic action of porphyrin molecules with different localization inside lipid bilayers of lecithin vesicles has been studied. The influence of an electrostatic field in the lipid membrane on the efficiency of electron photocatalytic transfer across the membrane is discussed.

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One-electron reduction of aqueous nitric oxide: a mechanistic revision

The pulse radiolysis of aqueous NO has been reinvestigated, the variances with the prior studies are discussed, and a mechanistic revision is suggested. Both the hydrated electron and the hydrogen atom reduce NO to yield the ground-state triplet (3)NO(-) and singlet (1)HNO, respectively, which further react with NO to produce the N(2)O(2)(-) radical, albeit with the very different specific rates, k((3)NO(-) + NO) = (3.0 +/- 0.8) x 10(9) and k((1)HNO + NO) = (5.8 +/- 0.2) x 10(6) M(-)(1) s(-)(1). These reactions occur much more rapidly than the spin-forbidden acid-base equilibration of (3)NO(-) and (1)HNO under all experimentally accessible conditions. As a result, (3)NO(-) and (1)HNO give rise to two reaction pathways that are well separated in time but lead to the same intermediates and products. The N(2)O(2)(-) radical extremely rapidly acquires another NO, k(N(2)O(2)(-) + NO) = (5.4 +/- 1.4) x 10(9) M(-)(1) s(-)(1), producing the closed-shell N(3)O(3)(-) anion, which unimolecularly decays to the final N(2)O + NO(2)(-) products with a rate constant of approximately 300 s(-)(1). Contrary to the previous belief, N(2)O(2)(-) is stable with respect to NO elimination, and so is N(3)O(3)(-). The optical spectra of all intermediates have also been reevaluated. The only intermediate whose spectrum can be cleanly observed in the pulse radiolysis experiments is the N(3)O(3)(-) anion (lambda(max) = 380 nm, epsilon(max) = 3.76 x 10(3) M(-)(1) cm(-)(1)). The spectra previously assigned to the NO(-) anion and to the N(2)O(2)(-) radical are due, in fact, to a mixture of species (mainly N(2)O(2)(-) and N(3)O(3)(-)) and to the N(3)O(3)(-) anion, respectively. Spectral and kinetic evidence suggests that the same reactions occur when (3)NO(-) and (1)HNO are generated by photolysis of the monoprotonated anion of Angeli's salt, HN(2)O(3)(-), in NO-containing solutions.     

Spin-forbidden deprotonation of aqueous nitroxyl (HNO)

The first mechanistic study of a spin-forbidden proton-transfer reaction in aqueous solution is reported. Laser flash photolysis of alkaline trioxodinitrate (N(2)O(3)(2)(-), Angeli's anion) is used to generate a nitroxyl anion in its excited singlet state ((1)NO(-)). Through rapid partitioning between protonation by water and electronic relaxation, (1)NO(-) produces (1)HNO (ground state, yield 96%) and (3)NO(-) (ground state, yield 4%), which comprise a unique conjugate acid-base couple with different ground-state multiplicities. Using the large difference between reactivities of (1)HNO and (3)NO(-) in the peroxynitrite-forming reaction with (3)O(2), the kinetics of spin-forbidden deprotonation reaction (1)HNO + OH(-) --> (3)NO(-) + H(2)O is investigated in H(2)O and D(2)O. Consistent with proton transfer, this reaction exhibits primary kinetic hydrogen isotope effect k(H)/k(D) = 3.1 at 298 K, which is found to be temperature-dependent. Arrhenius pre-exponential factors and activation energies of the second-order rate constant are found to be: log(A, M(-)(1) s(-)(1)) = 10.0 +/- 0.2 and E(a) = 30.0 +/- 1.1 kJ/mol for proton transfer and log(A, M(-)(1) s(-)(1)) = 10.4 +/- 0.1 and E(a) = 35.1 +/- 0.7 kJ/mol for deuteron transfer. Collectively, these data are interpreted to show that the nuclear reorganization requirements arising from the spin prohibition necessitate significant activation before spin change can take place, but the spin change itself must occur extremely rapidly. It is concluded that a synergy between the spin prohibition and the reaction energetics creates an intersystem barrier and is responsible for slowness of the spin-forbidden deprotonation of (1)HNO by OH(-); the spin prohibition alone plays a minor role.