We prepared three bifunctional redox protein maquettes based on 12-, 16-, and 20-mer three-helix bundles. In each case, the helix was capped with a Co(III) tris-bipyridyl electron acceptor and also functionalized with a C-terminal viologen (1-ethyl-1′-ethyl-4,4′-bipyridinium) donor. Electron transfer (ET) was initiated by pulse radiolysis and flash photolysis and followed spectrometrically to determine average, concentration-independent, first-order rates for the 16-mer and 20-mer maquettes. For the 16-mer bundle, the α-helical content was adjusted by the addition of urea or trifluoroethanol to solutions containing the metalloprotein. This conformational flexibility under different solvent conditions was exploited to probe the effects of helical secondary structure on ET rates. In addition to describing experimental results from these helical systems, this chapter discusses several additional metalloprotein models from the recent literature.
Radiation chemistry can be used to study reactions of free radicals and of metal ions in unusual valency states, including electron-transfer reactions. In some instances, radiation chemistry facilitates experiments that can not be studied by photochemistry, owing to differences in the fundamental physical processes in the two methods. Procedures have been developed to accurately determine radiolysis radical yields, and a variety of physical techniques have been used to monitor reactions. In particular, aqueous radiation chemistry has been extensively developed, and many free radicals can be generated in a controlled manner in aqueous solution. There are extensive literature resources for rate constants and for experimental design for a variety of radicals.
Flash photolysis and pulse radiolysis were used to generate reductants in situ to study the electron transfer (ET) reactivity of the FeIV==O heme centers in myoglobin and cytochrome c peroxidase. Reduction of a5RuIII groups covalently bound to surface histidines allowed intramolecular RuII → FeIV==O ET rates to be measured. Protonation of the oxene ligand was found to be largely rate determining in myoglobin, consistent with the lack of proton donors in its heme pocket. The large distance (21-23 Å) between surface histidines and the heme in wild-type cytochrome c peroxidase prevented the determination of the rate-limiting step(s) involved in FeIV==O reduction in this peroxidase, and strategies for attachment of an artificial redox center closer to its heme are outlined. From the work performed to date, pulse radiolysis appears to be a more versatile technique than flash photolysis for the study of FeIV==O heme reactivity in proteins.
The reactions of MnII TTHA (MnII-triethylenetetraminehexaacetate) complexes with HO2-O2- radicals were studied (pH 2.5-9.5), and a mechanism was suggested that involves the formation of a transient MnIItthaH(O2-)3- complex. At low pH, this complex is protonated, with the release of H2O2. At higher pH, the dismutation of O2- from the equilibrium complex (MnIITTHA(O2-)3- ⇌ MnIITTHA + O2-) is competitive with protonation. At low pH, the results indicate that there is a rapid first-order process that may be an isomerization from end-bound to side-bound of the attached superoxide radical. In contrast, the kinetics of the dismutation of superoxide radical by Escherichia coli MnSOD (manganese superoxide dismutase) were measured and shown to fit a mechanism involving the rapid reduction of Mn3+SOD by superoxide followed by both the direct reoxidation of Mn2+SOD by superoxide and the formation of a MnIISOD(O2-) complex. The differences in the mechanisms are discussed.
This chapter illustrates the complementarity of photochemical and radiation chemical techniques to elucidate elementary pathways in mechanistically rich systems. Some of the mechanistic conclusions that have resulted from these studies in aqueous media are presented. Extreme (both high and low) oxidation states of transition-metal complexes are included. Reactivity with respect to electron transfer reactions and small-molecule activation are addressed.
With the intensive development of ultrafast spectroscopic methods, reaction dynamics can he investigated at the subpicosecond time scale. Femtosecond spectroscopy of liquids and solutions allows the study of solvent-cage effects on elementary charge-transfer processes. Recent work on ultrafast electron-transfer channels in aqueous ionic solutions is presented (electron-atom or electron-ion radical pairs, early geminate recombination, and concerted electron-proton transfer) and discussed in the framework of quantum theories on nonequilibrium electronic states. These advances permit us to understand how the statistical density fluctuations of a molecular solvent can assist or impede elementary electron-transfer processes in liquids and solutions.
This chapter is intended as a guide to aid in the design of radiolysis experiments for researchers who are new to the field. It summarizes the features and limitations of the several kinds of particle accelerators and X-ray and gamma radiation sources used in the study of radiation chemistry. The chapter describes (1) the types of ionizing radiation and their interactions with matter, (2) the architecture and use of X-ray and gamma radiation sources, and (3) the electrostatic and radio-frequency methods of particle acceleration. The advantages and disadvantages of four types of accelerators are compared. A description of proton and heavy-ion accelerators completes the chapter.
One of the fundamental processes in biological energy-conversion systems is that of electron transfer. It takes place among and within proteins over considerable distances between active sites containing transition metal ions or organic cofactors and is generally characterized by relatively weak electronic interactions among them. Considerable research efforts have and are being invested in resolving the factors that control the rates of long-range electron transfer in proteins. Different expermental methods have been employed in these studies, ranging from fast mixing (stopped flow, rapid freeze EPR) and T-jump chemical relaxation to flash photolysis and pulse radiolysis. The latter two methods employ introduction of very short electromagnetic radiation pulses, absorbed by solutes in the former and by solvent in the latter. These in turn produce the electron donors or acceptors initiating the reaction of interest. The application of pulse radiolysis to studies of electron transfer within proteins is briefly reviewed to indicate its advantages. Results of its application to two types of copper-containing electron mediating proteins are presented and discussed.
When aqueous solutions containing Fe2+ ions are irradiated at <250 nm, photooxidation to Fe3+ occurs and molecular hydrogen is generated. This photoprocess has been studied extensively for over 60 years, but without agreement being reached about the nature of the primary step. Possible initial steps include metal-to-ligand charge transfer (MLCT), internal Fe2+ 3d → 4s absorption, direct electron photodetachment producing a partially solvated electron in a pre-existing solvent cavity, and polaron-type charge transfer to solvent (CTTS) absorption. We consider the energetics and solvent shift of the first three of these processes, concluding that the MLCT band is too high in energy, the 3d → 4s excitation could participate, and the direct photodetachment band is at the correct energy and intensity to account for all that is (as yet) observed of the absorption band. In general, a rather complicated picture of this process in inorganic complexes emerges. In this work, we apply a general method we have developed for estimating the effects of solvents on transitions of species that have strong specific interactions (e.g., hydrogen bonding) with the solvent molecules.
To understand electron transfer, it is important to separate the effects of the variables that control the rates. Pulse radiolysis offers the advantages of well-known free-energy changes (ΔG°) that are independent of distance and only weakly dependent upon solvent properties. It has the disadvantages of time resolution limited to 10 to 100 ps, the need for high concentrations to enable high time resolution, and concerns about sample degradation. Laser excitation offers much faster time resolution, less sample degradation, and modest needs for concentration. Difficulties are associated with both techniques in that electronic couplings and solvent reorganization energies depend upon distance. The advantages and capabilities of these two complementary techniques are compared. It is shown how pulse radiolysis can yield dependence of electron transfer rates on free energy, distance, temperature, and solvent reorganization energy. Recent results provide information about the distance dependence of the solvent reorganization energy. Examples are also given of the use of pulse radiolysis for measurement of free-energy changes and energetics of ion pairig.
When an acetonitrile solution containing Ni(bpy)32+ (bpy = 2,2′-bipyridine) triethylamine, and CO2 is irradiated at 313 nm, CO is produced with a quantum yield of ~0.1% (defined as CO produced/photons absorbed). Flash photolysis, electrochemistry, and pulse radiolysis experiments provide evidence for the formation of NiI(bpy)2+, as an intermediate, in the photochemical Ni(bpy)32+-triethylamine-CO2 system. Although Ni0(bpy)2 does react with CO2, NiI(bpy)2+ seems unreactive toward CO2 addition. The X-ray structure of (Ni3(bpy)6)(ClO4), which crystallizes as blue-violet needles, reveals the existence of a dimer in the solid. UV-vis spectra also indicate that reduced Ni(bpy)32+ solutions contain NiI(bpy)2+, Ni0(bpy)2, and [Ni(bpy)2]2 complexes in equilibrium.
We propose and apply a new tubular approach to compute the electron tunneling coupling in a protein. This approach goes beyond the single-pathway view to incorporate multiple-path effects and expose how interference arising from the structure can determine the coupling. An application to recent experiments in Ru-modified azurin is presented. The experimental data are used in a novel way to determine the proper effective electron tunneling energy to use in the model. The data are interpreted in terms of interfering tubes, and hydrogen bonds play a critical role in this interference. As tubes can be blocked or created by mutation, the theory suggests how experimental control of rates can be achieved.
Interpretation of the kinetic pulse radiolysis data for intramolecular Trp· → Tyr· radical transformation in aqueous solutions of linear H-Trp-(Pro)n-Tyr-OH, n = 0-5, is presented in terms of the Marcus electron transfer theory, taking into account conformational dynamics of the molecules. For this purpose, for each peptide, representative sets of low-energy conformers were selected with the help of experimental methods (1H and 13C NMR, and circular dichroism) and modeling methods (molecular mechanics and dynamics); and relative electron transfer rates averaged over all the conformers were calculated for two assumed competitive electron transfer pathways: through space (TS) and through the peptide backbone (TB). The TS rates were obtained by taking into account the overlap integrals of aromatic ring orbitals calculated quantum mechanically. By fitting the calculated rates to the experimental data for the rate constants for electron transfer, ket, with an exponential function appropriate for the two-pathway model, we have demonstrated that in linear short-bridged peptides (n = 0-2), electron transfer predominantly takes the TS pathway, which consists of van der Waals contacts between the aromatic rings, whereas in longer peptides (n = 3-5), it occurs exclusively by the TB pathway, which is made of a -(Pro)n-bridge in a helical conformation similar to that of all-trans poly-L-proline II. This pathway is characterized by a low value of the descriptor of the exponential distance dependence of the electron transfer rate, βTB = 2.5 ± 0.1 nm-1, suggesting that helical segments in proteins can function as efficient channels of long-distance electron transfer.
ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTCoordinated Photoinduced Electron and Proton Transfer in a Molecular TriadSu-Chun Hung, Alisdair N. Macpherson, Su Lin, Paul A. Liddell, Gilbert R. Seely, Ana L. Moore, Thomas A. Moore, and Devens GustCite this: J. Am. Chem. Soc. 1995, 117, 5, 1657–1658Publication Date (Print):February 1, 1995Publication History Published online1 May 2002Published inissue 1 February 1995https://pubs.acs.org/doi/10.1021/ja00110a030https://doi.org/10.1021/ja00110a030research-articleACS PublicationsRequest reuse permissionsArticle Views268Altmetric-Citations57LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts
The effect of pressure (up to 200 MPa) on the kinetics and thermodynamics of electron transfer reactions involving metal complexes and cytochrome c has been studied in detail for a number of systems. The observed activation and reaction volume data enable the construction of a reaction volume profile for each investigated system, and allow a detailed analysis of the partial molar volume changes associated with the electron transfer process. The reported results demonstrate the excellent agreement between data obtained using UV-vis, stopped-flow, pulse-radiolysis, flash-photolysis, and electrochemical techniques. The ultimate goal of this work is to contribute toward a better understanding of long-distance electron transfer reactions.
A multilayer film growth technique, in which single anionic sheets derived from inorganic solids are interleaved with cationic polyelectrolytes, has recently been developed. This method allows for the growth of concentric monolayers of redox-active polymers on high-surface-area silica supports, and for vectorial electron transfer reactions through the layers of the "onion". Transmission electron microscopy was used to probe the morphology of these lamellar heterostructures. Photoinduced charge separation has been observed in composites consisting of an inner polycationic layer of poly(styrene-co-N-vinylbenzyl-N′-methyl-4,4′-bipyridine) (PS-MV2+), and an outer polycationic layer of poly[Ru(bpy)2(vbpy)]2+, vbpy = 4-vinyl-4′-methyl-2,2′-bipyridine, bpy = 2,2′-bipyridine, which are separated by a thin inorganic sheet of α-Zr(PO4)22-. The thickness of the individual polymer layers was determined by ellipsometry for equivalent structures on planar supports. Electron transfer quenching of the Ru(II) polymer luminescence occurs upon addition of a solution-phase, reversible electron donor, disodium methoxyaniline-N,N′-diethylsulfonate (MDESA2-). In the absence of an inner viologen layer, this simple donor-acceptor charge-separated state decays in several microseconds to regenerate the ground state. In the triad system, which contains an inner viologen polymer layer, rapid electron transfer from Ru(I) to viologen creates a charge-separated state with a half-life of 21 µs. The simultaneous second-order decay (krecomb = 1 × 109 M-1 s-1) of signals from both MDESA·- and reduced MV·+ is consistent with escape of the former from the Ru(II) polymer and subsequent diffusion to MV·+ sites. Quantum yields for charge separation are ca. 30%.
Pulse radiolysis and laser photolysis were used to study intramolecular electron transfer (ET) reactions in solutions of electrolytes. The presence of counterions has been found to slow the rate of weakly exoergic (ΔG = 100 meV) electron transfer by as much as 3 orders of magnitude. The larger counterions resulted in a larger decrease of the ET rate, suggesting that the transfer rate is influenced by the mobility of ions. In a separate study, transient triplet charge-transfer (CT) absorption bands of p-aminonitroterphenyl were used as probes of ion-pairing dynamics and energetics. It has been found that the pairing process is diffusion controlled, and that in weakly polar media, association with ions can stabilize the photoinduced charge-separated state by up to 1 eV.
Cluster properties, mostly those that control electron transfer processes such as the redox potential in solution, are markedly dependent on their nuclearity. Therefore, clusters of the same metal may behave as electron donor or as electron acceptor, depending on their size. Pulse radiolysis associated with time-resolved optical absorption spectroscopy is used to generate isolated metal atoms and to observe transitorily the subsequent clusters of progressive nuclearity yielded by coalescence. Applied to silver clusters, the kinetic study of the competition of coalescence with reactions in the presence of added reactants of variable redox potential allows us to describe the autocatalytic processes of growth or corrosion of the clusters by electron transfer. The results provide the size dependence of the redox potential of some metal clusters. The influence of the environment (surfactant, ligand, or support) and the role of electron relay of metal clusters in electron transfer catalysis are discussed.
