Long-range intramolecular electron transfer in azurins.

Farver, Ole; Pecht, Israel

doi:10.1073/pnas.86.18.6968

Cited by 72 publications

(78 citation statements)

References 24 publications

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“…We found this rate to be the same, within experimental error, when gy 3 2 radicals were used as the electron donor. These results demonstrate that the T1 Cu(II) centre of AxNiR is the primary electron accepting site using both NMNA 3 and gy 3 2 radicals, with a reaction rate close to, or at the di¡usion controlled limit, since it is very similar to that observed for bimolecular reduction of the T1 Cu(II) in azurin by gy 3 2 radicals [15].…”

Section: Resultssupporting

confidence: 65%

The intramolecular electron transfer between copper sites of nitrite reductase: a comparison with ascorbate oxidase

et al. 1998

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The intramolecular electron transfer (ET) between the type 1 Cu(I) and the type 2 Cu(II) sites of Alcaligenes xylosoxidans dissimilatory nitrite reductase (AxNiR) has been studied in order to compare it with the analogous process taking place in ascorbate oxidase (AO). This internal process is induced following reduction of the type 1 Cu(II) by radicals produced by pulse radiolysis. The reversible ET reaction proceeds with a rate constant k i = k I3P +k P3I of 450 þ 30 s 3I at pH 7.0 and 298 K. The equilibrium constant K was determined to be 0.7 at 298 K from which the individual rate constants for the forward and backward reactions were calculated to be: k I3P = 185 þ 12 s 3I and k P3I 265 þ 18 s 3I . The temperature dependence of K allowed us to determine the v vH³ value of the ET equilibrium to be 12.1 kJ mol 3I . Measurements of the temperature dependence of the ET process yielded the following activation parameters: forward reaction, v vH g = 22.7 þ 3.4 kJ mol 3I and v vS g = 3126 þ 11 J K 3I mol 3I ; backward reaction, v vH g = 10.6 þ 1.7 kJ mol 3I and v vS g = 3164 þ 15 J K 3I mol 3I . X-ray crystallographic studies of NiRs suggest that the most probable ET pathway linking the two copper sites consists of Cys IQT , which provides the thiolate ligand to the type 1 copper ion, and the adjacent His IQS residue with its imidazole being one of the ligands to the type 2 Cu ion. This pathway is essentially identical to that operating between the type 1 Cu(I) and the trinuclear copper centre in ascorbate oxidase, and the characteristics of the internal ET processes of these enzymes are compared. The data are consistent with the faster ET observed in nitrite reductase arising from a more advantageous entropy of activation when compared with ascorbate oxidase.z 1998 Federation of European Biochemical Societies.

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Section: Resultssupporting

confidence: 65%

The intramolecular electron transfer between copper sites of nitrite reductase: a comparison with ascorbate oxidase

et al. 1998

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“…Direct self-assembly through the disulfide group orients the protein molecules with the copper center opposite to the electrode surface, with a 26-Å distance between the copper center and the electrode surface (35,36). An interfacial ET rate constant of Ϸ30 s Ϫ1 was observed, largely consistent with intramolecular ET between the copper center and the Cys 3 Cys 26 site measured by pulse radiolysis in homogeneous solution (44 s Ϫ1 ) (29). Interfacial ET can be significantly facilitated by wiring azurin molecules onto the gold surface by variable-length alkanethiols through noncovalent interactions of the hydrophobic patch around the copper ion with the terminal methyl group (37,38).…”

mentioning

confidence: 52%

“…Azurin is a blue single-copper protein that functions as an electron carrier physiologically associated with oxidative stress responses in bacteria (e.g., Pseudomonas aeruginosa) (26) and is a long-standing model for exploring electron tunneling through protein molecules (27)(28)(29)(30)(31)(32). Thanks to its intrinsic merits (e.g., high stability and excellent redox properties), azurin has recently emerged as a favorite target for nanoscale bioelectronics (33,34).…”

mentioning

confidence: 99%

Long-range protein electron transfer observed at the single-molecule level: In situ mapping of redox-gated tunneling resonance

Chi

Farver

Ulstrup

2005

Proc. Natl. Acad. Sci. U.S.A.

173

220

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A biomimetic long-range electron transfer (ET) system consisting of the blue copper protein azurin, a tunneling barrier bridge, and a gold single-crystal electrode was designed on the basis of molecular wiring self-assembly principles. This system is sufficiently stable and sensitive in a quasi-biological environment, suitable for detailed observations of long-range protein interfacial ET at the nanoscale and single-molecule levels. Because azurin is located at clearly identifiable fixed sites in well controlled orientation, the ET configuration parallels biological ET. The ET is nonadiabatic, and the rate constants display tunneling features with distance-decay factors of 0.83 and 0.91 Å ؊1 in H2O and D2O, respectively. Redoxgated tunneling resonance is observed in situ at the single-molecule level by using electrochemical scanning tunneling microscopy, exhibiting an asymmetric dependence on the redox potential. Maximum resonance appears around the equilibrium redox potential of azurin with an on͞off current ratio of Ϸ9. Simulation analyses, based on a two-step interfacial ET model for the scanning tunneling microscopy redox process, were performed and provide quantitative information for rational understanding of the ET mechanism.blue copper protein ͉ scanning tunneling microscopy ͉ nanoscale bioelectronics ͉ bioelectrochemistry C harge transfer plays key roles in many chemical and biological processes as well as in molecular electronics (1-5). For example, long-range protein electron transfer (ET) is central in aerobic respiration and photosynthesis. The importance of longrange ET is illustrated by a recent special issue of PNAS on this topic (6-12). Several articles reflect broadly the current status of this subject. However, one of the major objectives in nanoscale science and technology is to fabricate molecular electronic devices with specified functions. Molecular electronics is rooted in the concept of molecular charge transfer (particularly, molecular conductivity) (13,14). Two essential steps involved in bottom-up manipulations are (i) organizing molecules into nanoscale structures and (ii) interfacing such nanostructures with macroscopically addressable components (e.g., metal and semiconductor electrodes). Molecular electronic device function thus rests fundamentally on charge transfer through organic and͞or biological molecules and across the interface between molecules and macroscopic electrodes (15, 16). Understanding of charge transfer mechanisms has mostly been based on average results of macroscopic measurements. The advent of scanning probe microscopies, along with other supersensitive techniques, has made it possible to characterize or directly observe charge transfer through organic molecules down to the nanoscale and single-molecule levels, as illustrated by measurements of singlemolecule conductivity (17-21) and probing of molecular switching and resonant tunneling (22,23). This, however, remains a daunting challenge for proteins, with difficulties arising from the assembly of suitable st...

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“…Pulse radiolytically produced CO 2 Ϫ radicals reduce both the copper(II) site and the disulfide bridge in azurin at essentially diffusion-controlled rates (35)(36)(37)(38)(39)(40)(41). The Cu(II)3Cu(I) reduction results in an absorption decrease at 625 nm, while the RSSR Ϫ radicals formed absorb strongly around 410 nm.…”

Section: Resultsmentioning

confidence: 99%

“…All are rigid ␤-sheet proteins containing two redox centers: the copper ion coordinated directly to amino acid residues and a disulfide bridge (RSSR) at the opposite end of the barrel-shaped molecule. We have previously demonstrated that long-range ET between these two centers can be induced after pulse radiolytic single-electron reduction of RSSR (35)(36)(37)(38)(39)(40)(41). We examined the effects of specific structural changes on the rate of intramolecular ET between the RSSR Ϫ radical and the Cu(II) center by using wild-type and single-mutant azurins.…”

mentioning

confidence: 99%

Deuterium isotope effect on the intramolecular electron transfer inPseudomonas aeruginosaazurin

Farver

Zhang

Chi

et al. 2001

Proc. Natl. Acad. Sci. U.S.A.

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Intramolecular electron transfer in azurin in water and deuterium oxide has been studied over a broad temperature range. The kinetic deuterium isotope effect, k H͞kD, is smaller than unity (0.7 at 298 K), primarily caused by the different activation entropies in water (؊56.5 J K ؊1 mol ؊1 ) and in deuterium oxide (؊35.7 J K ؊1 mol ؊1 ). This difference suggests a role for distinct protein solvation in the two media, which is supported by the results of voltammetric measurements: the reduction potential (E 0 ) of Cu 2؉͞؉ at 298 K is 10 mV more positive in D2O than in H2O. The temperature dependence of E 0 is also different, yielding entropy changes of ؊57 J K ؊1 mol ؊1 in water and ؊84 J K ؊1 mol ؊1 in deuterium oxide. The driving force difference of 10 mV is in keeping with the kinetic isotope effect, but the contribution to ⌬S ‡ from the temperature dependence of E 0 is positive rather than negative. Isotope effects are, however, also inherent in the nuclear reorganization Gibbs free energy and in the tunneling factor for the electron transfer process. A slightly larger thermal protein expansion in H 2O than in D2O (0.001 nm K ؊1 ) is sufficient both to account for the activation entropy difference and to compensate for the different temperature dependencies of E 0 . Thus, differences in driving force and thermal expansion appear as the most straightforward rationale for the observed isotope effect. K inetic deuterium or tritium isotope effects (KIE) have long been recognized to reflect crucial mechanistic features in proton and hydrogen atom transfer processes (see, for example, refs. 1-6). KIE were also introduced early as a mechanistically diagnostic tool in hydrolytic metalloenzyme catalysis (7-8). Studies of KIE in metalloenzyme catalysis during the last decade have incorporated increasingly detailed theoretical notions (7-11), for example: proton͞deuteron tunneling and multiphonon environmental vibrational excitation (4-6, 12-16); gated proton͞deuteron transfer (6,(12)(13)(14); proton͞deuteron transfer and shallow barriers (17-20); coupled multiproton transfer (17, 21); and stochastic molecular (13-15) and bulk environmental control (12,22).Theoretical frames for proton tunneling and KIE are most straightforward when the barriers for proton transfer are high, yielding large values of the KIE (Ͼ Ͼ1), measured as the ratio between the rate constant for proton and deuteron (or triton) transfer. Proton͞deuteron tunneling is here strongly conspicuous. This partially adiabatic limit (4, 6), applies particularly to proton or hydrogen atom transfer between C-donor or -acceptor atoms. Proton transfer between O-and N-donor or -acceptor atoms mostly displays small values (KIE ϭ 1-2.5), reflecting strong hydrogen bond interactions, facile mutual approach between the donor and acceptor groups, and shallow barriers. These barriers correspond to the fully adiabatic limit (4), where the dominating effect of the KIE is reflected in isotopedependent splitting in the crossing region of the appropriate potential surfaces (6)...

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Long-range intramolecular electron transfer in azurins.

Cited by 72 publications

References 24 publications

The intramolecular electron transfer between copper sites of nitrite reductase: a comparison with ascorbate oxidase

The intramolecular electron transfer between copper sites of nitrite reductase: a comparison with ascorbate oxidase

Long-range protein electron transfer observed at the single-molecule level: In situ mapping of redox-gated tunneling resonance

Deuterium isotope effect on the intramolecular electron transfer inPseudomonas aeruginosaazurin

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