pH-dependent Electron Transfer Research Articles

Regulating optoelectronics of carbon dots with redox-active dopamine

The electron transfer (ET) processes in carbon-based quantum dots (CDs) have laid to extensive research endeavors due to their tunable optoelectronic properties with alteration in surface functionalization, doping, surface charge, etc. Here, we proffer shreds of evidence on pH-dependent electron transfer and transport properties of undoped CDs (UCDs) and amine-doped CDs (ACDs) with a redox-active neurotransmitter, dopamine (DA). A pronounced pH-dependent photoluminescence (PL) quenching is depicted for both the CDs with DA, where the quenching efficiency substantially increases in alkaline solvents. Taking advantage of the structural transformation of DA from hydroquinone to quinone, the electron accepting capacity can be improved for alkaline pH, which enhances the ET efficiency. The pH-regulated conductance measurements across the metal-CD-metal junction reveal a hike in the conductivity for acidic and alkaline pH relative to the neutral one. The I-V traces of UCDs and ACDs are contributed by an initial linear current rise and non-linear growth at higher bias explicating both direct and Fowler-Nordheim (F-N) tunneling mechanisms. In addition, the incorporation of DA into these CDs, not only significantly increases conductance in all pH media but also enables them to tunnel the potential barrier directly in acidic media, reducing F-N tunneling. These results could provide new avenues for developing reliable optoelectronic devices and sensing probes for a wide range of applications.

In-situ efficient electrosynthesis of H2O2–NaClO based on the media pH and catalyst mutual selection mechanism

Electrochemical two-electron oxygen reduction (2e− ORR) as a promising H2O2 synthesis strategy has been extensively explored, while the study on the relation between active sites at the catalyst surface and pH of applied medium is still scarce. Here in, the as-prepared catalysts by anodizing graphite felt in different media exhibit excellent H2O2 production activity in acidic (151.73 mg L−1 h −1 cm−2), neutral (53.72 mg L−1 h −1 cm−2) and alkaline (188.85 mg L−1 h −1 cm−2) media, which is superior to the current advanced felt-supported carbon-based catalysts. According to characterizations, a mutual selection mechanism between the catalysts and media pH is proposed, in which it is the catalysts’ specific functional groups and structures matching with the pH-dependent electron transfer mechanism leading to the efficient H2O2 production. When anolyte simulates the seawater environment in H-type cell, the simultaneous production of NaClO and H2O2 and degradation of rhodamine B are realized, and H2O2 accumulative concentration in solution reaches a breakthrough of 6.58 g L−1. In general, this work provides new insights into the relation between the pH-dependent surface properties of catalysts and 2e− ORR reaction mechanism. In-situ H2O2–NaClO synthesis is also a promising route for chemicals synthesis and pollutant degradation.

Role of Histidine 78 of leaf ferredoxin in the interaction with ferredoxin-NADP+ reductase: regulation of pH dependency and negative cooperativity with NADP(H).

In chloroplast stroma, dynamic pH change occurs in response to fluctuating light conditions. We investigated the pH-dependent electron transfer activity between ferredoxin-NADP+ reductase (FNR) and ferredoxin (Fd) isoproteins from maize leaves. By increasing pH (from 5.5 to 8.5), the electron transfer activity from FNR to photosynthetic-type Fd (Fd1) significantly increased while the activity to nonphotosynthetic type Fd (Fd3) decreased, which was mainly due to their differences in the pH dependency of Km for Fd. Mutation of His78 of Fd1 to Val, corresponding amino acid residue in Fd3, lost the pH dependency, indicating a regulatory role of the His78 in the interaction with FNR. We previously showed that the interaction between FNR and Fd was weakened by the allosteric binding of NADP(H) on FNR. His78Val Fd1 mutant largely suppressed this negative cooperativity. These results indicate the involvement of Fd1 His78 in pH dependency and negative cooperativity in the interaction with FNR.

Tuning Radical Relay Residues by Proton Management Rescues Protein Electron Hopping.

Transient tyrosine and tryptophan radicals play key roles in the electron transfer (ET) reactions of photosystem (PS) II, ribonucleotide reductase (RNR), photolyase, and many other proteins. However, Tyr and Trp are not functionally interchangeable, and the factors controlling their reactivity are often unclear. Cytochrome c peroxidase (CcP) employs a Trp191•+ radical to oxidize reduced cytochrome c (Cc). Although a Tyr191 replacement also forms a stable radical, it does not support rapid ET from Cc. Here we probe the redox properties of CcP Y191 by non-natural amino acid substitution, altering the ET driving force and manipulating the protic environment of Y191. Higher potential fluorotyrosine residues increase ET rates marginally, but only addition of a hydrogen bond donor to Tyr191• (via Leu232His or Glu) substantially alters activity by increasing the ET rate by nearly 30-fold. ESR and ESEEM spectroscopies, crystallography, and pH-dependent ET kinetics provide strong evidence for hydrogen bond formation to Y191• by His232/Glu232. Rate measurements and rapid freeze quench ESR spectroscopy further reveal differences in radical propagation and Cc oxidation that support an increased Y191• formal potential of ∼200 mV in the presence of E232. Hence, Y191 inactivity results from a potential drop owing to Y191•+ deprotonation. Incorporation of a well-positioned base to accept and donate back a hydrogen bond upshifts the Tyr• potential into a range where it can effectively oxidize Cc. These findings have implications for the YZ/YD radicals of PS II, hole-hopping in RNR and cryptochrome, and engineering proteins for long-range ET reactions.

L-cysteine capped ZnS:Mn quantum dots for room-temperature detection of dopamine with high sensitivity and selectivity

Dopamine (DA) is one of the most important catecholamine neurotransmitters of the human central nervous system, and is involved in many behavioral responses and brain functions. Below normal DA levels in biological fluids can lead to different neurodegenerative conditions. For excess DA levels, a failure in energy metabolism is indicated. In this study, a facile room-temperature phosphorescence sensor is developed to detect DA based on l-cysteine capped Mn doped ZnS quantum dots (l-cys ZnS:Mn QDs). The QDs display a prominent orange emission band peaking at ~598nm, which is strongly quenched upon addition of DA in alkaline medium. The sensor exhibits a linear working range of ~0.15–3.00μM, and a limit of detection of ~7.80nM. These results are explained in terms of a pH-dependent electron transfer process, in which the oxidized dopamine quinone functions as an efficient electron acceptor. The QDs-based sensor shows a high selectivity to DA over common interfering biomolecules (including some amino acids, ascorbic acid, chloride and glucose). The sensor has been successfully applied for the detection of DA in urine samples, yielding recoveries as high as 93%. Our findings indicate that our developed sensor exhibits high sensitivity and reproducibility to determine DA even in biological fluids where DA is at low levels, e.g., in the central nervous system, which is the usual clinical profile of a neurodegenerative disorder associated to the Parkinson's disease.

PH-dependent electron transfer reaction and direct bioelectrocatalysis of the quinohemoprotein pyranose dehydrogenase

A pyranose dehydrogenase from Coprinopsis cinerea (CcPDH) is an extracellular quinohemoeprotein, which consists a b-type cytochrome domain, a pyrroloquinoline-quinone (PQQ) domain, and a family 1-type carbohydrate-binding module. The electron transfer reaction of CcPDH was studied using some electron acceptors and a carbon electrode at various pH levels. Phenazine methosulfate (PMS) reacted directly at the PQQ domain, whereas cytochrome c (cyt c) reacted via the cytochrome domain of intact CcPDH. Thus, electrons are transferred from reduced PQQ in the catalytic domain of CcPDH to heme b in the N-terminal cytochrome domain, which acts as a built-in mediator and transfers electron to a heterogenous electron transfer protein. The optimal pH values of the PMS reduction (pH 6.5) and the cyt c reduction (pH 8.5) differ. The catalytic currents for the oxidation of l-fucose were observed within a range of pH 4.5 to 11. Bioelectrocatalysis of CcPDH based on direct electron transfer demonstrated that the pH profile of the biocatalytic current was similar to the reduction activity of cyt c characters.

Characterization of the Two Neurospora crassa Cellobiose Dehydrogenases and Their Connection to Oxidative Cellulose Degradation

The genome of Neurospora crassa encodes two different cellobiose dehydrogenases (CDHs) with a sequence identity of only 53%. So far, only CDH IIA, which is induced during growth on cellulose and features a C-terminal carbohydrate binding module (CBM), was detected in the secretome of N. crassa and preliminarily characterized. CDH IIB is not significantly upregulated during growth on cellulosic material and lacks a CBM. Since CDH IIB could not be identified in the secretome, both CDHs were recombinantly produced in Pichia pastoris. With the cytochrome domain-dependent one-electron acceptor cytochrome c, CDH IIA has a narrower and more acidic pH optimum than CDH IIB. Interestingly, the catalytic efficiencies of both CDHs for carbohydrates are rather similar, but CDH IIA exhibits 4- to 5-times-higher apparent catalytic constants (k(cat) and K(m) values) than CDH IIB for most tested carbohydrates. A third major difference is the 65-mV-lower redox potential of the heme b cofactor in the cytochrome domain of CDH IIA than CDH IIB. To study the interaction with a member of the glycoside hydrolase 61 family, the copper-dependent polysaccharide monooxygenase GH61-3 (NCU02916) from N. crassa was expressed in P. pastoris. A pH-dependent electron transfer from both CDHs via their cytochrome domains to GH61-3 was observed. The different properties of CDH IIA and CDH IIB and their effect on interactions with GH61-3 are discussed in regard to the proposed in vivo function of the CDH/GH61 enzyme system in oxidative cellulose hydrolysis.

PH-Dependent Electron Transfer from Re-bipyridyl Complexes to Metal Oxide Nanocrystalline Thin Films

Photoinduced interfacial electron transfer (ET) from molecular adsorbates to semiconductor nanoparticles has been a subject of intense recent interest. Unlike intramolecular ET, the existence of a quasicontinuum of electronic states in the solid leads to a dependence of ET rate on the density of accepting states in the semiconductor, which varies with the position of the adsorbate excited-state oxidation potential relative to the conduction band edge. For metal oxide semiconductors, their conduction band edge position varies with the pH of the solution, leading to pH-dependent interfacial ET rates in these materials. In this work we examine this dependence in Re(L(P))(CO)3Cl (or ReC1P) [L(P) = 2,2'-bipyridine-4,4'-bis-CH2PO(OH)2] and Re(L(A))(CO)3Cl (or ReC1A) [L(A) = 2,2'-bipyridine-4,4'-bis-CH2COOH] sensitized TiO2 and ReC1P sensitized SnO2 nanocrystalline thin films using femtosecond transient IR spectroscopy. ET rates are measured as a function of pH by monitoring the CO stretching modes of the adsorbates and mid-IR absorption of the injected electrons. The injection rate to TiO2 was found to decrease by 1000-fold from pH 0-9, while it reduced by only a factor of a few to SnO2 over a similar pH range. Comparison with the theoretical predictions based on Marcus' theory of nonadiabatic interfacial ET suggests that the observed pH-dependent ET rate can be qualitatively accounted for by considering the change of density of electron-accepting states caused by the pH-dependent conduction band edge position.

PH dependence of the reduction of dioxygen to water by cytochrome c oxidase. 1. The P(R) state is a pH-dependent mixture of three intermediates, A, P, and F.

Recent studies on cytochrome oxidase have indicated that the putative "peroxy" intermediate in the catalytic cycle (P(R)) is a mixture of intermediates, including P and F [Sucheta, A., et al. (1998) Biochemistry 37, 17905-17914], and the bench-made P and F forms appear to have the same redox state (Fe(a3)(4+)=O(2-)), but a different protonation state [Fabian, M., and Palmer, G. (2001) Biochemistry 40, 1867-1874]. To explore the possibility that the putative P(R) state is a pH-dependent mixture of intermediates, we investigated the reduction of dioxygen to water by the fully reduced cytochrome oxidase at pH 6.2, 7.5, and 8.5 in the visible and Soret regions (350-800 nm) using the CO flow-flash technique. Singular value decomposition and global exponential fitting of the time-resolved absorption difference spectra resolved five apparent lifetimes. The fastest three (1.5, 13, and 34 micros) were independent of pH, while the two slowest rates (80-240 micros and 1.1-2.4 ms) decreased by a factor of 2-3 as the pH increased. When the time-resolved spectra were analyzed using a unidirectional sequential model, the spectra of the reduced enzyme and the dioxygen-bound intermediate, compound A, were found to be pH-independent. However, the putative P(R) intermediate was best represented by a pH-dependent mixture of compound A, P, and F. The ferryl form was favored at low pH. The subsequent intermediate is a ferryl with a pH-dependent electron transfer equilibrium between heme a and Cu(A), the reduced heme a being favored at low pH. These results suggest a pH-dependent reaction mechanism of the reduction of dioxygen to water by the fully reduced enzyme that is more complex than previously proposed.

Chemosensors displaying pH controlled multistage fluorescence emission

A large number of systems displaying pH controlled multistages of fluorescence emission intensity, have not been recognized from this point of view. In this work we present two paradigmatic cases: (i) polyamine receptors linked to a fluorophore unit, in which the signal is modulated by an intra-molecular pH dependent electron transfer process, (ii) phenols presenting adiabatic excited state proton transfer. In the first example the chemosensors are constituted by a receptor unit containing a polyaza moiety suitable for binding protons (but also metal ions or anions). The receptor is linked to a benzene or naphthalene unit that acts as a fluorophore. The fluorescence signal is dependent on the protonation state of the receptor. In the second example, addition of buffers are used to tune the intensity of the fluorescence emission.

The pathway of electron transfer in NADH:Q oxidoreductase

The pre-steady-state reduction by NADPH of NADH:Q oxidoreductase, as present in submitochondrial particles, has been further investigated with the rapid-mixing, rapid-freezing technique. It was found that trypsin treatment, that had previously been used to inactivate the transhydrogenase activity (Bakker, P.T.A. and Albracht, S.P.J. (1986) Biochim. Biophys. Acta 850, 413-422), considerably affected the stability at pH 6.2 of the NAD(P)H oxidation activity of submitochondrial particles. Use of the inhibitor butadione circumvented this problem, thus allowing a more careful investigation of the kinetics at pH 6.2. In the presence of the inhibitor rotenone it was found that 50% of the Fe-S clusters 3 and all of the Fe-S clusters 2 and 4 could be reduced by NADPH within 30 ms at pH 6.2. The remainder of the Fe-S clusters 3 and all of the Fe-S clusters 1 were reduced slowly (complete reduction only after more than 60 s). It was concluded that these latter Fe-S clusters play no role in the NADPH oxidation activity. In the absence of rotenone at pH 6.2 only 50% of the Fe-S clusters 2-4 could be reduced within 30 ms, while Fe-S cluster 1 was again not reduced. This difference was attributed to the fast reoxidation of part of the Fe-S clusters 2 and 4 by ubiquinone. At pH 8.0, where the NADPH oxidation activity is almost zero, 50% of the Fe-S clusters 2-4 could still be reduced by NADPH within 30 ms, while Fe-S cluster 1 was not reduced. The presence of rotenone had no effect on this reduction. From these observations it is concluded that the Fe-S clusters 2 and 4, which were rapidly reduced by NADPH and reoxidised by ubiquinone at pH 6.2, could not be reduced by NADPH at 8.0. This provides an explanation why NADH:Q oxidoreductase was not able to oxidise NADPH at pH 8.0, while part of the Fe-S clusters were still rapidly reduced. As a working hypothesis a dimeric structure for NADH:Q oxidoreductase is proposed. One protomer (B) contains FMN and Fe-S clusters 1-4 in equal amounts; the other protomer (A) is identical except for the absence of Fe-S cluster 1. NADH is able to react with both protomers, while NADPH only reacts with protomer A. A pH-dependent electron transfer from protomer A to protomer B is proposed, which would allow the reduction of Fe-S clusters 2 and 4 of protomer B by NADPH at pH 6.2, which is required for NADPH:Q oxidoreductase activity.