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The anaerobic region of the gastrointestinal (GI) tract has been replicated in the anaerobic chamber of a microbial fuel cell (MFC). Electroactive biomolecules released by the facultative anaerobes (Providencia rettgeri) under anoxic conditions have been studied for their potential role for redox balance. MALDI study reveals the presence of vitamin B9 (folate), 6-methylpterin, para-aminobenzoic acid (PABA) and pteroic acid called pterin pool. ATR-FTIR studies further confirm the presence of the aromatic ring and side chains of folate, 6-methylpterin and PABA groups. The photoluminescence spectra of the pool exhibit the maximum emission at 420, 425, 440, and 445 nm when excited by 310, 325, 350, and 365 nm wavelengths (day 20 sample) highlighting the presence of tunable bands. The cyclic voltammetric studies indicate the active participation of pterin pool molecules in the transfer of electrons with redox potentials at − 0.2 V and − 0.4 V for p-aminobenzoate and pterin groups, respectively. In addition, it is observed that under prolonged conditions of continuous oxidative stress (> 20 days), quinonoid tetrahydrofolate is formed, leading to temporary storage of charge. The results of the present study may potentially be useful in designing effective therapeutic strategies for the management of various GI diseases by promoting or blocking folate receptors. [ABSTRACT FROM AUTHOR]

Green colored mononuclear copper(II) complexes viz. [Cu(L)(bpy)](ClO4) (1) or [Cu(L)(phen)](ClO4) (2) (where H(L) is 2-((2-dimethylamino)ethyliminomethyl)naphthol) show distorted square pyramidal (4 + 1) geometry with CuN4O chromophore. The existence of self-assembled molecular associations indicates the formation of the dimer. Dimeric nature in solution is retained due to the binding of the substrate, encourages steric match between substrate and Cu(II) active site, which favors electron transfer. Interestingly, both the complexes exhibit high-positive redox potential. Therefore, the presence of self-assembled molecular association along with the positive redox potential enhances the catalytic oxidation of ascorbic acid to dehydroascorbic acid or benzylamine to benaldehyde or catechol to o-quinone thereby model the functional properties of type 2 and type 3 copper oxidases. Notably, catalytic activity is effective when compared with other reported mononuclear copper(II) complexes and even superior to many binuclear copper(II) complexes. Existence of self-assembled molecular association in solution along with high-positive redox potential favors electron transfer process in mononuclear copper(II) complexes and models the functional properties of type 2 and type 3 copper oxidases. [ABSTRACT FROM AUTHOR]

The femtosecond dynamics of photoinduced electron transfers in supramolecular donor–acceptor complexes between (E)-bis(18-crown-6)stilbene (D) and tetraperchlorates of 2,7-di(2-ammonioethyl)(2,7-diazapyrenium) (A1), 3,3′-(E)-ethene-1,2-diylbis[1-(3-ammoniopropyl)pyridinium] (A2) and 4,4′-ethane-1,2-diylbis[1-(3-ammoniopropyl)pyridinium] (A3) was studied. The acceptors A2 and A3 are weak electron acceptors whose first reduction potentials are equal to −1.0 and −1.2 V (Ag), respectively, while A1 is a strong acceptor with a reduction potential of −0.42 V. It was shown that the back electron transfer time in CT-states of the complexes D·A2 and D·A3 is 30–40 ps, which is approximately 50 times greater than the analogous time for the charge transfer complexes studied earlier. The complex D·A1 is characterized by ultrafast back electron transfer (770 fs). The relaxation pathway of excited states of D·A1 depends on the wavelength of the excitation light. When excited at 356 nm, the accumulation of a transient locally excited (LE) state with a 250 fs lifetime was observed. But when excited at 425 nm, the formation of the LE-state was not observed. [ABSTRACT FROM AUTHOR]

Microbial fuel cells (MFCs) have emerged as a promising technology for sustainable wastewater treatment coupled with electricity generation. A MFC is a device that uses microbes as catalysts to convert chemical energy present in biomass into electrical energy. Among the various mechanisms that drive the operation of a MFC, extracellular electron transfer (EET) to the anode is one of the most important. Exoelectrogenic bacteria can natively transfer electrons to a conducting surface like the anode. The mechanisms employed for electron transfer can either be direct transfer via conductive pili or nanowires, or mediated transfer that involves either naturally secreted redox mediators like flavins and pyocyanins or artificially added mediators like methylene blue and neutral red. EET is a mechanism wherein microorganisms extract energy for growth and maintenance from their surroundings and transfer the resulting electrons to the anode to generate current. The efficiency of these electron transfer mechanisms is dependent not only on the redox potentials of the species involved, but also on microbial oxidative metabolism that liberates electrons. Attempts at understanding the electron transfer mechanisms will boost efforts in giving rise to practical applications. This article covers the various electron transfer mechanisms involved between microbes and electrodes in microbial fuel cells and their applications. [ABSTRACT FROM AUTHOR]

The electrochemical properties of N-methyl- and N-phenyl-2,4,6-triphenylpyridinium perchlorates were studied by cyclic voltammetry (CV). The substituent at the nitrogen atom strongly affects the potentials of salt reduction and the electrochemical reversibility of the transfer of the second electron. [ABSTRACT FROM AUTHOR]

Quantum chemical modeling of Fe(III), Fe(II), and Fe(I) aqua-, aqua-hydroxo-, and aquadihydroxocomplexes is presented. The mechanism of a consecutive transfer of two electrons is studied as these forms are electrochemically reduced from an aqueous solution. The reorganization energy of the solvent and the inner sphere of studied reagents is calculated, standard redox potentials are estimated. Based on Marcus theory, the activation energy of two steps of Fe(III) reduction is estimated and the second electron transfer is shown to be rate controlling, while the energy barrier is increased due to the products of Fe(III) hydrolysis. The model predictions are in qualitative agreement with previously reported experimental data. [ABSTRACT FROM AUTHOR]

Molecular phenomena involving electron transfer and reduction/oxidation processes are of the utmost importance in chemistry. However, accurate computational calculations of standard reduction potentials (SRPs) for transition metal complexes are still challenging. For this reason, some computational strategies have been proposed in order to overcome the main limitations in SRP calculations for copper complexes. However, these strategies are limited to particular coordination spheres and do not represent a general methodology. In this work, we present standard reduction potential calculations for copper complexes in aqueous solution covering a wide range of coordination spheres. These calculations were performed using the M06-2X density functional, and by employing the direct and isodesmic approaches. Result analysis reveals that values obtained with the use of the isodesmic method are in better agreement with experimental values than those obtained from the direct method (mean unsigned error 0.39 V with the direct and 0.08 V with the isodesmic method). This approach provides values with errors comparable to the experimental uncertainty due to the proper cancellation of computational errors. These results strongly suggest the isodesmic approach as an adequate methodology for the calculation of SRPs for copper complexes with diverse coordination spheres. [ABSTRACT FROM AUTHOR]

Neuroglobin (Ngb) is a recently identified hexa-coordinated globin, expressed in the nervous system of humans. Its physiological role is still debated: one hypothesis is that Ngb serves as an electron transfer (ET) species, possibly by reducing cytochrome c and preventing it to initiate the apoptotic cascade. Here, we use the perturbed matrix method (PMM), a mixed quantum mechanics/molecular dynamics approach, to investigate the redox thermodynamics of two neuroglobins, namely the human Ngb and GLB-6 from invertebrate Caenorhabditis elegans. In particular, we calculate the reduction potential of the two globins, resulting in an excellent agreement with the experimental values, and we predict the reorganization energies, λ, which have not been determined experimentally yet. The calculated λ values match well those reported for known ET proteins and thereby support a potential involvement in vivo of the two globins in ET processes. [ABSTRACT FROM AUTHOR]

The reduction potential of an electron transfer protein is one of its most important functional characteristics. Although the type of redox site and the protein fold are the major determinants of the reduction potential of a redox-active protein, its amino acid sequence may tune the reduction potential as well. Thus, homologous proteins can often be divided into different classes, with each class characterized by a biological function and a reduction potential. Site-specific mutagenesis of the sequence determinants of the differences in the reduction potential between classes should change the reduction potential of a protein in one class to that of the other class. Here, a procedure is presented that combines energetic and bioinformatic analysis of homologous proteins to identify sequence determinants that are also good candidates for site-specific mutations, using the [4Fe-4S] ferredoxins and the [4Fe-4S] high-potential iron-sulfur proteins as examples. This procedure is designed to guide site-specific mutations or more computationally expensive studies, such as molecular dynamics simulations. To make the procedure more accessible to the general scientific community, it is being implemented into CHARMMing, a Web-based portal, with a library of density functional theory results for the redox site that are used in the setting up of Poisson-Boltzmann continuum electrostatics calculations for the protein energetics. [ABSTRACT FROM AUTHOR]

The reduction potentials of electron transfer proteins are critically determined by the degree of burial of the redox site within the protein and the degree of permanent polarization of the polypeptide around the redox site. Although continuum electrostatics calculations of protein structures can predict the net effect of these factors, quantifying each individual contribution is a difficult task. Here, the burial of the redox site is characterized by a dielectric radius R (a Born-type radius for the protein), the polarization of the polypeptide is characterized by an electret potential ϕ (the average electrostatic potential at the metal atoms), and an electret-dielectric spheres (EDS) model of the entire protein is then defined in terms of R and ϕ. The EDS model shows that for a protein with a redox site of charge Q, the dielectric response free energy is a function of Q, while the electret energy is a function of Q. In addition, R and ϕ are shown to be characteristics of the fold of a protein and are predictive of the most likely redox couple for redox sites that undergo different redox couples. [ABSTRACT FROM AUTHOR]