Search

The effect of buffer pKaon the mechanism of electrochemical hydrogen evolution catalyzed by a cobalt porphyrin peptide (CoMP11-Ac) at constant pH is presented. The addition of buffer to CoMP11-Ac in water and KCl leads to an enhancement of the catalytic current of up to 200-fold relative to its value in the absence of a buffer. Two distinct catalytic regimes are identified as a function of the buffer pKa. In the presence of buffers with pKa≤ 7.4, a fast catalysis regime limited by diffusion of buffer is reached. The catalytic half-wave potential (Eh) shifts anodically (from −1.42 to −1.26 V vs Ag/AgCl/KCl(1M)) as the buffer pKadecreases from 7.4 to 5.6, proposed to result from fast Co(III)-H formation following the catalysis-initiating Co(II/I) reduction. With higher-pKabuffers (pKa> 7.7), an Eh= −1.42 V, proposed to reflect the Co(II/I) couple, is maintained independent of the buffer pKa, consistent with rate-limiting Co(III)-H formation under these conditions. We conclude that the buffer species pKaimpacts catalytic current and potential and the rate-determining step of the reaction.

Cobalt-mimochrome VI*a (CoMC6*a) is a synthetic mini-protein that catalyzes aqueous proton reduction to hydrogen (H2). In buffered water, there are multiple possible proton donors, complicating the elucidation of the mechanism. We have found that the buffer pKaand sterics have significant effects on activity, evaluated via cyclic voltammetry (CV). Protonated buffer is proposed to act as the primary proton donor to the catalyst, specifically through the protonated amine of the buffers that were tested. At a constant pH of 6.5, catalytic H2evolution in the presence of buffer acids with pKavalues ranging from 5.8 to 11.6 was investigated, giving rise to a potential–pKarelationship that can be divided into two regions. For acids with pKavalues of ≤8.7, the half-wave catalytic potential (Eh) changes as a function of pKawith a slope of −128 mV/pKaunit, and for acids with pKaof ≥8.7, Ehchanges as a function of pKawith a slope of −39 mV/pKaunit. In addition, a series of buffer acids were synthesized to explore the influence of steric bulk around the acidic proton on catalysis. The catalytic current in CV shows a significant decrease in the presence of the sterically hindered buffer acids compared to those of their parent compounds, also consistent with the added buffer acid acting as the primary proton donor to the catalyst and showing that acid structure in addition to pKaimpacts activity. These results demonstrate that buffer acidity and structure are important considerations when optimizing and evaluating systems for proton-dependent catalysis in water.

CoGGH, a Gly-Gly-His tripeptide coordinated to a cobalt ion, is shown to catalyze the reduction of aqueous protons to hydrogen (H2) in a light-driven reaction in water near neutral pH. Using [Ru(bpy)3]2+as a photosensitizer and ascorbate as an electron donor, a turnover number up to 2200 with respect to CoGGH has been observed with the system remaining active for more than 48 h. The reaction conditions that favor H2production are consistent with a reductive quenching mechanism. Results also suggest that CoGGH is robust under these reaction conditions and loss of activity over time results from [Ru(bpy)3]2+degradation.

Out-of-plane (OOP) deformations of the heme cofactor are found in numerous heme-containing proteins and the type of deformation tends to be conserved within functionally related classes of heme proteins. We demonstrate correlations between the heme ruffling OOP deformation and the 13C and 1H nuclear magnetic resonance (NMR) hyperfine shifts of heme aided by density functional theory (DFT) calculations. The degree of ruffling in the heme cofactor of Hydrogenobacter thermophiluscytochrome c552has been modified by a single amino acid mutation in the second coordination sphere of the cofactor. The 13C and 1H resonances of the cofactor have been assigned using one- and two-dimensional NMR spectroscopy aided by selective 13C-enrichment of the heme. DFT has been used to predict the NMR hyperfine shifts and electron paramagnetic resonance (EPR) g-tensor at several points along the ruffling deformation coordinate. The DFT-predicted NMR and EPR parameters agree with the experimental observations, confirming that an accurate theoretical model of the electronic structure and its response to ruffling has been established. As the degree of ruffling increases, the heme methyl 1H resonances move upfield while the heme methyl and meso 13C resonances move downfield. These changes are a consequence of altered overlap of the Fe 3d and porphyrin π orbitals, which destabilizes all three occupied Fe 3d-based molecular orbitals and decreases the positive and negative spin density on the β-pyrrole and meso carbons, respectively. Consequently, the heme ruffling deformation decreases the electronic coupling of the cofactor with external redox partners and lowers the reduction potential of heme.

A cobalt-tripeptide complex (CoGGH) is developed as an electrocatalyst for the selective six-electron, eight-proton reduction of nitrite to ammonium in aqueous buffer near neutral pH. The onset potential for nitrite reduction occurs at −0.65 V vs Ag/AgCl (1 M KCl). Controlled potential electrolysis at −0.90 V generates ammonium with a faradaic efficiency of 90 ± 3% and a turnover number of 3550 ± 420 over 5.5 h. CoGGH also catalyzes the reduction of the proposed intermediates nitric oxide and hydroxylamine to ammonium. These results reveal that a simple metallopeptide is an active functional mimic of the complex enzymes cytochrome cnitrite reductase and siroheme-containing nitrite reductase.

Heme peptides and their derivatives, also called microperoxidases (MPs), are employed as heme protein active site models, catalysts, and charge-transfer chromophores. In this work, two approaches to the biosynthesis of novel MPs are described. In one method, heme peptides are expressed as C-terminal tags to the protein azurin and the MP is liberated by proteolytic cleavage by an endopeptidase. In an alternative approach, heme peptides are expressed as N-terminal tags to the cysteine protease domain (CPD) of the Vibrio choleraeMARTX toxin. Once activated by inositol hexakisphosphate, CPD undergoes autocleavage at an N-terminal leucine residue to liberate the MP. Purification is aided by use of a histidine-immobilized Sepharose column that binds exposed heme [Asher, W. A., and Bren, K. L. (2010) Protein Sci. 19, 1830–1839]. These methods provide efficient and adaptable routes to the preparation of a wide range of biosynthetic heme peptides.

Zinc-substituted cytochrome c(Zn-cyt c) is noncovalently bound to single-walled carbon nanotubes (SWNTs), causing the Zn-cyt cfluorescence to be quenched by up to 95%, primarily due to photoinduced charge transfer. Deposition of Zn-cyt c/SWNT films onto conductive oxides allows for harvesting of photoexcited electrons with an internal quantum efficiency of over 5%.

Nuclear magnetic resonance spectroscopy is employed to characterize unfolding intermediates and the denatured state of horse ferricytochrome c in guanidine hydrochloride. Unfolded and partially unfolded species with non-native heme ligation are detected by analysis of hyperfine-shifted 1H resonances. Two equilibrium unfolding intermediates with His-Lys heme axial ligation are detected, as are two unfolded species with bis-His heme ligation. These results are contrasted with previous results on horse ferricytochrome c denaturation by urea, for which only one unfolding intermediate and one unfolded species were detected by NMR spectroscopy. Urea and guanidine hydrochloride are often used interchangeably in protein denaturation studies, but these results and those of others indicate that unfolded and intermediate states in these two denaturants may have substantially different properties. Implications of these results for folding studies and the biological function of mitochondrial cytochromes c are discussed.

Cytochrome rC557is an improperly matured, dimeric cytochrome cobtained from expression of the “signal peptide-lacking” Thermus thermophilus cycAgene in the cytoplasm of Escherichia coli. It is characterized by its Q00(or α-) optical absorption band at 557 nm in the reduced form (Keightley, J. A., Sanders, D., Todaro, T. R., Pastuszyn, A., and Fee, J. A. (1998) J. Biol. Chem.273, 12006–12016). We report results of a broad ranging, biochemical and spectral characterization of this protein that reveals the presence of a free vinyl group on the porphyrin and a disulfide bond between the protomers and supports His-Met ligation in both valence states of the iron. A 3-Å resolution x-ray structure shows that, in comparison with the native protein, the heme moiety is rotated 180° about its α,γ-axis; cysteine 14 has formed a thioether bond with the 2-vinyl of pyrrole ring I instead of the 4-vinyl of pyrrole ring II, as occurs in the native protein; and a cysteine 11 from each protomer has formed an intermolecular disulfide bond. Numerous, minor perturbations exist within the structure of rC557in comparison with that of native protein, which result from heme inversion and protein-protein interactions across the dimer interface. The unusual spectral properties of rC557are rationalized in terms of this structure.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.