Kendra Kathan-Galipeau, Sanjini Nanayakkara, Paul A.Journal of the American Chemical Society 2012, 134 Self-Assembly of Highly Ordered Peptide Amphiphile Metalloporphyrin Arrays. Protein Design: Toward Functional Metalloenzymes. Mocny, Leela Ruckthong, Hira Qayyum, and Vincent L. Electron Transfer in Binary Hemin-Modified Alkanethiol Self-Assembled Monolayers on Gold: Hemin’s Lateral and Interfacial Interactions. This article is cited by 33 publications. In contrast to coupled proton exchange, CO binding/release and ligand exchange are slow on the time scale of electron tunneling between the heme edge and the electrode. The CO is released upon heme oxidation at high potentials. Consistent with solution spectroscopy, CO must displace one axial histidine to the heme to form the His−CO form of the ferrous heme. Reduction of the heme in the presence of CO-saturated buffer shifted the oxidation peak from −0.2 to +0.35 V, indicating massive preferential CO binding to the reduced heme. The rate of electron transfer at zero driving force between the hemes and the gold electrode was determined to be 120 s -1, a rate consistent with tunneling through the mercaptoundecanoic acid spacer and suggesting that the coupled proton exchange is not rate limiting. The redox potentials correspondingly shift from −0.24 (pH > p K red, deprotonated) to −0.11 V (pH < p K ox, protonated). The pH dependency of redox midpoint potentials reveals a major (three pH units) shift of the p K a which matches the shift previously shown to originate in nearby glutamates 1. CV demonstrates the reversible electrochemistry typical for cytochrome b as well as the coupling of the b-heme oxidation and reduction to proton exchange. The positively charged residues aid adsorption to negatively charged surfaces, such as gold electrodes modified by 11-mercaptoundecanoic acid, and facilitate cyclic voltammetry (CV) measurements. Here we describe maquettes that bis-histidine ligate protoporphyrin IX (heme), much like native b cytochromes, as well as contain charged surface patches, much like native cytochrome c. The points of interest that can be now assessed are not only the processes that govern biological assembly of equilibrium structures, electrochemistry, and electron tunneling rates but also how these factors are coupled together to effect redox driven catalysis. We have designed and synthesized four-α-helix-bundle redox proteins, maquettes, that are much simplified and more robust than natural redox proteins and can be designed to bind onto electrode surfaces to facilitate systematic investigations. Experimental explorations of functional mechanisms in natural electron-transfer proteins are often frustrated by their fragility and extreme complexity.
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