A single protein redox ruler

1.3 V Inorganic Sequential Redox Chain with an All-Anionic Couple 1-/2- in a Single Framework

The relatively low symmetry of [3,3′-Co(1,2-C₂B₉H₁₁)₂]⁻ ([1]⁻), along with the high number of available substitution sites, 18 on the boron atoms and 4 on the carbon atoms, allows a fairly regioselective and stepwise chlorination of the platform and therefore a very controlled tuning of the electrochemical potential tuning. This is not so easily found in other systems, e.g., ferrocene. In this work, we show how a single platform with boron and carbon in the ligand, and only cobalt can produce a tuning of potentials in a stepwise manner in the 1.3 V range. The platform used is made of two icosahedra sharing one vertex. The E_1/2 tuning has been achieved from [1]⁻ by sequential chlorination, which has given potentials whose values increase sequentially and linearly with the number of chloro groups in the platform. [Cl₈-1]⁻, [Cl₁₀-1]⁻, and [Cl₁₂-1]⁻ have been obtained, which are added to the existing [Cl-1]⁻, [Cl₂-1]⁻, [Cl₄-1]⁻, and [Cl₆-1]⁻ described earlier to give the 1.3 V range. It is envisaged to extend this range also sequentially by changing the metal from cobalt to iron. The last successful synthesis of the highest chlorinated derivatives of cobaltabis(dicarbollide) dates back to 1982, and since then, no more advances have occurred toward more substituted metallacarborane chlorinated compounds. [Cl₈-1]⁻, [Cl₁₀-1]⁻, and [Cl₁₂-1]⁻ are made with an easy and fast method. The key point of the reaction is the use of the protonated form of [Co(C₂B₉H₁₁)₂]⁻, as a starting material, and the use of sulfuryl chloride, a less hazardous and easier to use chlorinating agent. In addition, we present a complete, spectroscopic, crystallographic, and electrochemical characterization, together with a study of the influence of the chlorination position in the electrochemical properties.

By sequential halogenation of [Co(C₂B₉H₁₁)₂]⁻ ([1]⁻) with chlorine, the [Cl₈-1]⁻, [Cl₁₀-1]⁻, and [Cl₁₂-1]⁻ derivatives of [1]⁻ have been prepared and isolated. The E_1/2 values increase sequentially and linearly with the number of chloro groups in the platform. If these potentials are added to the existing E_1/2 values due to [Cl-1]⁻, [Cl₂-1]⁻, [Cl₄-1]⁻, and [Cl₆-1]⁻ described earlier, a 1.3 V range is obtained. This allows tuning of the desired potentials for the purposes of nature.

Design of a Fe4 S4 cluster into the core of a de novo four-helix bundle

We explore the capacity of the de novo protein, S824, to incorporate a multinuclear iron-sulfur cluster within the core of a single-chain four-helix bundle. This topology has a high intrinsic designability because sequences are constrained largely by the pattern of hydrophobic and hydrophilic amino acids, thereby allowing for the extensive substitution of individual side chains. Libraries of novel proteins based on these constraints have surprising functional potential and have been shown to complement the deletion of essential genes in E. coli. Our structure-based design of four first-shell cysteine ligands, one per helix, in S824 resulted in successful incorporation of a cubane Fe₄ S₄ cluster into the protein core. A number of challenges were encountered during the design and characterization process, including nonspecific metal-induced aggregation and the presence of competing metal-cluster stoichiometries. The introduction of buried iron-sulfur clusters into the helical bundle is an initial step toward converting libraries of designed structures into functional de novo proteins with catalytic or electron-transfer functionalities.

Redox Potentials and Electronic States of Iron Porphyrin IX Adsorbed on Single Crystal Gold Electrode Surfaces

Metalloporphyrins are active sites in metalloproteins and synthetic catalysts. They have also been studied extensively by electrochemistry as well as being prominent targets in electrochemical scanning tunneling microscopy (STM). Previous studies of FePPIX adsorbed on graphite and alkylthiol modified Au electrodes showed a pair of reversible Fe(III/II)PPIX peaks at about -0.41 V (vs NHE) at high solution pH. We recently used iron protoporphyrin IX (FePPIX) as an intercalating probe for long-range electrochemical electron transfer through a G-quadruplex oligonucleotide (DNAzyme); this study disclosed two, rather than a single pair of voltammetric peaks with a new and dominating peak, shifted 200 mV positive relative to the ≈-0.4 V peak. Prompted by this unexpected observation, we report here a study of the voltammetry of FePPIX itself on single-crystal Au(111), (100), and (110) and polycrystalline Au electrode surfaces. In all cases the dominating pair of new Fe(III/II)PPIX redox peaks, shifted positively by more than 200 mV compared to those of previous studies appeared. This observation is supported by density functional theory (DFT) which shows that strong dispersion forces in the FePPIX/Au electronic interaction drive the midpoint potential toward positive values. The FePPIX spin states depend on interaction with the Au(111) interface, converting all the Fe(II)/(III)PPIX species into low-spin states. These results support electrochemical evidence for the nature of the electronic coupling between FePPIX and Au-surfaces, and the electronic states of adsorbate molecules, with a bearing also on recent reports of magnetic FePPIX/Au(111) interactions in ultrahigh vacuum (UHV).