Alternating Current Electrolysis for Individual Synthesis of Methanol and Ethane from Methane in a Thermo-electrochemical Cell

The individual synthesis of methanol and ethane from methane was investigated using a thermo-electrochemical cell in gas flow mode over the temperature range of 150–200 °C. Methane was directly oxidized at an anode consisting of sub-10 nm Pt and Fe particles. In the electrolysis of humidified methane, methanol was produced through the formation of active oxygen intermediates from water vapor. In the electrolysis of unhumidified methane, ethane was produced via the dissociation of C–H bonds, followed by dimerization of the resultant •CH3 radicals. However, the formation rates for the target products decreased with time during electrolysis because of overoxidation of the anode by the direct-current (DC) voltage. The alternating current (AC) electrolysis of methane avoided this problem. Under optimized AC polarization conditions at a square waveform voltage of ±2.5 V with a pulse time of 5 s, this cell generated methanol and ethane at rates of 5.1 × 10–5 mol m–2 s–1 (92 mmol gcat–1 h–1) and 3.8 × 10–5 mol m–2 s–1 (69 mmol gcat–1 h–1), respectively, without a substantial loss of activity during continuous electrolysis

Superposition of EcTI in free and bound state.

<p>(a) Interface between EcTI (yellow ribbon) and trypsin (green space filling representation) in the inhibitor-trypsin complex. The side chains of Arg64 and Leu115 of EcTI are shown as sticks. A superimposed tracing of molecule B of free EcTI is shown in red, clearly indicating that the conformational changes in the reactive loop around Arg64 are needed in order to avoid clashes and allow the proper fit of its side chain into the S1 pocket of the enzyme. (b) Steroview of the superposition of trypsin-bound EcTI and STI (PDB ID: 1AVW). Color highlights are for the loops which are most different between these two proteins, blue for EcTI and red for STI, whereas other parts of the chain are gray.</p

Interactions between EcTI and trypsin.

<p>Interactions between EcTI and trypsin.</p

EcTI and its complex with trypsin.

<p>(a) Ribbon representation of the overall three-dimensional structure of EcTI. β-strands labeled β1–12 are shown in red, and loops labeled L1–13 are in green. Loop L10 is only marked for consistency with the other related structures, since the main chain is broken in this region. (b) Pseudo-dimer of the two crystallographically independent molecules of EcTI in the asymmetric unit. The main contact loops are labeled. (c) Overall structure of the EcTI-trypsin complex shown as a cartoon diagram. EcTI is colored green, while trypsin is red, and the reactive loop is labeled.</p

Inhibitory properties of EcTI and the related inhibitors.

<p>n.i – no inhibition. The values in parentheses correspond to the standard deviations of the calculated data. Blank fields – data not collected or available.</p>*<p>Taken from Patil et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0062252#pone.0062252-Patil1" target="_blank">[19]</a>.</p

Preparation of the EcTI-trypsin complex.

<p>Free EcTI and its complex with trypsin were run on the same Sephacryl S-100 HR column, with the same buffer. The two curves were overlaid, with the blue one representing EcTI-trypsin and the red one representing free EcTI.</p

The reactive loop of EcTI.

<p>(a,b) 2Fo-Fc electron density maps contoured at 1.2σ of the reactive loop area of (a) free EcTI and (b) its complex with trypsin. (c) A steroview of the hydrogen networks of the reactive loop region of a molecule of free EcTI. Blue dashed lines indicate the hydrogen bond network formed by the side chain of Asn13 and the interacting residues, and red dashed lines indicate the hydrogen bond network formed by the reactive loop region residues and their partners. Green spheres represent water molecules.</p

Data collection and structure refinement.

*<p>The highest resolution shell is shown in parentheses.</p>†<p><i>R</i>_merge = ∑_h∑_i|<i>I</i>_i–〈<i>I</i>〉|/∑_h∑_i<i>I</i>_i, where I_i is the observed intensity of the i-th measurement of reflection h, and 〈I〉 is the average intensity of that reflection obtained from multiple observations.</p>‡<p><i>R</i> = ∑||<i>F_o</i>|–|<i>F_c</i>||/∑<i>|F_o|,</i> where F_o and F_c are the observed and calculated structure factors, respectively, calculated for all data. <i>R</i>_free was defined in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0062252#pone.0062252-Brnger1" target="_blank">[33]</a>.</p

Rigid body movement of the Kunitz inhibitors bound to trypsin.

<p>(a) Superposition of the trypsin complexes with EcTI, STI, and TKI based on the Cα coordinates of trypsin reveals overall rigid body shifts of the inhibitors. The tracing of the trypsin chain in the complex with EcTI is shown as green ribbon, EcTI is yellow, STI red, and TKI magenta. (b) Superposition of EcTI, STI, and TKI based on their Cα coordinates (colored as in panel (a)) reveals a small change in the orientation of the reactive loop (residues 61–67). (c) Superposition of EcTI bound to trypsin with the STI complex. STI is shown in orange and trypsin in space-filling green. In order to show the reason for the rigid body movement of EcTI it is shown twice, in yellow when superimposed directly on STI, and in blue when the trypsin molecules of the two complexes were superimposed.</p

Close-up of the interactions of EcTI and trypsin.

<p>(a) Steroview of the region of interactions, with residues belonging to trypsin shown as cyan sticks and the disulfide bond colored yellow. The residues of EcTI are shown as green sticks and the gray spheres represent water molecules. Hydrogen bonds are indicated by red dashed lines. (b) Interface of the EcTI-trypsin complex, with EcTI shown as a green ribbon with selected side chains in stick representation, whereas the surface of trypsin is colored according to charge (blue positive, red negative, gray uncharged).</p