Biomimetic peptide-based models of [FeFe]-hydrogenases: utilization of phosphine-containing peptides

Abstract

Two synthetic strategies for incorporating diiron analogues of [FeFe]-hydrogenases into short peptides via phosphine functional groups are described. First, utilizing the amine side chain of lysine as an anchor, phosphine carboxylic acids can be coupled via amide formation to resin-bound peptides. Second, artificial, phosphine-containing amino acids can be directly incorporated into peptides via solution phase peptide synthesis. The second approach is demonstrated using three amino acids each with a different phosphine substituent (diphenyl, diisopropyl, and diethyl phosphine). In total, five distinct monophosphine-substituted, diiron model complexes were prepared by reaction of the phosphine-peptides with diiron hexacarbonyl precursors, either (μ-pdt)Fe2(CO)6 or (μ-bdt)Fe2(CO)6 (pdt = propane-1,3-dithiolate, bdt = benzene-1,2-dithiolate). Formation of the complexes was confirmed by UV/Vis, FTIR and (31)P NMR spectroscopy. Electrocatalysis by these complexes is reported in the presence of acetic acid in mixed aqueous-organic solutions. Addition of water results in enhancement of the catalytic rates.

Hydrophilic Quaternary Ammonium‐Group‐Containing [FeFe]‐Hydrogenase Models: Synthesis, Structures, and Electrocatalytic Hydrogen Production

“…However, in contrast to this, Table and Figure indicate that the oxidation potentials of 2 and 4 are almost unaffected by water. Similar electrochemical behavior in the mixed MeCN/H 2 O solvent was also observed for other diiron complexes with hydrophilic phosphine ligands . Although Darensbourg and co‐workers proposed that the positive shift of the reduction potentials is due to hydrogen‐bonding interactions with the nitrogen atom of the PTA ligand, Jones and co‐workers suggested that the positive shift of the reduction potentials is due to interaction of the reduced species with the MeCN/H 2 O solvent .…”

“…Similar electrochemical behavior in the mixed MeCN/H 2 O solvent was also observed for other diiron complexes with hydrophilic phosphine ligands . Although Darensbourg and co‐workers proposed that the positive shift of the reduction potentials is due to hydrogen‐bonding interactions with the nitrogen atom of the PTA ligand, Jones and co‐workers suggested that the positive shift of the reduction potentials is due to interaction of the reduced species with the MeCN/H 2 O solvent . According to the suggestions indicated above, we suggest that such unusual electrochemical behavior displayed by our models 2 and 4 might be attributed to hydrogen‐bonding interactions of the nitrogen atom in the PNP ligand with water and interactions of the reduced species with water in the mixed solvent, although this hypothesis needs to be investigated further…”

“…To evaluate the electrocatalytic abilities of 2 and 4 , we determined the ratio of the catalytic current ( i cat ) to reductive peak current in the absence of the added acid ( i p ; note that a higher values of i cat / i p indicates faster catalysis) . At the highest concentration of HOAc (50 m m ), the i cat / i p values for 2 and 4 were 9.18 and 10.62, respectively (see Figure S6 in the Supporting Information).…”

“…[38,58,59,[63][64][65][66] To evaluatethe electrocatalytic abilities of 2 and 4,wedetermined the ratio of the catalytic current( i cat )t or eductive peak current in the absence of the added acid (i p ;notethat ah igher values of i cat /i p indicates faster catalysis). [62,67,68] At the highest concentrationo fH OAc (50 mm), the i cat /i p values for 2 and 4 were 9.18 and 10.62,r espectively (see Figure S6 in the Supporting Information). We further determined the overpotentials of 2 and 4 in MeCN according to the Artero relationship (note that the overpotential is equal to the difference between the potentiala th alf-maximum of the catalytic current and the theoreticalh alf-waver eduction potential (E 1/2 T )o ft he acid).…”

Catalytic Metallopolymers from [2Fe‐2S] Clusters: Artificial Metalloenzymes for Hydrogen Production

“…to covalently attach a [2Fe‐2S] cluster onto relatively larger peptides, and they demonstrated catalytic activity with this [2Fe‐2S]‐peptide system with a TON of 84 over 2 hours in aqueous solution . In a follow‐up paper, phosphine functional groups were employed in the short peptides, instead of 1,3‐dithiol groups as in the previous paper, to replace one of the carbonyl ligands on the [2Fe‐2S] cluster and to form a [2Fe‐2S]‐peptide system …”