Armed by Asp? C-terminal carboxylate in a Dap-branched peptide and consequences in the binding of Cu^II and electrocatalytic water oxidation

Abstract

The imidazole ring of histidine (His) and the methylcarboxylate function of aspartic acid (Asp) are crucial for crucial for catalysis, but also acts as inhibitor anion according to cyclic voltammetry observations. The system is highly tolerant to the presence of chloride, which is a feature of practical importance in efficient water oxidation catalysis.

Copper Containing Molecular Systems in Electrocatalytic Water Oxidation—Trends and Perspectives

“…In contrast to former studies on bpy-based systems (4-6) adding the metallopeptoid as solid to an aqueous solution at pH 12.5 resulted in a brown precipitate. On the other hand the complex is stable in aqueous phosphate buffer up to pH 11.5, conditions more similar to those applied in the case of peptide-containing systems (discussed later) [32,[41][42][43]. Under these conditions oxygen evolution occurred with kobs of 5.8 s −1 (determined by foot-of-the-wave, i.e., FOWA analysis [44]) and Faraday efficiency of up to 91%.…”

“…This equilibrium was also proposed to be responsible for the film formation, owed to the local pH decrease during the controlled potential electrolysis. Accumulation of molecular catalysts on electrode surfaces has been proposed to cause deviations from the diffusion controlled behavior upon electrolysis [31,32] and in other cases coordination polymerization was exploited to generate catalytic films [33][34][35]. These observations may inspire further investigations on the pH-dependent speciation and redox transformations of molecular systems and how these properties influence their affinity for surfaces that can potentially promote their immobilization at the electrode via adsorption instead of covalent linkage.…”

“…A set of C-terminally amidated homolog ligands were therefore reported by utilizing a branching unit, Dap = L-2,3-diaminopropionic acid that has been extended at the two vicinal N-termini and the C-terminus with various combinations of amino acids. The imidazole ring of histidine (His) and the methylcarboxylate function of aspartic acid (Asp) are typical anchoring groups for Cu binding in enzymes, artificial proteins and other bio-inspired systems and this fact has inspired a set of ligands including Dap and His, Asp, or Gly amino acids [32,72,73].…”

“…The effect of Asp at the C-terminus was investigated later by applying H-Gly-Dap(H-Gly)-Asp-NH 2 (2GD) to ligate Cu II [32]. At pH < 8, 2GD formed a dimeric 2Cu:2L complex that showed no catalytic activity.…”

New peptide-type tripodal ligands and their metal complexes

“…Branched peptides investigated by Szyrwiel and colleagues can be considered as close relatives of tripodal peptides; they -especially the trihistidyl compound 3H -were shown to possess remarkable binding affinity towards Cu(II) ions [37,38]. They also described a Asp-functionalized ligand, which forms catalytically active copper(II) complexes for electrocatalytic water oxidation [39]. As example for non-histidyl compounds, a tren-based alanine ligand has been synthesized by Mohamadou et al, and characterized in presence of Cu(II) and Ni(II) ions.…”