Enantioselective Adsorption of Laccase on Homocysteine-Modified Au(111) Single-Crystalline Electrodes for Oxygen Reduction

Efficient direct electron transfer (DET) of metalloenzyme-modified electrodes is critical for high performance of bioelectrochemical devices including enzymatic fuel cells. In this work, we investigated the effect of enantioselectivity on the DET-based electrocatalytic activity of metalloenzyme-modified electrodes for the oxygen reduction reaction (ORR). Au(111) single-crystalline electrodes were modified with a metalloenzyme of laccase (Lac), isolated from Rhus vernicifera, via covalent bonding with a self-assembled monolayer (SAM) of amino acids of l- or d-homocysteine (l-/d-Hcy). The Lac-d-Hcy/Au(111) electrode showed higher electrocatalytic current densities for the ORR in comparison to the Lac-l-Hcy/Au(111) electrode. Electrochemical measurements and high-speed atomic force microscopy (HS-AFM) observation confirmed the immobilization of Lac on the d-Hcy/Au(111) surface with an increased amount of DET-active Lac via enantioselective adsorption: a 4-fold greater adsorbed amount of DET-active Lac. The comparison between current densities for the ORR normalized by the amount of DET-active Lac revealed that the normalized current densities of Lac-d-Hcy/Au(111) and Lac-l-Hcy/Au(111) electrodes were comparable but higher than those of the Lac-modified Au(111) electrodes via a racemic Hcy SAM (Lac-dl-Hcy/Au(111)). These results indicate that not only the enantioselective adsorption but also the chirality-induced spin selectivity (CISS) effect govern apparent current densities of the Hcy-modified Au(111) electrodes for the ORR. Our findings clearly demonstrate the chirality-driven enhancement of DET in enzyme electrocatalysis and will serve as the basis for designing and developing bioelectrochemical devices leveraging enantioselective interfaces.