Electron Transfer across Helical Peptides

Electron transfer (ET) across proteins is one of the fundamental processes in nature. While trying to decipher and understand natural ET, much scientific effort has been employed in scaling down the process to the role of the elementary secondary-structure units of proteins, that is, β sheets and α helices. Among these two motifs, the vast majority of studies have focused on α-helical peptides because they require fewer amino acids for formation, they can be easily assembled on surfaces, and they can be easily functionalized. Herein, the focus is only on ET across α-helical peptides, not only because of the reasons discussed, but also because they are one of the most promising biological molecules for integration into ET-based bioelectronic devices. The different methodologies used to follow ET across the peptides, namely, photoinduced ET, electrochemistry, and solid-state conductivity measurements (referred to as electron transport, ETp), are reviewed and discussed. In this context, the fundamental differences between the different methodologies, the appropriate interpretation of the results, and possible mechanisms to describe the ET(p) process in each methodology are discussed. Furthermore, possible functionalization of the helical peptides to modify and control their (opto-)electronic properties is reviewed.