The design of new artificial photoenzymes, in which electron transfer (ET) pathways are engineered on the surface of the protein, often yields active sites with properties that are substantially different from natural templates. In the studied azurin construct, one or two tryptophans were inserted between the appended Re photosensitiser and the copper centre, forming a hole transfer pathway. Although the presence of aromatic amino acids significantly accelerated charge transfer, the hole transfer rates were lower than those of tuned natural enzymes, and the quantum yield was reduced [1]. To understand these experimental observations, we combine quantum chemical calculations with QM/MM dynamics to characterise the ET mechanism [2]. We compare the empirical Pathways model [3] with well-established quantum approaches for evaluating electronic coupling matrix elements using geometries sampled from QM/MM molecular dynamics. Additionally, we employ a tunnelling-currents approach to calculate ET rates at the ab initio/DFT level. This method provides valuable insight in the form of spatially resolved electronic current flow, supplementing electronic coupling information [4]. We believe this approach is a valuable tool for identifying key active sites and understanding the ET mechanism in large systems, which is essential for the design of novel photoenzymes.
Tomáš Grycz