Theoretical Evidence for Multiple Charge Transfer Pathways in Bacteriorhodopsin

The development of molecular-scale junctions utilizing biomolecules is a challenging field that requires intimate knowledge of the relationship between molecular structure and conductance characteristics. One of the key parameters to understanding conductance efficiency is the charge mobility, which strongly influences the response time of electronic devices. The charge mobility of bacteriorhodopsin (bR), a membrane protein that has been studied experimentally in detail, was theoretically investigated using extended Marcus-Hush theory. Charge mobilities of 1.3 × 10(-2) and 9.7 × 10(-4) cm(2)/(V s) for hole and electron transfer, respectively, were determined. The computed electron mobility is comparable to experimentally measured values (9 × 10(-4) cm(2)/(V s)). Interestingly, the pathways for hole and electron hopping were very distinct from each other, utilizing different transmembrane helices to traverse the protein. In particular, only the electron transfer pathway involved the retinal chromophore, indicating that the efficiency of charge transfer is directly affected by the tertiary arrangement of proteins. Our results provide a template for obtaining the molecular and electronic-level details that can reveal fundamental insights into experimental studies on protein electron transport and inform efficient design of biomolecular-based junctions on the nanoscale.