The Role of Vesicle Fusion Temperature in Dictating the Efficiency of Charge Transport via Bacteriorhodopsin-Based Molecular Junctions

The retinal protein bacteriorhodopsin is the major photosynthetic protein of the archaeon Halobacterium salinarum. In recent years, there is an emerging interest in understanding the long-range charge transport through single and multiple bilayers of bacteriorhodopsin (bR) with unusually small decay coefficients. Here, the charge transport is studied via a single bR vesicle monolayer prepared at different vesicle fusion temperatures in Au/cysteamine/bR/AuNW junction configuration. The variable-temperature current–voltage measurements are systematically combined with multiple complimentary surface-sensitive techniques such as Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and infrared absorption spectroscopy (PM-IRRAS). The photoelectron spectroscopy measurements carried out at different vesicle fusion temperatures indicate that the changes occurring at the amide I region of the protein's secondary structure and the positional change of C═C stretching and CH3 rocking peaks of the retinal chromophore shift the position of the HOMO onset to lower binding energies. This reduction in the HOMO onset values with increasing fusion temperature is further supported by the barrier height and coupling parameter values extracted from a Taylor expansion polynomial fit of the current–voltage relationship. Overall, the findings demonstrate that the efficiency of charge transport via bR molecules increases with the vesicle fusion temperature, a phenomenon not previously reported for protein junctions.