Temperature-dependent solid-state electron transport through bacteriorhodopsin: experimental evidence for multiple transport paths through proteins

Electron transport (ETp) across bacteriorhodopsin (bR), a natural proton pump protein, in the solid state (dry) monolayer configuration, was studied as a function of temperature. Transport changes from thermally activated at T > 200 K to temperature independent at <130 K, similar to what we have observed earlier for BSA and apo-azurin. The relatively large activation energy and high temperature stability leads to conditions where bR transports remarkably high current densities above room temperature. Severing the chemical bond between the protein and the retinal polyene only slightly affected the main electron transport via bR. Another thermally activated transport path opens upon retinal oxime production, instead of or in addition to the natural retinal. Transport through either or both of these paths occurs on a background of a general temperature-independent transport. These results lead us to propose a generalized mechanism for ETp across proteins, in which tunneling and hopping coexist and dominate in different temperature regimes.