Charge transport in bacteriorhodopsin monolayers: The contribution of conformational change to current-voltage characteristics

When moving from native to light activated bacteriorhodopsin, modification of charge transport consisting of an increase of conductance is correlated to the protein conformational change. A theoretical model based on a map of the protein tertiary structure into a resistor network is implemented to account for a sequential tunneling mechanism of charge transfer through neighbouring amino acids. The model is validated by comparison with current-voltage experiments. The predictability of the model is further tested on bovine rhodopsin, a G-protein coupled receptor (GPCR) also sensitive to light. In this case, results show an opposite behaviour with a decrease of conductance in the presence of light. Introduction. – In recent years, several papers reported on the electrical properties of single proteins inserted into hybrid systems [1–5]. The principal aim of these researches is to explore the possibility to design nanodevices, mainly nanobiosensors, with extreme sensibility and specificity. Most work was devoted to metalloproteins, which exhibit semiconductor-like conductivities, but increasing attention has been also addressed to bacteriorhodopsin (bR), the light sensitive protein present in Archaea. Bacteriorhodopsin exhibits a conductivity close to that of an insulator, and is robust against thermal, chemical and photochemical degradation [3,4]. It is a protein (opsin)-chromophore complex that underlies photonactivated modifications in which the chromophore changes its structure from the all-trans to the 13-cis. The chromophore evolution induces multiple protein transformations, which fastly drive the protein from the K590 to the L550 state. In the slow transition from this state to the M410, a proton is released. Junctions prepared with bR monolayers contacted with Au electrodes, show non-linear I-V characteristics both in the dark (or blue light) and in the presence of green light [3–5]. It is generally assumed that green light produces a more intense current as a consequence of the modification of the chromophore-opsin complex. Although the experimental results are not completely assessed from the quantitative side [4], the main results of available experiments can be summarized as follows. (i) A conductivity about four orders of magnitude higher than that of a homogeneous layer of similar dielectric material was observed. (ii) The current voltage (I-V ) characteristics exhibit a super-Ohmic behaviour pointing to a tunnelling charge-transfer mechanism. (iii) The presence of green light significantly increases the electrical conductivity up to about a factor of three with respect to the dark value. (iv) The I-V sensitivity to the green light is washed out by substituting the original chromophore with one that is not sensible to light. (v) When the chromophore of the protein is completely removed, the conductivity is suppressed by three orders of magnitude lower, taking the value of a standard dielectric. Despite the presence of these experiments, the existing theoretical approaches only provide a phenomenological interpretation of some I-V characteristics in terms of metal-insulatormetal tunneling theories [5, 6]. The aim of this work is to develop a theoretical model able to capture the main features of the above experiments and sufficiently general to constitute a new framework for describing electron transport properties through proteins. To this purpose the protein conductivity is modelled by a sequential tunneling mechanism through the protein amino acids, whose positions pertain to a specific conformation. Thus, the conductivity is directly connected to