Non-Gaussian fluctuations in opsins.

We investigate conductance fluctuations of two transmembrane proteins, bacteriorhodopsin and proteorhodopsin, belonging to the family of opsins. These proteins are sensitive to visible light and are promising biomaterials for the realization of novel photodevices. The conductance exhibits, over a threshold bias value, a rapid increase which is well described by a power-law behaviour. In the same way, over the threshold value the variance fast decreases following a power-law. Furthermore, the conductance fluctuations evidence a non-Gaussian behaviour with a probability density function (PDF) which follows a generalized Gumbel distribution, typical of extreme-value statistics. The theoretical model is validated on existing current-voltage measurements and the interpretation of the PDF of conductance fluctuations is proven to be in line with the microscopic mechanisms responsible of charge transport.