Giant enhancement of fluctuation in small biological systems under external fields

The giant enhancement (GE) of fluctuation under an external field is a universal phenomenon predicted by the theoretical analysis given in (Reimann et al 2001 Phys. Rev. Lett.). Here, we propose the application of the theory of the GE of fluctuation to estimate the energy barrier of a biomolecule that exhibits its function subject to thermal noise. The rotary motor protein F₁ was used as a model, which is a component of FoF₁ adenosine triphosphate (ATP)-synthase. In the single-molecule experiment on F₁, the diffusion coeffcients of a rotary probe attached to F₁, which characterised the fluctuation of the system, were measured under the influence of an electrorotary field. These diffusion coeffcients were then used to estimate a highenergy barrier of the rotary potential of F₁ based on the theory of the GE of fluctuation. Furthermore, the ion channel protein channelrhodopsin (ChR) was used as another research model. The current fluctuations of ions moving through ChR were numerically investigated using a simulation model of the protein in the presence of an external voltage. The energy barrier for ion conduction is discussed based on the current fluctuations.