Chemically driven electron tunnelling pumps

Abstract

The simplest mechanism for molecular electron pumps is discussed which is based on nonadiabatic electron tunnelling and nonequilibrium conformational fluctuations. Such fluctuations can be induced, e.g. by random binding of negatively charged adenosintriphosphate (ATP) molecules to the electron-transferring molecular complex, their subsequent hydrolysis and the products dissociation. The pumping rate can be controlled by the ATP concentration in solution. Depending on the model parameters there may exist a critical ATP concentration for the pump to function. Alternatively, nonequilibrium fluctuations can be induced by externally applied stochastic electric fields. For realistically chosen parameters, the mechanism is shown to be robust and highly efficient.

Keywords:

• Electron transfer
• Tunnelling
• Dissipation
• Nonequilibrium fluctuations
• Molecular electron pumps

Notes

In the living cells ATP is perpetually synthesised from ADP and by the ATP-syntase membrane complexes fuelled by the electrochemical proton gradient [3,5].

Non-Markovian generalisation of this scheme accounting for complex dynamics with memory within a conformation can be done, e.g. in the framework of a stochastic trajectories description, cf. Refs. 48-50 25.

In the case of nitrogenase, where the ATP binding site is located at the distance of about from the electron donor site, the actual electrostatic mechanism is more intricate [14]. Namely, binding of ATP molecules causes a structural rearrangement of the protein complex with the donor metallocluster relocating from the protein surface, where it is exposed to water (with a large dielectric constant of about ), into the protein interior (with much smaller dielectric constant, ). Roughly speaking, the increase of the electron donor energy in the conformation 2 can be related to the extra change of Born solvation energy of the donor metallocluster due to the excess, transferring electron (i.e. change of the charging energy due to the desolvation of metallocluster).

It compares well with the efficiency of various ionic pumps which can be as high as [5].