Solid-state electron transport in Mn-, Co-, holo-, and Cu-ferritins: force-induced modulation is inversely linked to the protein conductivity.

In this work, solid-state electron transport through the three metal core reconstituted ferritins, namely, Mn(III)-ferritin, Co(III)-ferritin, and Cu(II)-ferritin, has been probed and compared to the electron transport via the naturally-occurring iron-containing holoferritin and the metal-free apoferritin using current sensing atomic force spectroscopy (CSAFS), which allows direct contact to be established with the protein molecules. The CSAFS results reveal that by applying compressional force, in varying degrees (17-66 nN) and for varying durations (1 min, 2 min, and 3 min), the electronic conductivity of these proteins can be increased (for greater amount of force applied or for prolonged application of force) or decreased (for lesser amount of force applied or for shorter application time). The compressional modulation of the electronic conductivities appears to be due to compression of the protein part. The observation of the order of electronic conductivities of Mn-, holo-, Co-, and Cu-ferritins at almost any specific force value being similar to that of the free metal conductivities indicates that the absolute conductivity values are directly influenced by the metal core. Importantly, we found that more conductive the protein is, less modulated it can be. These findings could be highly relevant in realizing metalloprotein-based bioelectronic devices, especially where the electrode-protein-electrode sandwich configurations are employed.