Fig. 4: Model for origin of photoconductivity in protein nanowires.

a Simplified energy level diagram for hemes depicting the changes that occur upon photoexcitation in transient absorption and their respective decay times. b The dark current in the ground state arises due to propagation of a reduced state created by electron injection from the electrode. c The photocurrent is due to the laser excitation initiating an ultrafast charge transfer between hemes, creating newly reduced (red) and double oxidized hemes (blue). The photoreduction provides additional charge carriers and larger driving force for charge transfer, which therefore increases the current under bias. d Quantum dynamics simulations of ultrafast charge transfer between hemes in protein nanowires, forming a doubly oxidized heme and an excited state of a reduced heme.