Abstract
The proton pump photocycle of bacteriorhodopsin (bR) produces photocurrent on a microsecond time scale which is assigned to the deprotonation step forming the M412 intermediate. The return of the M412 intermediate to the bR ground state (bR570) has two pathways: (1) thermally via multiple intermediates (which takes 15 ms) or (2) by a more rapid and direct process by absorbing blue light (which takes hundreds of nanoseconds). By using nanoparticles (Ag, Ag-Au, and Au NPs) having different surface plasmon resonance extinction spectra, it is found that Ag NPs whose spectrum overlaps best with the M412 absorption regions enhance the stationary photocurrent 15 times. This large enhancement is proposed to be due to the accelerated photoexcitation rate of the M412 (in the presence of the plasmon field of the light in this region) as well as short-circuiting of the photocycle, increasing its duty cycles.