Kinetics of the N intermediate and the two pathways of recovery of the ground‐state of bacteriorhodopsin

Absorption kinetic measurements at alkaline pH, in which bacteriorhodopsin (BR) is pre-excited by another flash, indicate that a part of the recovery of the BR ground-state is faster than the decay of the N intermediate of the photocycle. This fact proves the existence of a parallel pathway in the late part of the BR photocycle (the decay of Ms into BR), which does not include the N intermediate. We demonstrate that the decay of the Mf intermediate does not lead to any direct recovery of the BR ground-state, and that excitation of N does not form an M-like intermediate. Mf decays directly into the N intermediate, and photoexcitation of N leads to the formation of a red-shifted form, O*. The kinetics of this red-shifted intermediate are also presented.