  Theoretical characterizations of the various intermediates in the proton pump
cycle of the retinal Schiff base in the Halobacterium salinarium have been
performed. Contrary to the general belief over the years that the most stable
intermediate, the M-state, is a non-protonated cis-isomer, we find that the
M-state is a polarized cis-isomer stabilized due to interactions of the
dissociating proton with the pi-electrons. The role of proton in the pump cycle
is found to be profound leading to the stabilization or in certain cases
destabilization of the intermediates. We propose the chemical structure of the
M-state for the first time.
