Proton-pump mechanism in retinal Schiff base: On the molecular structure of the M-state

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 π-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.