S-cis form. As the proton starts to dissociate from the N, the M-state gets progressively unstable as the planarity of the 5 member ring is lost. This results in the back-isomerisation of the 13cis, 14S-cis form to the 13-cis, 14S-trans form. This conformation corresponds to the late M-state.
Proton-Pump Mechanism in Retinal Schiff Base: On the molecular structure of the M-state