Small structural changes in the vicinity of ASP-96 may lead to a significant decrease in its pK in the N state.





Changes in the relative energies of protonation states (as calculated from structural changes in the photo-cycle)  can explain the vectoriality of proton transfer. No need for change of accessibility during the cycle.


Completion of the cycle (O -> BR)  may involve proton transfer from  ARG-82 to the release group and subsequent re-protonation of ARG-82 from ASP-85.



The protonation state of the O-intermediate can be reproduced by assuming BR-like structure in the Schiff  base region and M-like in the release group region.