Electrostatic protein-chromophore interactions promote the all-trans→13-cis isomerization of the protonated retinal Schiff base in bacteriorhodopsin:: An ab initio CASSCF/MRCI study

Ab initio calculations of the potential energy surfaces of the states So, SI, and St of protonated Schiff base model molecules containing three, four, and five conjugated double bonds have been carried out at the HF/MRCI and CASSCF/MRCI levels of theory. Our calculations demonstrate that predictions of crossings of electronic stares depend on the method of calculation and are different at the CASSCF and CASSCF/MRCI levels. Moreover, when a counterion is added in the vicinity of a protonated Schiff base, HF/MRCI and CASSCF/MRCI calculations predict different regions for S-0/S-1 crossing. The length of the conjugated system seems not to affect such qualitative results considerably. Our calculations suggest that (i) the second excited state is of no importance for the primary step of the photocycle of bacteriorhodopsin, (ii) an efficient decay into the electronic ground state during an all-trans --> 13-cis isomerization is only possible due to the interaction of the protonated Schiff base with a counterion, (iii) this isomerization reaction can occur spontaneously only after a preceding relaxation of bond lengths in the excited state, and (iv) an all-trans --> 13,14-di-cis double isomerization is most likely inefficient due to a nonvanishing barrier in the excited state.