"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."M. Nonella
J. Phys. Chem. 104B, 2000, 11379-11388.
Ab initio calculations of the potential energy surfaces of the states S0, S1, and S2 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 level of theory. Our calculations demonstrate that predictions of crossings of electronic states 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 S0/S1 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, that (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, that (iii) this isomerization reaction can occur spontaneously
only after a preceding relaxation of bond lengths in the excited state, and that (iv) an all-trans → 13,14-dicis double isomerization is most likely inefficient due to a non-vanishing
barrier in the excited state.
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