THE C5H6NH2- AN AB-INITIO MINIMAL MODEL FOR RETINAL PHOTOISOMERIZATION( PROTONATED SHIFF BASE )

The minimum energy path for photoisomerization of the minimal retinal protonated Shiff base model tZt-penta-3,5-dieniminium cation (cis-C5H6 H2+) is computed using MC-SCF and multireference Moller-Plesset method s. The results show that, upon excitation to the spectroscopic state, this molecule undergoes a barrierless relaxation toward a configuratio n where the excited and ground states are conically intersecting. The intersection point has a similar to 80 degrees twisted central double bond which provides a route for fully efficient nonadiabatic cis --> t rans isomerization. This mechanism suggests that Cis-C5H6NH2+ provides a suitable ''ab initio'' model for rationalizing the observed ''ultra fast'' (sub-picosecond) isomerization dynamics of the retinal chromoph ore in rhodopsin. The detailed analysis of the computed reaction coord inate provides information on the changes in molecular structure and c harge distribution along the isomerization path. It is shown that the initial excited state motion is dominated by stretching modes which re sult in an elongation of the central double bond of the molecule assoc iated with the change in bond order in the excited state. Thus, the ac tual cis --> trans isomerization motion is induced only after the bond stretching has been completed. It is also demonstrated that, along th e excited state isomerization coordinate, the positive charge is progr essively transferred from the -CH-CH=NH2 to the CH2=CH-CH- molecular f ragment. Thus, at the intersection point, the charge is completely loc alized on the CH2=CH-CH- fragment. This result suggests that strategic ally placed counterions can greatly affect the rate, specificity, and quantum yield of the photoisomerization.