| Abstract
| - In this paper, ab initio CASSCF computations are used toinvestigate the photoisomerization pathof the protonated Schiff base (PSB)4-cis-γ-methylnona-2,4,6,8-tetraeniminium cation: a fiveconjugated doublebond model of the retinal chromophore of rhodopsin (the human retinavisual pigment).We show that, afterinitial skeletal relaxation from the Franck−Condon region (whichinvolves a large increase in the centralCC bond length), the system is “trapped” in an energy plateau onthe S1 energy surface which may be theorigin of the “slow” cis → transisomerization dynamics observed in retinal PSBs in solution. Theenergyplateau is absent in shorter retinal chromophore models which have asteeper S1 isomerization path. Therhodopsin cavity (where the native chromophore is embedded) may havethe effect of removing the energyplateau from the S1 potential thus dramatically increasingthe photoisomerization rate from picoseconds tofemtoseconds.
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