3 ELECTRONIC-STATE MODEL OF THE PRIMARY PHOTOTRANSFORMATION OF BACTERIORHODOPSIN

The primary all-trans --> 13-cis photoisomerization of retinal in bact eriorhodopsin has been investigated by means of quantum chemical and c ombined classical/quantum mechanical simulations employing the density matrix evolution method. Ab initio calculations on an analog of a pro tonated Schiff base of retinal in vacuo reveal two excited states S-1 and S-2, the potential surfaces of which intersect along the reaction coordinate through an avoided crossing, and then exhibit a second, wea kly avoided, crossing or a conical intersection with the ground state surface. The dynamics governed by the three potential surfaces, scaled to match the in situ level spacings and represented through analytica l functions, are described by a combined classical/quantum mechanical simulation. For a choice of nonadiabatic coupling constants close to t he quantum chemistry calculation results, the simulations reproduce th e observed photoisomerization quantum yield and predict the time neede d to pass the avoided Grossing region between S-1 and S-2 states at ta u(1) = 330 fs and the S-1 --> ground state crossing at tau(2) = 460 fs after light absorption. The first crossing follows after a 30 degrees torsion on a flat S, surface, and the second crossing follows after a rapid torsion by a further 60 degrees. tau(1) matches the observed fl uorescence lifetime of S-1. Adjusting the three energy levels to the s pectral shift of D85N and D212N mutants of bacteriorhodospin changes t he crossing region of S-1 and S-2 and leads to an increase in tau(1) b y factors 17 and 10, respectively, in qualitative agreement with the o bserved increase in fluorescent lifetimes.