W. Humphrey, H. Lu, I. Logunov, H. J. Werner, and K. Schulten.
Three electronic state model of the primary phototransformation of
bacteriorhodopsin.
Biophysical Journal, 75:1689-1699, 1998.
HUMP98
The primary all-trans13-cis photoisomerization of retinal in bacteriorhodopsin has been investigated by means of quantum chemical and combined classical/quantum mechanical simulations employing the desnity matrix evolution method. Ab initio calculations on an analogue of a protonated Schiff base of retinal in vacuo reveal two closely lying excited states and , the potential surfaces of which intersect along the reaction coordinate through an avoided crossing, and then exhibit a second, weakly 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 analytical functions, are described by a combined classical/quantum mechanical simulation in the average state picture. For a suitable choice of non-adiabatic coupling constants, the simulations reproduce the observed photoisomerization quantum yield and predict the time passing the avoided crossing region between and states at fs and the
ground state crossing at =460 fs after light absorption. The first crossing follows after torsion on a flat surface, while the second crossing follows after a rapid torsion by further 60^. matches the observed fluorescence life time of . Adjusting the three energy levels to the spectral shift of D85N and D212N mutants of bacteriorhodopsin changes the crossing region of and and leads to an increase of by factors seventeen and ten, respectively, in qualitative agreement with the observed increase of fluorescent life times.
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