TCB Publications - Abstract

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-trans$\rightarrow$13-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 $S_{1}$ and $S_{2}$, 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 $S_{1}$ and $S_{2}$ states at $\tau_{1}=330$ fs and the $S_{1} \rightarrow$ ground state crossing at $\tau_{2}$=460 fs after light absorption. The first crossing follows after $30^\circ$ torsion on a flat $S_{1}$ surface, while the second crossing follows after a rapid torsion by further 60^$\circ$. $\tau_{1}$ matches the observed fluorescence life time of $S_{1}$. Adjusting the three energy levels to the spectral shift of D85N and D212N mutants of bacteriorhodopsin changes the crossing region of $S_{1}$ and $S_{2}$ and leads to an increase of $\tau_{1}$ by factors seventeen and ten, respectively, in qualitative agreement with the observed increase of fluorescent life times.

Download Full Text

The manuscripts available on our site are provided for your personal use only and may not be retransmitted or redistributed without written permissions from the paper's publisher and author. You may not upload any of this site's material to any public server, on-line service, network, or bulletin board without prior written permission from the publisher and author. You may not make copies for any commercial purpose. Reproduction or storage of materials retrieved from this web site is subject to the U.S. Copyright Act of 1976, Title 17 U.S.C.

Download full text: PDF (253.5KB), PS (890.1KB)