Ilya Logunov and Klaus Schulten.
Quantum chemistry of in situ retinal: Study of the spectral
properties and dark adaptation of bacteriorhodopsin.
In D. Bicout and M. J. Field, editors, Proceedings of the Ecole
de Physique des Houches, pp. 235-256, Paris, 1995. Les Editions de
Physique, Springer.
LOGU95
A combination of molecular dynamics and quantum chemistry techniques have been employed to study the electronic excitation and conformational potential surface of retinal in the binding site of bacteriorhodopsin (bR). The CASSCF (6,9)/6-31G level of ab initio calculations (within Gaussian92) has been used for the treatment of both the ground (S0) and excited (S1) states of retinal. Charges of all atoms in the protein are represented by spherical Gaussians and explicitly included in the electronic Hamiltonian of retinal. Spectral properties have been analyzed for the native bR pigment as well as for its D85N mutant. The calculated relative shift in the absorption maxima between the two pigments is in better agreement with experiment than the computed absolute parameters of the absorption line shapes. The dark adaptation processes in bacteriorhodopsin (which involves rotation around the 13-14 and the 15-N retinal double bonds) has been modelled by following the pre-defined reaction coordinate. Our simulations support the notion that the isomerization process is catalyzed by the protonation of an aspartic acid (Asp85) side group of bacteriorhodopsin.
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