posted on 2013-05-14, 00:00authored byOmar Valsson, Pablo Campomanes, Ivano Tavernelli, Ursula Rothlisberger, Claudia Filippi
Bovine rhodopsin is the most extensively
studied retinal protein
and is considered the prototype of this important class of photosensitive
biosystems involved in the process of vision. Many theoretical investigations
have attempted to elucidate the role of the protein matrix in modulating
the absorption of retinal chromophore in rhodopsin, but, while generally
agreeing in predicting the correct location of the absorption maximum,
they often reached contradicting conclusions on how the environment
tunes the spectrum. To address this controversial issue, we combine
here a thorough structural and dynamical characterization of rhodopsin
with a careful validation of its excited-state properties via the
use of a wide range of state-of-the-art quantum chemical approaches
including various flavors of time-dependent density functional theory
(TDDFT), different multireference perturbative schemes (CASPT2 and
NEVPT2), and quantum Monte Carlo (QMC) methods. Through extensive
quantum mechanical/molecular mechanical (QM/MM) molecular dynamics
simulations, we obtain a comprehensive structural description of the
chromophore–protein system and sample a wide range of thermally
accessible configurations. We show that, in order to obtain reliable
excitation properties, it is crucial to employ a sufficient number
of representative configurations of the system. In fact, the common
use of a single, ad hoc structure can easily lead to an incorrect
model and an agreement with experimental absorption spectra due to
cancelation of errors. Finally, we show that, to properly account
for polarization effects on the chromophore and to quench the large
blue-shift induced by the counterion on the excitation energies, it
is necessary to adopt an enhanced description of the protein environment
as given by a large quantum region including as many as 250 atoms.