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The Importance of Charge Transfer between the Retinal Chromophore and the Protein Environment in Bacteriorhodopsin: A Theoretical Analysis on Reduced and Oxidized Chromophores
Author(s) -
Sakar K.,
Vacek G.,
Luthi H. P.,
Nagashima U.
Publication year - 1997
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1997.tb03185.x
Subject(s) - chromophore , bacteriorhodopsin , chemistry , photochemistry , homo/lumo , isomerization , molecular orbital , opsin , retinaldehyde , ground state , chemical physics , rhodopsin , crystallography , retinal , molecule , atomic physics , physics , organic chemistry , biochemistry , membrane , catalysis
— The importance of charge transfer(CT) between the retinal chromophore and the protein environment in the ground state of bacteriorhodopsin(BR) has been verified by using ab initio and semiempirical molecular orbital methods. We hypothesize that the chromophore is stabilized in BR by highest occupied molecular orbital‐lowest unoccupied molecular orbital(HOMO‐LUMO) interaction with the protein environment. If sufficient charge is transferred between two sites due to the strong HOMO‐LUMO interaction, the chromophore might be treated as a one‐electron reduced species(when it behaves as an electron acceptor), or as a one‐electron oxidized one (when it acts as an electron donor).In both optimized geometries, the ‐conjugated systems exhibit a drastic decrease in bond alternation. To estimate the rotational barrier for thermal isomerization between the al‐trans and the 13,15‐dicis form, the potential energy curve around these two bonds was computed. The first ‐ * transition energy was also calculated for an inspection of the opsin shift. The barrier height and the transition energy became much lower as a result of the chromophore reduction. The site selectivity in photo‐ and thermal isomerization and the opsin shift in BR can be well explained by considering CT from the protein environment to the chromophore.