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Original scientific paper

On the Possible Role of an Intermolecular Charge Transfer State in the Excitation of the Biologically Active Bond of the Retinal Chromophore-counterion Pair

Jurica Novak ; Higher Medical and Biological School, Laboratory of Computational Modeling of Drugs, South Ural State University, 20-A, Tchaikovsky Str., Chelyabinsk 454080, Russia
Momir Mališ ; Department of Chemistry, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
Nađa Došlić orcid id ; Department of Physical Chemistry, Ruđer Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia

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In non-polar solvents the protonation of the all-trans Schiff base of retinal (SBR+) using strong acids leads to formation of retinal chromophore-counterion pairs. Previously we have shown that the main non-reactive deactivation channel of these ion pairs involves internal conversion from the initially excited ππ* state to an inter-molecular charge transfer state (inter-CT) with subsequent dissociation of the chromophore-counterion pair. In solution this leads to the abortion of isomerization. Motivated by the recent X-ray diffraction experiments showing that the disruption of the hydrogen-bonded network of counterions precedes the isomerization of all-trans SBR+ in bacteriorhodopsin we decided to take a closer look at the dynamics of the chromophore-counterion pair in the inter-CT state. Using constrained non-adiabatic dynamics simulations in which the dissociation of the chromophore-counterion pair was impeded, we show that the charge distribution in the inter-CT state leads to site-specific elongation of the biologically active C13=C14 bond. On this basis we hypothesize that an inter-molecular charge transfer state involving the retinal chromophore and the H-bonded counterions (S2) may play an active role in the photoisomerization reaction in a constraint environment.

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retinal; trifluoroacetic acid; hydrogen bond; charge transfer; photoisomerization; non-adiabatic molecular dynamics

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