Novak, Jurica; Mališ, Momir; Došlić, Nađa
(2020)
On the Possible Role of an Intermolecular Charge Transfer State in the Excitation of the Biologically Active Bond of the Retinal Chromophore-counterion Pair.
Croatica chemica acta, 93
(4).
pp. 303-310.
ISSN 0011-1643
Abstract
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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