Abstract
The conformational preferences of all-trans retinal (referred to as retinal) have been investigated by the PCILO (Perturbative Configuration Interaction using Localized Orbitals) method. The results indicate that the all-trans isomer (the conformer in which all the double bonds of the polyene chain intrans conformation) is relatively higher in energy than the one in which the terminalC=O bond assumes acis-conformation. The β-ionone ring and the polyene chain are nonplanar and the angle between the two planes is 90°. The all-trans isomers withθ 5 = 180° or 0° are energetically more preferred than the other isomers of retinal, the 11-cis isomer being the least stable. A rotation, however, around C12–C13 single bond (θ 4) in 11-cis retinal predicts two distorted 11-cis, 12-s-conformers withθ 4 = 90° and 270° which are as stable as 9-cis or 13-cis isomers. The theoretically predicted distorted 11-cis conformers are in agreement with experimentally observed conformers of 11-cis retinal in solution by NMR studies.
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