The molecular structure of a curl-shaped retinal isomer

Abstract

Computational studies of retinal protonated Schiff base (PSB) isomers show that a twisted curl-shaped conformation of the retinyl chain is a new low-lying minimum on the ground-state potential energy surface. The curl-shaped isomer has a twisted structure in the vicinity of the C(11)=C(12) double bond where the 11-cis retinal PSB isomerizes in the rhodopsin photoreaction. The twisted configuration is a trapped structure between the 11-cis and all-trans isomers. Rotation around the C(10)-C(11) single bond towards the 11-cis structure is prevented by steric interactions of the two methyl groups on the retinyl chain and by the torsion barrier of the C(10)-C(11) bond in the other direction. Calculations of spectroscopic properties of the 11-cis, all-trans, and curl-shaped isomers provide useful data for future identification of the new retinal PSB isomer. Circular dichroism (CD) spectroscopy might be used to distinguish between the retinal PSB isomers. The potential energy surface for the orientation of the beta-ionone ring of the 11-cis retinal PSB reveals three minima depending on the torsion angle of the beta-ionone ring. Two of the minima correspond to 6-s-cis configurations and one has the beta-ionone ring in 6-s-trans position. The calculated CD spectra for the two 6-s-cis configurations differ significantly indicating that the sign of the beta-ionone ring torsion angle could be determined using CD spectroscopy. Calculations of the CD spectra suggest that a flip of the beta-ionone ring might occur during the first 1 ps of the photoreaction. Rhodopsin has a negative torsion angle for the beta-ionone ring, whereas the change in the sign of the first peak in the experimental CD spectrum for bathorhodopsin could suggest that it has a positive torsion angle for the beta-ionone ring. Calculated nuclear magnetic resonance (NMR) shielding constants and infrared (IR) spectra are also reported for the retinal PSB isomers.