Stereoisomers of carotenoids: spectroscopic properties of locked and unlocked cis-isomers of spheroidene.

A systematic optical spectroscopic and computational investigation of a series of locked-cis-isomers of spheroidene has been carried out with the goal being to better understand the relationships between stereochemistry, photochemistry, photophysics and biological function of geometric isomers of carotenoids. The spectroscopic properties of 15,15'-locked-cis-spheroidene, 13,14-locked-cis-spheroidene, 11, 12-locked-cis-spheroidene in solution are compared with those observed for unlocked spheroidene. The locked-cis bonds are incapable of undergoing cis-to-trans isomerization and therefore provide an effective means of exploring the relationship between specific stereoisomers and molecular spectroscopy. Samples of the molecules were purified using a high performance liquid chromatography (HPLC) apparatus equipped with a diode array detector, which records the absorption spectra immediately as the molecules emerge from the column and prior to any isomerization that might occur. For several stable isomers, resonance Raman (rR) spectroscopy was carried out to assign their configurations. Quantum computations of absorption spectra were performed using ZINDO/S and also MNDO-PSDCI methods employing nearly full single and double configuration interaction within the pi-electron manifold. Also, for a few test cases, ground state minimizations were done using density functional methods (B3LYP/6-31G(d)). The MNDO-PSDCI methods coupled with the density functional ground state minimization provide an accurate assignment of the positions of the 2(1)Ag - , 1(1)Bu +, and 1(1)Ag + excited states and also address the nature of the forbidden 1(1)Bu - state, whose location is uncertain for polyenes and carotenoids. We demonstrate that the configurational description of the 1(1)Bu - state is sufficiently unique to preclude assignment of its energy based on the characterization of surrounding excited singlet states. The experimental and computational data also offer important insights into the photochemical and photophysical properties of stereoisomers of carotenoids.