An all-trans-retinal (ATR) dimer (1) isolated from photoreceptor outer segments was found to have a stereogenic center at C13' flanked by tetraene (295 nm) and hexaenal (438 nm) chromophores. Analytical chiral HPLC (Chiralcel OD) revealed that the isolated retinoid had formed in 13% enantiomeric excess. Using a combination of 1H-1H NOESY constraints, molecular modeling, and CD exciton coupling analysis, it was determined that the favored enantiomer was 13V(R). Three low-energy conformers of the 13V(S) model were found with MMFF/DFT and were used to calculate the CD spectrum of the ATR dimer (DeVoe method). The Boltzmann weighted spectrum was found to exhibit a positive exciton couplet, in excellent agreement with the experimental spectrum for the first eluted enantiomer. This further suggested that despite the large energy difference between the two interacting chromophores, the dominant source of optical activity in the CD spectrum is the nondegenerate exciton mechanism.
Absolute Configurational Determination of an All-trans-Retinal Dimer Isolated From Photoreceptor Outer Segments
PESCITELLI, GENNARO;
2004-01-01
Abstract
An all-trans-retinal (ATR) dimer (1) isolated from photoreceptor outer segments was found to have a stereogenic center at C13' flanked by tetraene (295 nm) and hexaenal (438 nm) chromophores. Analytical chiral HPLC (Chiralcel OD) revealed that the isolated retinoid had formed in 13% enantiomeric excess. Using a combination of 1H-1H NOESY constraints, molecular modeling, and CD exciton coupling analysis, it was determined that the favored enantiomer was 13V(R). Three low-energy conformers of the 13V(S) model were found with MMFF/DFT and were used to calculate the CD spectrum of the ATR dimer (DeVoe method). The Boltzmann weighted spectrum was found to exhibit a positive exciton couplet, in excellent agreement with the experimental spectrum for the first eluted enantiomer. This further suggested that despite the large energy difference between the two interacting chromophores, the dominant source of optical activity in the CD spectrum is the nondegenerate exciton mechanism.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
