Unique properties of the 11-cis and 11,11′-di-cis isomers of β-carotene as revealed by electronic absorption, resonance Raman and 1H and 13C NMR spectroscopy and by HPLC analysis of their thermal isomerization
In comparison with the all-trans and other cis isomers of β-carotene, the 11-cis and 11,11′-di-cis isomers exhibited the following unique properties. (1) The wavelengths of the Bu+â†�Ag– (0–0) absorption of these two isomers are similar to that of the all-trans isomer, and do not follow the general rule of its blue shift found in other mono-cis and di-cis isomers. Their extinction coefficients are significantly lower than those of other isomers. (2) The frequencies of the CC stretching Raman lines of these isomers are the same as that of the all-trans isomer, and do not follow the general trend of high-frequency shifts found in other mono-cis and di-cis isomers. The Raman lines due to the out-of-plane C–H wagging and methyl rocking modes appear in the 11,11′-di-cis isomer. (3) The 1H chemical shifts of these isomers indicate severe steric interaction between the methyl and the olefinic 1H atoms in the concave side of the 11-cis bend, whereas their 13C chemical shifts suggest twisting and polarization of the cis C11C12 bond. (4) The rates of thermal isomerization of these isomers are much higher than that of the 15-cis isomer, i.e. the least stable isomer previously known.
The results lead us to the conclusion that the 11-cis configuration has inherent twisting around the double and single bonds in the cis bend due to the severe steric interaction between the 13-methyl and the 10-olefinic hydrogens, and that it is unstable enough to disappear thermally at room temperature.