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 beta-carotene, the 11-cis and 11,11'-di-cis isomers exhibited the following unique properties. (1) The wavelengths of the B-u(+)<--A(g)(-)(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 C=C 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 Il,11'-di-cis isomer. (3) The H-1 chemical shifts of these isomers indicate severe steric interaction between the methyl and the olefinic H-1 atoms in the concave side of the 11-cis bend, whereas their C-13 chemical shifts suggest twisting and polarization of the cis C11=C12 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 ll-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.