CONFORMATION AND OPTICAL-ACTIVITY OF ALL-TRANS, MONO-CIS, AND DI-CIS CAROTENOIDS - TEMPERATURE-DEPENDENT CIRCULAR-DICHROISM

The CD. spectra of carotenoids with an asymmetric centre at C(3) have the following unusual features: (1) All‐trans and di‐cis compounds with two end‐rings, at least one of which possesses an asymmetric C‐atom, have very similar CD. spectra, whereas the corresponding mono‐cis compounds give mirror‐image CD. spectra; (2) In carotenoids or apocarotenoids with only one end‐ring all‐trans and mono‐cis compounds have the same CD. spectra; (3) The CD. spectra of such carotenoids are strongly temperature dependent either increasing in magnitude or completely changing in sign upon cooling. These properties have been rationalized with the aid of a model with takes the total chromophore of the carotenoid as being intrinsically chiral with symmetry C2. It seems that the chirality arises not only from the presence of the hydroxyl group of an asymmetric carbon atom, C(3), which occupies an equatorial position thereby locking the conformation of the end‐ring, but also from the steric hindrance across the formal single bond C(6), C(7), linking the end‐ring to the chain and thus creating a chiral π‐system. (The twist about the C(6), C(7)‐bond acquires a handedness because of the predominance of one conformational form of the end‐ring. In this way, the double bonds of the end‐ring become twisted out of the plane of the chain with one hand predominating. Thus the whole conjugated system becomes chiral). The reversal of sign between the trans (and di‐cis) and mono‐cis compounds is due to a tilt of the 2‐fold symmetry axis and thereby a change of chirality. The temperature dependence stems from the varying population of forms of different twist of the end‐group relative to the chain. Compounds with 7, 8‐triple bonds also show distinct CD. spectra and a sign change between all‐trans and mono‐cis isomers in addition to temperature dependence. The latter property demonstrates that some steric hindrance between the end‐ring and the main chain is present in these compounds. Some suggestions for the origins of the sign patterns and band intensities of the CD. and absorption spectra are included. Copyright © 1979 Verlag GmbH & Co. KGaA, Weinheim