A Computational Study of End‐Group Conformational Energy Barriers in Carotenoids

Abstract

Introduction Carotenoids possessing different end-groups are known to be selectively metabolized in biological systems and there is evidence demonstrating that end-groups affect the ability of carotenoids to adopt specific geometries within larger biological structures such as membranes. In order to further assess the importance of the end-group structure in carotenoids we have investigated the influence of the end-group type and functionality on the conformational energy barrier. Methods We used the density functional method and the program GAUSSIAN to calculate the conformational energy curves for rotation of the beta-ring or the epsilon-ring relative to short polyene chains around the C6-C7 single bond. Results Epsilon rings have been shown to possess a dramatically larger conformational energy barrier to rotation than beta rings. The rotational energy barrier of the epsilon ring relative to the polyene chain around the C6-C7 bond has been found to range from 8.65 to 9.05 kcal/mole whereas the barrier for rotation around the beta-ring varies from 3.06 to 3.40 kcal/mole. Conclusions Conjugation of the beta-ring has no influence on the conformational energy barrier to rotation in carotenoid systems possessing this end-group. The length of the polyene chain (extent of conjugation) and substitution on the ring have little influence on the rotational energy barrier. The most important factor influencing the conformational energy for ring rotation about the C6-C7 bond is the steric interaction between the ring and polyene methyl groups. The high barrier to rotation for the epsilon ring provides a physical basis for the observed differences in the metabolism and properties of carotenoids possessing this end-group in biological systems.