Exploring Molecular Structure Influence on Effective Conjugate Length of Carotenoids

The functions of carotenoids, including light collection, protection, antioxidation and anticancer activities, are closely related to their molecular structures. The conjugated systems confer notable electronic and photoelectricity properties. We employed UV-vis and Raman spectroscopies to characterize the conjugation of five carotenoids. Their conjugated lengths were determined as zeaxanthin < β-carotene < canthaxanthin < astaxanthin < lycopene. The molecular structures of these five carotenoids were optimized by density functional theory. Based on the optimized structures, we obtained parameters such as the HOMO-LUMO gap, bond-length alternation, and the torsion angle of ionone rings. Through analysis, the effects of hydroxyl and carbonyl groups on the effective conjugation length were obtained. In addition, the hydroxyl and carbonyl groups in astaxanthin form a unique intramolecular hydrogen bond structure, which decreases the torsion of the ionone rings. The intramolecular hydrogen bonding energy of astaxanthin was calculated to be 5.03 kcal/mol through the molecular tailoring approach.