Photon-and Singlet-Oxygen-Induced Cis-Trans Isomerization of the Water-Soluble Carotenoid Crocin

Studying the cis-trans isomerization process in crocin (CR), one of the few water-solublecarotenoids extracted from saffron, is important to better understand the physiological role ofcis-carotenoids in vivo and their potential as antioxidants in therapeutic applications. For that,cis-trans isomerization of both methanol- and water-dissolved CR was induced by light or thermallygenerated singlet oxygen (1O2). The kinetics of molecular concentrations were monitored by bothhigh-performance liquid chromatography (HPLC) and non-destructive spectrophotometric methods.These last made it possible to simultaneously follow the cis-trans isomerization, the possible bleachingof compounds and the amount of thermally generated 1O2. Our results were in accordance with acomprehensive model where the cis-trans isomerization occurs as relaxation from the triplet stateof all-trans- or 13-cis-CR, whatever is the way to populate the CR triplet state, either by photon or1O2 energy transfer. The process is much more (1.9 to 10-fold) efficient from cis to trans than viceversa. In H2O, a 1O2-induced bleaching effect on the starting CR was not negligible. However, theCR "flip-flop" isomerization reaction could still occur, suggesting that this process can represent anefficient mechanism for quenching of reactive oxygen species (ROS) in vivo, with a limited need ofcarotenoid regeneration.