NMR structure of chromeraxanthin - a unique allenic carotenoid of the apicomplexan alga Chromera velia

Chromera velia is photosynthetic alga that is evolutionary closest to the group of het- erotrophic apicomplexan parasites. The main light harvesting carotenoid of C. velia has been described as isofucoxanthin-like pigment, however its structure remained unknown. Here, we purified this pigment from a 5 L culture of C. velia using a two-step HPLC procedure and its structure determined using combination of tandem mass spectrometry and 2D- and 3D-NMR techniques. The data identified this pigment as a new allenic xanthophyll which we denoted chromeraxanthin. Although previously predicted to be structurally similar to isofucoxanthin, chromeraxanthin possess different arrangement of the terminal cycles and thus it likely not derived directly from fucoxanthin biosynthe- sis pathway. The six membered ring connected to the carotenoid backbone via allenic bond is identical to that of fucoxanthin including its O-acetylation. However, the second six-membered ring bearing epoxy group in fucoxanthin is replaced by a capsanthin-like cyclopentane ring in chromeraxanthin. Notably, similar structure lacking only the O- acetylation has been previously described as allenicmytiloxanthin isolated from Pacific oyster.