DFT and Raman study of all-trans astaxanthin optical isomers

Astaxanthin (AST) is a xanthophyll carotenoid widely distributed in aquatic animals, which has many physiological functions such as antioxidant, anti-inflammatory, anti-hypertensive and anti-diabetic activities. Astaxanthin has three optical isomers, including a pair of enantiomers (3S,3'S and 3R,3'R) and a meso form (3R,3'S). Different optical isomers have differences in a variety of physiological functions. Traditionally, High Performance Liquid Chromatography (HPLC) can be used to distinguish these isomers. In this work, it's found that Raman spectroscopy can be employed to distinguish the three optical isomers. Because the intensities of two Raman bands at 1190 cm-1 and 1215 cm-1 of three isomers are different. DFT calculations are performed and used to analyze the spectral differences. The calculation results show that the structures of these chiral isomers are not strictly mirror-symmetrical to each other, which leads to the difference in their Raman spectra.