Structural changes of carotenoid astaxanthin in a single algal cell monitored in situ by Raman spectroscopy

The changes of structure of astaxanthin (AXT), a superpotent antioxidant, upon thermal stress were investigated in unicellular microalgae Haematococcus pluvialis by measuring Raman spectra in situ and analyzing obtained results with DFT calculations. Although no visual changes are observed in the Haematococcus cells upon heating, discernible changes in Raman spectra occur from -100 °C systematically up to 150 °C. The exponential increase of the Raman shift of the ν C═C band at ca. 1520 cm(-1) along with the change of the intensity ratio of bands at 1190 and 1160 cm(-1) is observed, that correlates with the changes predicted by calculations for astaxanthin conformers ordered by decreasing energy. It is assumed that AXT molecules, initially in the form of H-aggregates with the trans conformations of the end-rings, interconvert toward more stable gauche forms upon thermal stress of the algae. The applied approach enables one to follow structural changes of the carotenoid upon temperature stress both in a single algal cell and in a multicellular sample in situ. Obtained information might be of use to improve the industrial process of extraction of AXT in its most bioavailable form.