Polymorphism of 3,3′-dihydroxy-β,β-carotene-4,4′-dione (Astaxanthin)

Astaxanthin (AXT) 3,3′-dihydroxy-β,β-carotene-4,4′-dione, from the group of carotenoid, specifically as xanthophyll, is investigated in this work on its polymorphism. The method to obtain a second polymorph of AXT is described. Polymorph I can be purchased as a commercial product or obtained via phase transformation from polymorph II. Polymorph II can be prepared from polymorph I by a thermal treatment carried out in a differential scanning calorimetry (DSC). The resulting crystals are characterized by different analytical techniques. X-ray powder diffractometer (XRPD) and resonance Raman spectroscopy prove the existence of two different crystalline polymorphs. Simultaneous thermal analysis (STA) gives additional information on their thermal behaviors. Polymorph I melts at 230 °C and polymorph II melts at 216 °C. Phase transition experiment carried out by slurry conversion experiment reveals two polymorphs are monotropically related. Polymorph II is the metastable form.