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Keywords:

  • carotenoid;
  • transastaxanthin;
  • aggregates;
  • lattice phonon Raman spectroscopy;
  • astaxanthin crystals

Astaxanthin is a carotenoid naturally found in microbial organisms, microalgae, and many crustaceans. Its consumption has led to beneficial effects such as pigmentation of marine animals, and it favorably addresses several human health issues as a result of its high important antioxidant property. Several companies produce synthetic trans-astaxanthin for dietary purposes in aquaculture, where it is mainly used for pigmentation. It is known that trans-astaxanthin manifests itself as a monomer in organic solvents, as aggregates in aqueous solutions of organic solvents, or as crystalline solids. These forms display unique optical and structural properties, which have an impact on biological systems. In this work, we report on detailed Raman investigations, in conjunction with optical absorption spectroscopy, of monomer, aggregates, and crystalline forms of trans-astaxanthin. The Raman and optical absorption spectroscopic investigations of trans-astaxanthin aggregates were performed as a function of time, showing the formation of card-packed aggregates after 2 h, and head-to-tail aggregates after 24 h in a 10% acetone–water astaxanthin solution. For the crystalline trans-astaxanthin, a pointwise Raman mapping evidenced the presence of two distinct crystal structures. The Raman modes of these crystal structures (A and B) were correlated with the intermolecular interactions present in chloroform solvated (AXT-Cl) and unsolvated (un-AXT) trans-astaxanthin single crystals. Both crystal structure A and the card-packed aggregates have similar intermolecular π stacking interactions as AXT-Cl. The crystal structure B and the head-to-tail aggregates showed linear chain features as in un-AXT. This work also clearly demonstrates that Raman spectroscopy is a powerful tool to distinguish the crystal structures present in crystalline powder of trans-astaxanthin. Copyright © 2012 John Wiley & Sons, Ltd.