Raman spectroscopic investigations on intermolecular interactions in aggregates and crystalline forms of trans‐astaxanthin

Abstract

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.

Investigations on the geometrical isomers of astaxanthin: Raman spectroscopy of conjugated polyene chain with electronic and mechanical confinement

“…), and it belongs to all‐ trans ‐isomer of AST, as it has its absorption maximum at 474 nm with no cis peak. This observation is consistent with our previous work . UV–Vis spectra (right sided triangle, downward triangle, and square traces) from the fractions F4, F2, and F1 (peak #4, #2, and #1 in Fig.…”

“…Another low intensity mode is seen around 1275 cm −1 and it is labeled as ν 3 (Figure S1 (supporting information)). The Raman and the UV–Vis absorption spectra of fraction F3 are similar to that of all‐ trans ‐AST described in our previously published work . Hence, fraction F3 is attributed to the all‐ trans ‐AST isomer.…”

“…In these works, the high intensity peaks at ~1520 and 1157 cm −1 , were attributed to C=C and C–C stretching modes, respectively, and resonance enhancement was considered the main reason for their high intensity. Recently, our report and other previous reports have shown that ν 1 and ν 2 Raman modes are of high intensity, even if they are probed by laser wavelengths such as 632.8 and 785 nm, which are far from the resonance wavelength . These reports clearly suggest that a satisfactory explanation for the observed high intensity of ν 1 and ν 2 Raman modes of carotenoids in general, and particularly for AST, is very much needed.…”

“…Traditionally, the intensity of these peaks was attributed to the resonance Raman effect. Recent reports, including our work, have shown that these modes exhibit strong intensity even if carotenoids are probed by laser wavelengths whose energy is considerably lower than that of the electronic absorption energies of carotenoids . A comprehensive experimental result and underlying mechanism for the intensity of CC stretching modes for cis ‐isomers of carotenoids, particularly for AST, is not yet available …”

Recent advances in linear and nonlinear Raman spectroscopy. Part VIII

“…Subramanian and coworkers carried out Raman spectroscopic investigations of intermolecular interactions in aggregates and crystalline forms of trans ‐astaxanthin. Their results demonstrate that Raman is a powerful tool for distinguishing the crystal structures present in crystalline powders of trans ‐astaxanthin . Taddei and coworkers studied the affinity of protein fibres toward sulfation by treating wool, Bombyx mori, and Antheraea pernyi (Tussah) silk fibers with chlorosulfonic acid in pyridine using FT‐IR and FT‐Raman spectroscopies.…”