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

Abstract

Recent research interests in geometrical isomers of astaxanthin (AST) are motivated by their metabolic activities in aquatic animals and human. It has been established that cis‐isomers of AST are selectively absorbed in human plasma during the metabolic process; however, exact absorption mechanism is still unclear. Hence, a detailed investigation of the structural and optical properties of geometrical isomers of AST is required. Among the techniques available for the study of AST and other carotenoids, Raman spectroscopy has been much acclaimed. Raman spectra have been shown to be influenced by the electronic and mechanical confinement effects arising from the conjugated polyene chain of carotenoids. In this work, we present Raman studies of geometrical isomers of AST, along with their optical absorption characteristics. Geometrical isomers of AST were prepared by heating all trans‐AST in solution form, and the isomers were separated using high performance liquid chromatography. Optical absorption spectra of cis‐isomers of AST showed hypsochromic shifts in the main absorption band and formation of new bands at lower wavelengths. A detailed Raman spectral analysis performed on the cis‐isomers of AST showed new modes which have not been observed and accounted for so far. In addition, we demonstrate that the electronic and mechanical confinement effects in the polyene chain of AST play an important role in the Raman spectra of geometrical isomers of AST. It is anticipated that this work will demonstrate that Raman spectroscopy is an important diagnostic tool in distinguishing and identifying the geometrical isomers of AST. Copyright © 2014 John Wiley & Sons, Ltd.

Thermal‐ and Photo‐Induced Isomerization of All‐E‐ and Z‐Isomer‐Rich Xanthophylls: Astaxanthin and Its Structurally‐Related Xanthophylls, Adonirubin, and Adonixanthin

“…In adonirubin and adonixanthin, five and seven Z ‐isomer peaks were observed, respectively, which were identified by absorption maxima, shape of spectrum, and relative intensities of the Z ‐peak to the absorption maximum of the isomer (% D B /D II ) (Table 1 and Figure 2; Figures S2,S4, Supporting Information). [ 32,34,36–44 ]…”

“…The xanthophyll isomer peaks were identified according to HPLC retention times, visible spectral data (absorption maxima and shape of spectrum), and relative intensities of the Z ‐peak at around 360–370 nm to the main absorption peak of the isomer (Figure S1, Supporting Information; % D B / D II ). [ 32,34,36–44 ] Total Z ‐isomer ratio (%) and remaining ratio (%) of xanthophylls after the storage test were assessed as shown below: [ 10,22 ] …”

Structure of supramolecular astaxanthin aggregates revealed by molecular dynamics and electronic circular dichroism spectroscopy

“…19,22 These variations of structure influence the bioavailability of AXT. 16,20,21,23 We and others also noticed the ability of chiral xanthophyll molecules to form a wide range of supramolecular assemblies. [24][25][26][27][28][29][30][31][32][33][34][35] Typically, aggregation occurs when water is added to an AXT solution in an organic solvent.…”

“…AXT is often attached to proteins (carotenoproteins) and lipoproteins (carotenolipoproteins), or esterified with fatty acids (monoester or diester forms), which makes it more resistant to oxidization, heat and light degradation. 2,4,5,[13][14][15][16][17][18] The conjugated bonds can adopt both the trans and cis conformations, 19,20 although the trans-form prevails. 14,21 Each molecule has two chiral centres at the C3 and C3 0 atoms giving rise to two enantiomers (3R,3 0 R) and (3S,3 0 S) and one meso form (3R,3 0 S).…”

Thermal stability and oral absorbability of astaxanthin esters from Haematococcus pluvialis in Balb/c mice

“…9 The all-trans isomer of astaxanthin predominates in nature, although the 9-cis, 13-cis, and 15-cis isomers are also present. 10 Haematococcus pluvialis is a common, green alga and is considered to contain the highest known natural levels of astaxanthin, with a content of up to 40 g kg −1 on a dry weight basis. 11 In H. pluvialis, astaxanthin is chemically bound to various types of fatty acids, such as oleic, eicosanoic, palmitic and stearic acids.…”