The Fascinating Structure of Astaxanthin
Stereoisomers:
The Mystery of Left-handed, Right-handed, and Meso Forms
Astaxanthin is like a little guard with a unique structure. Its molecular formula is , and its relative molecular mass is 596. Most of it exists in the form of esters in nature. It has super strong antioxidant ability, much stronger than β-carotene and vitamin E. That's why it's called "super vitamin E".
In terms of stereoisomers,
Haematococcus pluvialis is a great producer of natural astaxanthin. The astaxanthin it contains is mainly in the trans configuration (3S, 3’S), which is the same as that in aquatic animals like salmon and Antarctic krill. Also, its ester ratio is similar, making it easily absorbed by organisms. It's better than synthetic and other fermented sources of astaxanthin.
Fermented astaxanthin comes from sources like Xanthophyllomyces dendrorhous, Saccharomyces cerevisiae, Yarrowia lipolytica, and Paracoccus. The optical isomer of astaxanthin from Xanthophyllomyces dendrorhous is (3R, 3’R), but there are also some that can be modified to produce the (3S, 3’S) configuration. The ones from Saccharomyces cerevisiae, Yarrowia lipolytica, and Paracoccus are all (3S, 3’S). Note that the astaxanthin from Xanthophyllomyces dendrorhous and Paracoccus can only be used in feed for now. The astaxanthin from Saccharomyces cerevisiae and Yarrowia lipolytica has not passed any safety evaluation and cannot be used for people or feed.
Synthetic astaxanthin is common in aquafeed. But because of possible food safety issues, its application in many fields is limited. Its content can be more than 10%. However, there are two chiral centers (C - 3 and C - 3’) in the astaxanthin molecule, which results in three stereoisomers: (3S, 3’S), (3R, 3’S), and (3R, 3’R). Most natural bioactive substances are left-handed. The astaxanthin from algae is 100% left-handed (3S, 3’S), the one from Xanthophyllomyces dendrorhous is 100% right-handed (3R, 3’R), and synthetic astaxanthin is a mixture, with only 1/4 being the left-handed (3S, 3’S) isomer. These different stereoisomeric states lead to differences in their biological activities, physiological functions, tissue distribution, and coloring abilities. And they can't be converted into each other in animals. Meso-astaxanthin (3R, 3’S) has no biological activity, and the antioxidant activity of synthetic astaxanthin is only about 1/4 that of natural algal astaxanthin. It may also be bad for the health of ornamental fish.
Geometric Isomers:
The Difference between Z and E Structures
Astaxanthin has multiple conjugated double bonds in its molecule. The different arrangements of atoms bonded to the carbon - carbon double bonds form the Z structure (cis structure) and the E structure (trans structure). The all - E structure is the most stable because the branched groups don't take up space. Most of the
synthetic and natural astaxanthin is in the all - E structure (with a little E/Z mixture). The all - Z structure can't be absorbed and used by animals, and the FDA has banned the marketing of all - cis astaxanthin.
Free and Esterified Astaxanthin:
Big Differences in Stability
The hydroxyl groups in the two ring structures at the ends of the astaxanthin molecule can form esters with fatty acids, resulting in monoester or diester. After esterification, the hydrophobicity of astaxanthin is enhanced, and the lipophilicity of the diester is stronger than that of the monoester. Astaxanthin is absorbed in the midgut in free or esterified form and is transported in the blood combined with lipoproteins. The liver is the main organ for astaxanthin metabolism. In immature salmonids, astaxanthin is mainly in the free form in the muscle and moves to the skin and ovaries when they become sexually mature. Free astaxanthin is unstable and easily oxidized. Synthetic astaxanthin is 100% free, which is why it needs to be microencapsulated. In
Haematococcus pluvialis, astaxanthin monoester accounts for more than 77%, diester is about 20%, and free astaxanthin is about 3%. The main carotenoid in Xanthophyllomyces dendrorhous is (3R, 3’R) free astaxanthin. The astaxanthin from Saccharomyces cerevisiae, Yarrowia lipolytica, and Paracoccus is all (3S, 3’S) free astaxanthin. Free astaxanthin has a smaller molecular size and a much faster metabolic rate than esterified astaxanthin.
How to Identify Astaxanthin from Different Sources
Stereoisomer Identification Method (Optical Isomer Identification Method)
This method can tell the difference between synthetic astaxanthin and astaxanthin from
Xanthophyllomyces dendrorhous. Synthetic astaxanthin has three stereoisomers: (3S, 3’S), (3R, 3’S), and (3R, 3’R). The stereoisomer of astaxanthin from Xanthophyllomyces dendrorhous is (3R, 3’R). But the astaxanthin from Paracoccus, Saccharomyces cerevisiae, Yarrowia lipolytica, and Haematococcus pluvialis is all in the (3S, 3’S) configuration, and this method can't accurately distinguish them.
Natural Esterified Astaxanthin Detection Method
After confirming that the astaxanthin is in the (3S, 3’S) configuration, this method can tell the difference between astaxanthin from
Haematococcus pluvialis and fermented sources. More than 97% of the astaxanthin from Haematococcus pluvialis is esterified astaxanthin, and its peak time is between 20 and 26 minutes. The astaxanthin from fermented sources (Paracoccus, Saccharomyces cerevisiae, and Yarrowia lipolytica) is all free astaxanthin, mainly all - trans free astaxanthin, and its peak time is about 11 minutes. Also, the astaxanthin from Saccharomyces cerevisiae and Yarrowia lipolytica contains more other carotenoids. In the astaxanthin from Paracoccus, all - trans free astaxanthin accounts for about 52% of the total carotenoids, and other carotenoids account for about 40%. Now there are products on the market that are a mixture of synthetic astaxanthin and astaxanthin from Haematococcus pluvialis, and it's harder to identify them. We mainly judge by the peak area of all - trans free astaxanthin.
Detection Method of Astaxanthin in Oily Samples of
Haematococcus pluvialis
Also after confirming the (3S, 3’S) configuration, this method can further tell the difference between synthetic astaxanthin, astaxanthin from Haematococcus pluvialis, and fermented sources. We mainly look at the peak area of other carotenoids in the chromatogram. The proportion of astaxanthin from Haematococcus pluvialis in the total carotenoids is usually more than 90%, and it has three geometric isomers of double bonds: all - trans, 13 - cis, and 9 - cis. Its peak time is between 10 and 15.5 minutes. The astaxanthin from fermented sources is mainly all - trans free astaxanthin, and the proportion is generally less than 75%. It also contains a lot of other carotenoids. Synthetic astaxanthin is basically 98% all - trans free astaxanthin.
Knowing these structures and identification methods of astaxanthin from different sources is very helpful. It can help researchers study the characteristics of astaxanthin and also help consumers choose products containing astaxanthin. This way, we can better understand and use this amazing substance, astaxanthin.
For more information:
Steven Yang
Steven.yang@vitasourcebio.com