Carotenoid sulphate

Takaichi, S., K. Furihata, J.-I. Ishidsu, and K. Shimada. 1991. Carotenoid sulphates from the aerobic photosynthetic bacterium Erytkrobacter longus. Phytochemistry 30 3411-3415. 

Since the first reported naturally occurring carotenoid sulphate by our group in 1981 [67], several examples have been reported from marine sponges, echinoderms and bacteria [68], The first structure established was for bastaxanthin C (27) [69]. The naturally occurring mono- and disulphates so far known contain either... 

The ionised sulphate group makes carotenoid sulphates strongly polar. This results in significant solubility in water with values up to 0.4 mg/ml. The high polarity and solubility properties determine the choice of isolation procedure. Special methods are employed and detailed isolation procedures available [68]. The isolation methods include column chromatography on Sephadex or silica, TLC and reversed phase HPLC. Sulphur analysis is also desirable. Further characterisation is carried out by spectroscopic methods. 

The ionised carotenoid sulphate may be converted into the hydrogen sulphate by ion exchange. Hydrolysis of the sulphate may be accomplished enzymatically, or the sulphate group may alternatively be removed by acid hydrolysis [68]. 

Partial synthesis of carotenoid sulphates is effected from carotenols by reaction with a sulphur trioxide/pyridine complex prepared from chlorosulphinic acid and pyridine, followed by sodium salt formation by... 

Sulphate formation has been studied for carotenols with different end groups [73,74], Carotenoids with non-allylic secondary hydroxy groups form stable sulphates. Less stable sulphates have been obtained from tert-carotenols, phenolic carotenols and non-allylic a-glycols. Unstable carotenoid sulphates are formed from sec or tert allylic carotenols. Solvolysis products of unstable carotenoid sulphates have been studied in some detail [73]. 

Two closely related carotenoid sulphates have been recently isolated from ophiuroids. Ophioxant-hin (266) is the major pigment from both Ophioderma longicaudum (6) and Ophiocomina nigra (183), of which the latter has also furnished dehydroophioxanthin (267), with an acetylenic functionality. 

The isolation of and the special physical, chemical and enzymic methods employed in the isolation and characterization of naturally occurring carotenoid sulphates have been dealt with in Vol. lA, Worked Example 11. 

Naturally occurring carotenoid sulphates hitherto described contain non-allylic secondary sulphate groups as in the bastaxanthins 352, 365, 411, 420, 423, 431, 431.1 and caloxanthin (182) sulphate (end group a) or as in ophioxanthin (205) (end group b) or a secondary sulphate group a to a carbonyl as in erythroxanthin sulphate (348.3) (end group c). 

The identification of natural carotenoid sulphates [1-7] prompted their chemical synthesis. So far carotenoid monosulphates and disulphates with end groups a and c have been prepared by chemical synthesis. In addition other semisynthetic carotenoid sulphates have been prepared as discussed in Section D and E. 

All carotenoid sulphates have been prepared by partial synthesis from the corresponding carotenols by reaction with a sulphur trioxide/pyridine complex prepared from chloro-sulphonic acid and pyridine [8-10], followed by sodium salt formation by addition of NaOH or, for alkali-labile carotenoids, NaCl [11], Scheme 1. The presumed mechanism is that S-0... 

Carotenoid sulphates that can be stored unchanged in methanol solution for at least 3 months at -10°C are considered as stable [11]. 

The first carotenoid sulphates to be prepared were zeaxanthin (119) disulphate and lycoxanthin (109) sulphate, in moderate yields [14]. Improved yields of zeaxanthin (119) disulphate and monosulphate were subsequently obtained [11] by using about a 20-fold excess of chlorosulphonic acid. In experiments on the mg scale the yields reported were zeaxanthin (119) disulphate ca. 70%, 119 monosulphate ca. 10%, alloxanthin (117) disulphate 12%, 117 monosulphate 39%, fucoxanthin (369) 3-sulphate 64%, peridinin (558) 3-sulphate 60%, capsorubin (413) disulphate 53%, 413 monosulphate 11%, astaxanthin (406) disulphate ca. 65% and 406 monosulphate ca. 4%. Astaxanthin (406) disulphate has also been prepared in the Roche laboratories (see IR spectrum, Vol. IB, p.l33). 

The less stable carotenoid sulphates are those which upon storage in methanol or water are partly converted into less polar solvolysis products [11]. 

E. Preparation and Solvolysis Reactions of Unstable Carotenoid Sulphates... 

Tertiary carotenoid sulphates obtained from carotenoids with end group m, secondary allylic sulphates derived from carotenols with end groups n, o and p and primary allylic sulphates obtained from an apocarotenol with end group q, are unstable and undergo methanolysis during the work-up procedure [15]. 

Chromatographic properties of carotenoid sulphates are described in Vol. lA, Worked Example 11, which also describes chemical conversion into the free hydrogen sulphates by ion exchange and removal of sulphate groups by acid hydrolysis to provide the parent carotenol, as well as enzymic hydrolysis. 

The characteristic infrared absorption of the sulphates is treated in Vol. IB, Chapter 4 and MS data in Vol. IB, Chapter 7. Recently, field desorption MS for carotenoid sulphates has been discussed [18]. It should be noted that molecular ions are not observed upon ordinary electron impact MS. NMR data are given in the original literature [2,5-7,11,14]. 

The water solubility of carotenoid sulphates is a special property among carotenoids and depends not only on the number of sulphate groups present, but on the total carotenoid structure. The following water solubilities in mg/ml have been reported r,2 -dihydro-( ), /-caroten-T-ol (88) sulphate > 0.01, astaxanthin (406) disulphate > 0.02, zeaxanthin (119) disulphate > 0.05, capsorubin (413) disulphate >0.14, fucoxanthin (369) sulphate > 0.20 and peridinin (558) sulphate > 0.36 [12]. It should be noted that the water solubility is drastically reduced in the presence of inorganic salts, as expected from solubility product considerations. 

D Auria, M.V., Ricdo, R., and Minale, L. (1985) Ophioxanfhine, a new marine carotenoid sulphate from the ophiuroid Ophioderma longicaudum. Tetrahedron Lett., 26, 1871-1872. 

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