The structure of canthaxanthin

ARCHIVES

OF

BIOCHEMISTRY

AND

BIOPHYSICS

The Structure

61, 137-139

(1956)

of Canthaxanthin

F. J. Petracek and L. Zechmeister Frortl

the Batrs

and

Crellin Laboratories of Chemistr!/, of Technology, Pasadena, C”ali~ornial Received

August

(itrlijornia

Inafitrrfe

23, 1955

Canthaxanthin, the main carotenoid of the edible mushroom “cilinabar chanterelle” (C an th are 11us cinnabarinus, Basidiomycet’e), was detected and obtained in crystalline form by Haxo (1). It is a neut)ral xanthophyll forming t,he main polyene of the fruiting bodies in the mushroom. Later, Saperstein and Starr (2,3) isolated the same pigment from some mutant strains of Corynebacterium michiganense. They rharacterized canthaxanthin, secured its empirical formula, C4,,H520n, and recognized its ketone charact’er. Furthermore, they proposed, with all reservations, a hydroxyketone strucbure for canthaxant,hin containing bot,h functional groups at the same end of the molecule. Recently, the writers have observed (4) that, under certain conditions, X-bromosuccinimide converts p-carotene, in part, illto three carbonyl derivatives, viz., 4,4’-diketo-&carotene, 4-keto+carot,ene, and 4-keto- 3’) 4’-dehydro-P-carotene. The properties of our diketone (see t,he formula) mere indicative of a possible identit,y with cantha-

_ CH-CR-

_- C-CH

_ CFI- -

I CH,

Cnnthaxanthin

(4,4’-diketo-P-carotene)

xanthin, and this was substantiated by a direct comparison of our syn1 Publication

No.

2020. 137

138

F. J. PETRACEK

AND L. ZECHMEISTER

thetic product with a sample ex Corynebacterium, kindly sent us by Drs. Saperstein and Starr. This identification was based, first of all, on crystal forms, solubilities, melting points, mixed chromatogram tests, and the spectral properties of the ketone samples themselves as well as of their reduction products. Saperstein and Starr (2) had reduced canthaxanthin with aluminum isopropoxide under rather energetic conditions and obtained a new pigment (maxima in hexane, 487, 457, 432 mp). We believe, however, that this pigment cannot represent the diol corresponding to canthaxanthin. The true diol, i.e., 4,4’-dihydroxy-P-carotene, shows, as expected, the p-carotene spectrum (479, 450 mp). It was obtained in our laboratory by a mild lithium aluminum hydride treatment both from natural and synthetic canthaxanthin. These two reduction products were found to be identical. The product of the aluminum isopropoxide treatment cannot be clarified at the present time but it may well be a retro compound, originating, in part, from some conversion other than a simple reduction of the diketone. As is well known, allylic hydroxyl groups are sensitive and may undergo easy dehydration. Finally, it should be mentioned that besides canthaxanthin, some other naturally occurring polyene ketones, such as echinenone (5, 6) and astaxanthin (7), contain carbonyls in the 4- (and/or 4’) position, i.e., at that sensitive point of the carotene molecule which is attacked in vitro by N-bromosuccinimide. EXPERIMENTAL

A comparison of naturally occurring and synthetic canthaxanthin is given in Table I. Lithium aluminum hydride reductions were effected in absolute ether solution at room temperature, and the excess reagent was destroyed with methanol after 15 min. The yields were almost quantitative. The mixed chromatogram test of diketone samples was carried out by developing the pigment mixture with benzene on a lime-Celite (2 : 1) column, while for the analogous test applied to the two diol samples, benzene + 5 % acetone and a calcium carbonate-lime-Celite (1: 1: 1) column was used. Even after a long development in narrow columns, no separation took place. The partition values were obtained by shaking the hexane solution (saturated with 95 % methanol) with an equal volume of 95 % methanol

STRUCTURE

OF

TABLE Comparison

of Natural

139

CANTHAXANTHIN

I

and Synthetic

Canthaxanthin

Samples Synthetic

Natural

Crystal form (from benzene-methanol) Melting point (COT.)” Composition (found) * ET&. at h,, , 480 rn$ (in benzene) Position of x,,, in hexane Position of x,,, in ethanol Partition, in hexane: 95% methanol t’rovitamin A assay

Trapezoidal

prisms

Trapezoidal

prisms

213” C, 85.31; H, 9.49 11.8 x 10’

213” C, 85.23; H, 9.41 11.2 x 104

466 mpc 477 mp Slightly more phase Negative

466 rnF 478 rnfi 50:50

in the

hypo-

Negative

a These melting points were determined, under strictly identical conditions, in an electrically heated Berl block, in evacuated and sealed capillaries. Saperstein and Starr used a Fisher-Johns apparatus and found 218” for the natural product. * The compositions given represent, respectively, the average of two analyses reported by Saperstein and Starr, and by the presentwriters. c The value, 468 mp was reported by Saperstein and Starr. The respective curves were now found to be identical.

(saturated with hexane). The loss of extinction in the epiphase was then determined photometrically. Found for 4,4’-dihydroxy-P-carotene, 22:78 in hexane: 95 % methanol. ACKNOWLEDGMENTS

We wish to thank Dr. H. J. Deuel, Jr. and Mr. Southern California for bioassays. The microanalyses Swinehart, in Dr. 8. J. Haagen-Smit’s laboratory.

A. Wells of the University were carried out by Mr.

of G.

SUMMARY

The structure of the naturally occurring carotenoid pigment canthaxanthin, C40Hsz02,is that of 4,4’-diketo-P-carotene. REFERENCES 1. HAXO,

2. 3. 4. 5. G. 7.

F., SAPERSTEIN, SAPERSTEIN, (1954). PETRACEK, GOODW-IX, GANGULY, phys., in I~IJIIN, R., (1938).

Botan. Gaz. 112, 228 (1950). S., AND STARR, M. P., Biochem. S., STARR, M. P., AND FILFUS, F. J., AND ZECHMEISTER, T. W., AND TAHA, M. M., J., KRINSKY, N. I., AND press. AND SOaBNsEN, N. A., Z.

J. 67, 273 (1954). J. A., J. Gen. Microbial.

L., 1. 117n. Chem. Sot., in press, Biochem. .T. 47, 244 (1950). PINCKARD, .J. II., Arch. Riochrm. ange?u.

Chem.

51. 465 (1938);

ner.

10, 85 1956. and

Rio-

71, 1879