228 CAROTENOIDS CONTAINING CARBONYL GROUPS XII 



For the isolation of the pigment the hypophasic portion of oscillatoria-extracts 

 (cf. p. 225) are used. The pigment is dissolved in ether, the solution is washed free 

 from alkali, and dried over sodium sulphate, and the solvent is distilled off in vacuum. 

 The deep red, resinuous residue is dissolved in chloroform, and chromatographed 

 on calcium carbonate. After elution and removal of the solvent the myxoxanthophyll 

 is dissolved in pyridine. Colourless impurities are removed by freezing and the 

 mother liquors are then diluted with petroleum ether, when the pigment crystallises 

 on cooling. 



The structure of myxoxanthophyll has been investigated mainly by Karrer 

 and RuTSCHMANN^*. The molecular formula of the pigment is C^f^H^^O^^^- ^^. 

 On microhydrogenation, lo mols of hydrogen are absorbed rapidly and an 

 additional mol of hydrogen more slowly^*, indicating the presence of lo 

 double bonds and one carbonyl group. Of the remaining six oxygen atoms, 

 four are present as secondary hydroxyl groups, since myxoxanthophyll forms 

 a tetraacetate. The latter still contains two free hydroxyl groups, as shown by 

 Zerewitinoff determinations, but these cannot be esterified and are therefore 

 probably tertiary in character^*. It should be remarked that the presence of a 

 carbonyl group has not been definitely established, although the behaviour of 

 the pigment and the long-wavelength absorption renders it very probable. 

 Karrer and Rutschmann^^ tentatively proposed the following formula for 

 myxoxanthophyll, which is in agreement with the properties of the pigment: 



CH, CH, CH3 CH3 



OH C OH CH, CH, CH, CH3 CH OH 



\/\/ I I I I \/ 



CH C-CH=CH-C=CHCH=CH-C=CHCH=CHCH=C-CH=CHCH=C-CH=CH-CH CH 



CH C-CHa HaC-C CH 



X\/'\ Myxoxanthophyll( ?) \/\ 



OH CHa OH CO OH 



It should be emphasised, however, that this constitution is by no means 

 certain. 



Myxoxanthophyll crystallises from acetone in violet needles, m.p. 182° 

 (uncorr., in vacuum)*. According to Heilbron and Lythgoe, the pigment is 

 laevo-rotatory, [aj^d = -255°' (in ethanol). The pigment is readily soluble in 

 pyridine and ethanol, more sparingly soluble in chloroform and acetone, and 

 insoluble in petroleum ether, ether and benzene. Concentrated sulphuric acid 

 produces a deep blue colouration in a chloroform solution of the pigment. No 

 colouration is produced by concentrated hydrochloric acid. 



Solvent Absorption maxima 



Pyridine 526 489 458 m/i 



Chloroform 518 484 454 m/z 



Ethanol 503 471 445 m/^ 



* I. M. Heilbron and B. Lythgoe, /. Chem. Soc. 1936, 1376 record m.p. 169-170° C; 

 but the determination was not carried out in vacuum and the sample was probably less pure. 



References p. 253-255. 



