CHAPTER XXI 

 THE PIGMENTS OF MICRO-ORGANISMS 



OUR present knowledge of the pigments of micro- 

 organisms is in an unsatisfactory state. The 

 constitution of comparatively few of them is known, 

 and their classification is an arbitrary one depending on 

 solubility relationships. However, there are three main 

 chemical t\pes into which they fall, namely : — 



(a) Carotenoid Pigments. — These are red, orange or 

 yellow pigments soluble in the fat solvents, ether, alcohol 

 and chloroform. They are named after the type pigment 

 carotene, an unsaturated hydrocarbon, C56H40, present 

 as the red colouring matter of carrots. Hydroxyl deriva- 

 tives of carotene, the xanthophylls, also belong to this 

 group. Usually they occur together as more or less 

 complex mixtures, which until recently were almost 

 impossible to separate. Nowadays they are separated by 

 means of chromatographic analysis, which depends on 

 differential adsorption of the pigments on an appropriate 

 adsorbent, such as calcium phosphate, alumina, kaolin and 

 others, a principle originally developed by Tswett. A 

 solution of the pigment in a suitable solvent is poured 

 through a column of the adsorbant and the chromatogram 

 " developed " by washing with the same or different 

 solvents. The pigments separate into coloured bands at 

 various depths in the column, which is then sliced between 

 the bands and the separated pigments eluted. The caro- 

 tenoids are usually characterised by the bands in their 

 light absorption spectra. Many carotenoid pigments are 

 readily bleached on exposure to atmospheric oxidation. 



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