30 CARBON ASSIMILATION [CH. 



fluorescent. Chlorophyll a may be represented as the methyl phytyl 

 ester of an acid chlorophyllin (phytol is a primary alcohol, see p. 38): 



/COOCHj ,COOH 



C : > 2 H 30 ON 4 Mg/ C 32 H ;w ON 4 Mg<' 



N COOC. 20 H 39 X COOH 



Chlorophyll a Chlorophyllin 



On treatment in the cold with alkali, the ester is saponified, and the 

 alkali salt of chlorophyllin is formed. During saponification, there is a 

 change of colour in the pigment, the so-called brown phase, followed by 

 a return to green. 



ExpL 22. Saponification of a mixture of the green pigments. Pour a little of the 

 ether solution obtained in Expt. 18 into a test-tube, and in a pipette take a little 

 30 % solution of potash in methyl alcohol. Place the lower end of the pipette at the 

 bottom of the test-tube and allow the potash to run in below the chlorophyll solu- 

 tion. At the interface between the solutions there appears immediately a brown - 

 coloured layer which diffuses on shaking. In about ten minutes it changes back 

 through an olive-green colour to pure green. 



The chlorophyll has been saponified to the potassium salt of the acid chlorophyl- 

 lin. This salt is insoluble in ether, so if water is added to bring about a separation 

 of the two layers, the green colour is no longer present in the ethereal layer. 



The change of colour on saponification is different for the two 

 chlorophylls, the brown phase produced in the above mixture of chloro- 

 phylls being due to a yellow phase produced by chlorophyll a, and a 

 brown-red phase produced by chlorophyll b. To demonstrate this the 

 phase test (Expt. 22) may also be carried out separately on the two 

 chlorophylls. 



Expt. 23. Saponification of chlorophylls a and b separately. The methyl alcohol 

 .solution obtained in Expt. 19 is transferred to ether as in Expt. 18. Both the latter 

 and the petrol ether solution of chlorophyll a are saponified as in the previous 

 experiment. 



As already demonstrated the potassium salts of the chlorophyllins 

 which are produced by saponification of the mixture of green pigments 

 in the cold are not fluorescent. By saponification of chlorophyll with 

 hot alkali, isochlorophyllins are formed (see Expt. 24 below) which are 

 fluorescent. 



On heating chlorophyllins with concentrated alcoholic alkalies, a series 

 of decomposition products, phyllins (also acids) are obtained by removal 

 of carboxyl groups. The final phyllin has only one carboxyl group. When 

 this is removed, a substance, aetiophyllin, C 31 H 34 N 4 Mg, is obtained which 

 contains no oxygen (see Scheme 1, p. 34). 



Another difference between the results of treating chlorophyll with 

 hot and cold alkali is that in the former process the yellow pigments are 



