Carbon Assimilation. 
87 
N-Mg X 
/ \ 
HC CH 
II II 
HC-CH 
NH 
/ \ 
HC C—MgX 
II II 
HC —CH 
NH 
/ \ 
C COOMgX 
The pyrrole magnesium derivatives have thus—analogous with 
sodiumacetic ester—behaved like any Grignard body with binding 
of the metal to carbon. 
Chlorophyll can be regarded as of the same class of organo- 
magnesium compounds, and it seems unjustified to draw a sharp 
line between magnesium phenyl iodide, pyrrole magnesium iodide 
and chlorophyll, only chlorophyll is characterised by a greater 
stability of magnesium towards water than the ordinary organo- 
magnesium compounds on account of the complex binding of the 
metal. 
This comparison does not require that the pigment in the 
process of assimilation should take the carbon dioxide into its 
molecule. This can be prevented by substitution in the magnesium¬ 
carrying pyrrole nuclei. Rather the function of chlorophyll may be 
imagined thus : that the carbon dioxide is attracted by the affinity 
of the magnesium compounds, and that its reduction is effected by 
the chlorophyll component a in the process which uses the absorbed 
light energy. Chlorophyll a is hereby oxidised to chlorophyll b, and 
this is again transformed to the first component with evolution of 
oxygen. Between the two components an equilibrium condition is 
obtained. 
It is possible that this evolution of oxygen either takes place 
direct or that the yellow pigments, carotin and xanthophyll take 
part in the re-formation of chlorophyll a. As the yellow pigments 
constantly accompany the green pigments in the chloroplasts, it is 
probable that they have a function Perhaps this is to regulate the 
ratio of the chlorophyll components, perhaps by the withdrawal of 
oxygen from chlorophyll b by carotin, this oxygen being then evolved 
from xanthophyll by means of the action of an enzyme.” 
It will be seen that Willstatter here first defends his conception 
of chlorophyll as a Grignard body. In spite of his elaboration of 
this conception, it appears to us that his arguments amount to this : 
In the Grignard syntheses a good many curious things may happen. 
Carbon assimilation is a curious process which involves the complex 
organic magnesium compound chlorophyll. Why should not 
carbon assimilation be a Grignard synthesis 
