20 PHOTOSYNTHESIS 



even with excess of magnesium salts; it is however slowly 

 resynthesized in absence of water by heating phaeophytin 

 with a modified Grignard reagent, e.g. ethoxy magnesium 

 bromide. Ordinary Grignard reagents react immediately at 

 ordinary temperatures to give the magnesium derivative, 

 but in this case the side chains are also attacked. In contrast 

 with its inertness towards magnesium ions, phaeophytin 

 readily reacts with other metallic salts, e.g. copper salts. 

 Hence when carrying out experiments with chlorophyll 

 derivatives copper and other metals must be rigorously 

 excluded. The copper derivative of phaeophytin is green, 

 stable to light, and not affected by acids; the zinc derivative 

 is like chlorophyll in that acid readily removes the zinc; the 

 iron derivative is dark green and shows some likeness to the 

 iron porphyrin compounds which are the prosthetic groups 

 of respiratory pigments. 



The atom of magnesium is situated in the centre of the 

 molecule and is surrounded by four pyrrole nuclei, and thus 

 chlorophyll has a similar structure to haemin, the latter 

 however containing a central iron atom (Fig. 3.2). In 

 general plan phaeophytin and phaeophorbide (the result of 

 removing magnesium from a chlorophyllide) are similar to 

 the porphyrins in structure but differ in the addition of two 

 more hydrogen atoms to the pyrrol system at C atoms 7 and 

 8. This alters a sequence of conjugated double bonds, which 

 in the porphyrins is part of the ring system, and results in a 

 marked change in the type of absorption spectrum, the 

 chlorophyll derivatives absorbing much more strongly at 

 longer wavelengths than the corresponding porphyrin de- 

 rivatives. The simpler chlorins, derived from chlorophyll 

 derivatives by pyrolysis, are readily oxidized to the corre-. 

 sponding porphyrins. The chlorins derived from chloro- 

 phyll h (i.e. with a formyl group at carbon atom 3) are 

 sometimes referred to as rhodins. The introduction of the 

 two hydrogen atoms which occurs in the pyrrol ring IV of 

 the porphyrin results in the appearance of two asymmetric 

 carbon atoms, and in agreement with this is the fact that not 

 only chlorophyll, but also the pheophorbides and simpler 

 derivatives of the parent 'chlorin' show optical activity. 



