110 CHEMISTRY OF PLANT LIFE 



Haemoglobin is a complex compound, consisting of about 96 per 

 cent of albumin (a protein, see Chapter XIII) united with about 

 4 per cent of hcematin, a brilliant red pigment which has the 

 formula FeClC32H32O4N4. When treated with acids, the iron 

 (and its accompanying Cl) is removed, and haematoporphyrin, 

 C32H3eO4N4, is obtained. When either haematin, or haemato- 

 porphyrin is oxidized, haematinic acid imide identical with that 

 obtained from aetioporphyrin is obtained. Also, when haema- 

 toporphyrin is reduced, haemopyrrole identical with that from 

 aetioporphryin is obtained. Thus, it would appear that the unit 

 structural groups in haematin and in chlorophyll are identical; 

 although chlorophyll may exhibit more variations in isomeric 

 arrangement of these structural units than have been found in 

 haematin. Hence, it is apparent that the only essential difference 

 in composition between chlorophyll and haematin is that in the 

 former the structural units are linked together by iron, while in 

 the latter, the same units are united through magnesium as the 

 linking element. Further, it is known that while iron is not a 

 constituent element in the chlorophyll molecule, it is, in some 

 unknown way, absolutely essential to the production of chloro- 

 phyll in plants; plants furnished with an iron-free nutrient solu- 

 tion rapidly become etiolated and photosynthesis stops. 



The following skeleton formulas have been suggested to indi- 

 cate the way in which these elements are linked between the struc- 

 tural units in their respective compounds. 



C\ /C 



>N N N N 



- 



ex \ / c c \ 



Mg Fe 



~ C / \ /C- -Ov / 1 \ 



\ / 



>N N< N Cl N 



C/ X C C 



Chlorophyll Hsematin 



It is understood, of course, that the mineral element does not 

 furnish the definite means of holding the structural units together 

 as otherwise it would not be possible to remove the iron, or mag- 

 nesium, without breaking down the molecule, as is done in the 

 case of the porphyrins. The actual binding linkage is undoubtedly 

 between carbon atoms, as indicated in Willstatter's formulas for 

 aetiophyllin and aetioporphyrin (see page 109). The attach- 



