248 FINE-STRUCTURE OF PROTOPLASM II 



and Wiedemann (1941) succeeded in producing this protein con- 

 taining chlorophyll in its pure state. They call the resulting chromo- 

 protein "chloroplastin'". Its molecular weight in the ultracentrifuge was 

 found to be roughly five million. This compound was obtained from 

 thirty different plant species; it shows, as do the haemoglobins of 

 various vertebrata, slight differences, according to the plant species. 

 The chloroplastin of Aspidistra contains about 69% of protein 

 (plastin), 21% of lipids and 8% of pigments, 6% of which, approxi- 

 mately, is chlorophyll. Menke (1940b), finding 7-8% of chlorophyll 

 in toto in the chloroplasts, doubts whether the chloroplastin contains 

 a pure chromoprotein. As, however, the chloroplastin is free from 

 iron, it may nevertheless be assumed that it does not contain all 

 essential constituents of the stroma. 



We are better informed as to the structure of the pigments in chloro- 

 plasts than on the molecular structure of the protein. One reason for 

 this is that the pigments are easier to isolate, another being that they 

 are of considerable physiological interest. 



The chlorophyll molecule C55H7205N4Mg is like a tadpole in appearance, 

 having a large head and a long tail (Fig. 1 27). The head consists of four 

 rings of pyrrole linked together to form one porphin ring. This harbours 

 a magnesium atom in the centre and at its periphery are, in chlorophyll <2, 

 four methyl, one ethyl and one vinyl groups and also three oxygenic side 

 chains, viz., one butyric acid, one acetic acid and one formaldehyde residue. 

 The two latter are interconnected laterally (shown by 9 and 10 in Fig. 127); 

 an isocyclic ring is therefore formed, to which has been ascribed the process 

 of assimilation on account of its labile acetic acid-ester configuration 

 (Fischer, 1935 ; Stoll, 1936). The acid groups are esterified with methanol 

 and phytol (CgoHggOH). Chlorophyll b differs from chlorophyll a merely 

 by the substitution of the methyl group at the 3. C atom, shown by a circle 

 in Fig. 127, by a formaldehyde residue -CH = O. 



There are ten double bonds in the polycyclic ring; they are conjugated, 

 which means to say that they alternate regularly with simple bonds. Systems 

 of conjugated double bonds like this cause absorption of light in short-wave 

 light. Strong absorption in the far red is furthermore induced by the effect 

 of porphin ring formation upon the system of unsaturated bonds. The 

 presence of magnesium only slightly shifts the position of the various 

 absorption bands of this system, but it does affect their intensity. It is there- 

 fore responsible for the green colour of chlorophyll. If the magnesium is 

 removed from the porphin nucleus, the brilliant colouring fades and changes 

 to a dirty olive brown (phaeophorbids). The sUght morphological difference 

 as between chlorophyll h and chlorophyll a suffices to change the bluish 



