PROBLEMS IN PHOTOSYNTHESIS 



H-C=CH2 R 



HaC-C'^ 1 C-^ ^C-^ii C-C2H5 

 \ I I ^4 



C— N N-C 



// \ / \ 



H-C Mg C-H 



\ / \^ // Fig. 2. Constitution of chlorophyll. R-e-CHsin 



H C=n' U—C chlorophyll a; R -=- CHO in chlorophyll b. 



\/ I II \5 



H3C-C IV C;:^ ^C III ^C-CH3 



8\ / ^C^ \/ 



7C-H Ce 



CH2 H-C^-5 C=0 



' I 9 



CH2 IICOOCH3 



COOC20H39 



AhIh hhIh hh'hh h'hh h 

 HjC c-cic-c=c-c=c-c=c-cic-c=c-c=c-c=c-c 



I II I I I I I I I I 



H2C C I CH3 ' CH3 ' CH3 ' CHa 



HsQ^ ^CHa 

 H /C 



C-CH CH2 



I I 



C CH3 H3C c 



H. H 



Fig. 3. (3-Carotene consisting of eight isoprene groups, four of which form two ionone rings. 



HaC-C C C C ^^^^. ^ 



HC OH HC; :C-CHa 



\ e CH CH 



^. 



C 



C C-CH3 Hq, C CH 



^V V" c 



^c c 



H 



c 



H 

 CHa 



Fig. 4. Structural relationship between chlorophyll and carotene. Left: chlorophyll 

 without Mg atom, N atoms and side chains. Right: |3-carotene without ionone rings (Szent- 

 Gyorgyi). 



Conant (6, 7) believed the isocyclic ring with the Cg and Cio atoms to be of 

 importance in photosynthesis. 



The yellow carotenoids occur in all plants together with the chlorophylls. 

 These substances are unsaturated hydrocarbons composed of isoprene 

 groups. Some are aliphatic molecules with open carbon chains, some are 

 molecules containing one or two ionone rings. Of the more than 60 carot- 

 enoids known jS-carotene is the most abundant. It is composed of eight 

 isoprene groups, four of which form two ionone rings (Fig. 3). 



It is surprising to see how the chlorophylls and the carotenoids differ in 

 molecular structure. There seems to be no structural relationship whatever 

 between the four pyrrole nuclei coordinated by a metal in the chlorophyll 

 molecule and the isoprene chains of the carotenoids. However, Szent- 

 Gyorgyi (38) pointed out that the carbon chain of j8-carotene — without the 

 ionone rings — when written as a ring bears a striking resemblance to the 



