22 PROBLEMS IN PHOTOSYNTHESIS 



that free radicals give rise to polymerization of vinyl compounds. The 

 reaction of ferrous ions with H2O2 (6, 7, 20) gives us an interesting example 



Fe2+ + H,,02 -^ Fe'+ + OH^ + HO- 



The HO radicals formed react further cither with ferrous ions 



Fe-'+ + HO. -^ Fe^+ + OH- (a) 



or with HoOo 



H2O2 + HO. -^ HoO + HO2. (b) 



In the presence of sufficient vinyl compounds of the general formula CHo^ 

 CHX the reactions a and h will be suppressed as the HO radical will be used 

 for the polymerization reaction 



HO. + CH2=CHX -^ HO— CH2— CHX— 



and 



HO— CH,— CHX— + CH2=CHX -^ HO— CH2— CHX— CH.— CHX— etc. 



The property of free radicals of inducing polymerization thus provides an 

 indirect method of determining these substances. Uri found that the addi- 

 tion of methylmethacrylate* reduces the amount of O2 developed in photo- 

 synthesis {Chlorella) to about one-third. The polymer produced from methyl- 

 methacrylate had a molecular weight of about 10^. As the niolecular weight 

 of methylmethacrylate is 100, the polymerization degree was thus 1000. 

 No polymerization was observed in the dark under the same experimental 

 conditions or upon illumination of killed algae. In the absence of CO2 no 

 polymerization could be obtained, probably because photosynthesis proceeds 

 too slowly. The addition of methylmethacrylate had no influence upon the 

 O2 production in the Hill reaction (see § 49) so that apparently no polymers 

 are formed. This negative result was independent of the hydrogen acceptors 

 used and it may thus be concluded that no free radicals are necessary in the 

 Hill reaction. Uri considers this highly improbable and assumes that the 

 free radicals produced during the Hill reaction may not be able to form long 

 chains of polymers but may act as terminal groups of such chains. This 

 behavior has been found for the free radical SON* and for the Br atom (20, 

 54). There is, however, insufficient evidence to assume this. 



In § 9 we discussed the work of Commoner et al. (14, 15) dealing with 

 electron spin resonance in enzyme systems. These authors succeeded in 

 observing electron spin resonance in chloroplasts. Calvin and Sogo (12) 

 confirmed these findings so that the theoretical assumption that chlorophyll 

 is brought into its triplet state upon illumination seems to be experimentally 

 proven in vivo.** However, it must be pointed out that electron spin reso- 



* CH2=C— COOCH3. 



CH3 



** Fujimori and Livingston (25) succeeded in determining in vitro the half-life of the triplet state of 

 chlorophyll in organic solvents. 



