21 



B. Eumberg, F. Schmidt-Mende , J. l^eikard and H.T. Witt 



a) The shift from negative to positive values indicates which 

 added oxidized substances are electron-acceptors of Z~ (5). So 

 far as investigated all substances surnamed under ox.Sj in fig. 

 1o react with Z. S.B. Henriques in our laboratory showed that 

 also TPN-reductase - but not TPN alone - traps electrons from Z~ 

 From the highest redox potential of ox.Sj (methyl viologen) 

 follows for the redox potential of Z~/Z a value of ^-o,^^ 

 Volt (4). Because TPN-reductase is reduced by Z~, TPN-reductase 

 must contain a redox system (1). Arnon (7) et al have shown that 

 this is ferredoxin. TPN-reductase was already described in de- 

 tail (7). 



b) By shifting the absorption changes of Chl-a-j- from negative 

 to positive values it is possible to separate the difference 

 spectrum of Chl-aj from the overall difference spectrum (fig.1 

 top) under complete natural conditions (fig. 7) (compare 1,c). 



8 . Water as the ultimate electron donor of Chl-a -j- 



It is long known that CMU blocks especially the oxidation of 

 water (9) • If water is the ultimate electron donor for Chl-aj, 

 any reaction of Chl-aj should vanish in the presence of CIi/IU. 



hvj ^"^TT 

 oxS-;^ < Z < — Chl-aj < ••• a^O 



This is true for chlorella but this is not the case in fresh 

 chloroplasts (fig. 8 mid). The reason is again the backflow of 

 electrons from Z~ to Ghl-at , which keeps the cycle in action. 

 Trapping the electrons of Z~ by addition of oxidized substances 

 ox.S-p (i.e. benzyl viologen), results in the disappearance of 

 the changes caused by Chl-aj (fig. 8 bottom). This fact is a fur- 

 ther possibility for the determination of those substances which 

 can accept electrons from Z~ (5). 



In aged chloroplasts, where Chl-a-p is supplied directly by 

 electrons of red PMS (see 4), the adaition of CMU has no influ- 

 ence on the absorption changes of Chl-a-j- (4) . 



9. Some properties of the Chl-aj-reaction 



by the depe 



the light-induced changes at 7o5 mu on the ratio of ferro/ferri- 

 cyanide . From such measurements Kok (2) estimated a value of 

 + 0,43 Volt (pH 5 - 9>5)« Our measurements gave nearly the same 

 potential + o,^ Volt (pH 6-8) (fig. 9) (-4-). The pH-independen- 

 ce indicates a pure electron transfer. 



b) Temperature of inactivation . The Chl-a-p-reaction is stable 

 up to 65''C (1). ^ 



c) pH-Range . The Chl-a-^ reaction is stable between pH 4 and 

 pH 11 (1). ^~ 



by means of hVj-background light (7oo - 72o mu) 



