II. Die Eigenschaften des Reaktionscyclus von Chlorophyll-aI-430-703
Authors: Rumberg B. 
Published: February 2015

The effect of far-red background-light. In chlorella or chloroplasts with added Hill - oxidants far-red actinic light between 700 and 730 mμ oxidizes Chl-aI; in the dark Chl-aI
⊕ stays in this oxidized state. Actinic light in the range λ<700 mμ results also in an oxidation of Chl-aI, but in the dark Chl-aI
⊕ is reduced. Therefore light of λ<700 mμ is channeled into two reaction centres. One part (hνI)is channeled to Chl-aI (with the effect of oxidation of Chl-aI), a second part (hνII) is channeled to a second reaction centre where it provides electrons for the reduction of Chl-aI
⊕ [s. scheme (1)]. Under the influence of far-red background-light (700 -730 mμ) Chl-aI accumulates in its oxidized form. The magnitude of its oxidation depends on the intensity of the background-light. a) In soft far-red background-light practically no Chl-aI
⊕ is accumulated (s. Fig. 1 a). A supplementary red flash (<700 mμ) at t1 with hνI and hνII-light should have the following effect: The hνI-light oxidizes Chl-aI immediately. The hνII-light provides electrons from a second reaction centre for the reduction of Chl-aI
⊕. b) In stronger far-red background-light Chl-aI accumulates in its oxidized form (Fig. 1 b and Fig. 1 c). A supplementary red flash (<700 mμ) at t3 or t5 oxidizes the rest of Chl-aI immediately. Afterwards all Chl-aI
⊕ is again reduced. - Changing from soft to stronger far-red background-light a shift from negative changes of absorption of Chl-aI to positive ones is aspected (Fig. 1 a′ and 1 b′). However, in chloroplasts without any added Hill- oxidants no shift takes place (Fig. 2 top). This indicates that no accumulation of Chl-aI
⊕ has occurred. The reason is, that Chl-aI
⊕ can be reduced by a backflow of electrons from the electron acceptor Z [s. scheme (2)]. Trapping the electrons of Z⊖ by electron acceptors (ox. SI) prevents the backflow [s. scheme (3)] and positive changes of absorption occur (Fig. 2 bottom). By shifting the changes of absorption of Chl-aI from negative to positive values it is possible to separate the difference-spectrum of Chl-ai from the overall difference-spectrum under complete natural conditions (s. Fig. 4 and 1. c. 1).

Electron-acceptors of Z⊖. All substances surnamed under ox. SI (Table I and Fig. 11) trap electrons of Z⊖ (shift from negative to positive changes of absorption at 703 mμ). In this way it was shown, that also photosynthetic-pyridine-nucleotide-reductase (PPNR) - but not TPN alone - traps electrons from Ze. Therefore PPNR must contain a redox system [ferredoxin] [s. scheme (4)]. From the highest redox potential of ox. Si (methyl viologene) that of Z/Z⊖ (≲ -0,44 volt) was estimated.

Water as the ultimate electron-donor for Chl-aI. CMU blocks oxidation of water. If water is the the ultimate electron donor for Chl-aI, turnover of Chl-aI should vanish in the presence of CMU [s. scheme (5)]. This is true for chlorella, but not for chloroplasts without ox. Si (Fig. 5 midth). The reason is again the backflow of electrons from Z⊖ to Chl-aI
⊕ which keeps the cycle in action. Trapping the electrons of Z⊖ by addition of ox. Si results in the disappearence of the changes of Chl-ai (Fig. 5 bottom). In aged chloroplasts, where Chl-aI
⊕ is supplied directly by electrons of PMSH⊖, addition of CMU should have no influence on the changes of Chl-aI (Fig. 6).

a) Redox potential. By addition of different ratios of ferro/ferricyanide the ratio of Chl-aI/Chl-aI
⊕ can be changed in the dark. This is shown by the dependence of the light induced changes at 703 mμ on the ratio of ferro/ferricyanide (Fig. 7). The oxidation of Chl-ai occurs by a single electron transfer. The redox potential of Chl-aI
⊕/Chl-aI was determined to +0,46 ± 0,1 volt. b) The Chl-ai-reaction is stable between pʜ = 3 and pʜ = 11 (Fig. 8). c) The Chl-ai-reaction is stable up to 65°C (Fig. 9). d) Aging up to 7 days at 5°C has no influence upon the Chl-ai-reaction (Fig. 10).

Reaction Scheme. The results are summarized in Fig. 11 and Table 2. In chlorella and fresh chloroplasts Chl-aI
⊕ is reduced by electrons originating from water with the help of hνII-light. After suppressing water-oxidation (by aging, extraction of plastoquinone, treatment with digitonin, addition of CMU)1 Chl-aI
⊕ can be reduced by backflow of electrons from Z⊖ or by added electron-donators as reduced PMS or reduced DPIP. The electrons of Z⊖ can be trapped by TPN (via ferredoxin) or substances surnamed under ox. SI.