II. Die Eigenschaften des Reaktionscyclus von Chlorophyll-a_I-430-703

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-a_I; in the dark Chl-a_I^⊕ stays in this oxidized state. Actinic light in the range λ<700 mμ results also in an oxidation of Chl-a_I, but in the dark Chl-a_I^⊕ is reduced. Therefore light of λ<700 mμ is channeled into two reaction centres. One part (hν_I)is channeled to Chl-a_I (with the effect of oxidation of Chl-a_I), a second part (hν_II) is channeled to a second reaction centre where it provides electrons for the reduction of Chl-a_I^⊕ [s. scheme (1)]. Under the influence of far-red background-light (700 -730 mμ) Chl-a_I 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-a_I^⊕ is accumulated (s. Fig. 1 a). A supplementary red flash (<700 mμ) at t₁ with hν_I and hν_II-light should have the following effect: The hν_I-light oxidizes Chl-a_I immediately. The hν_II-light provides electrons from a second reaction centre for the reduction of Chl-a_I^⊕. b) In stronger far-red background-light Chl-a_I 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-a_I immediately. Afterwards all Chl-a_I^⊕ is again reduced. - Changing from soft to stronger far-red background-light a shift from negative changes of absorption of Chl-a_I 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-a_I^⊕ has occurred. The reason is, that Chl-a_I^⊕ 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. S_I) prevents the backflow [s. scheme (3)] and positive changes of absorption occur (Fig. 2 bottom). By shifting the changes of absorption of Chl-a_I 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. S_I (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 Z^e. 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-a_I. CMU blocks oxidation of water. If water is the the ultimate electron donor for Chl-a_I, turnover of Chl-a_I 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-a_I^⊕ 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-a_I^⊕ is supplied directly by electrons of PMSH^⊖, addition of CMU should have no influence on the changes of Chl-a_I (Fig. 6).

a) Redox potential. By addition of different ratios of ferro/ferricyanide the ratio of Chl-a_I/Chl-a_I^⊕ 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-a_I^⊕/Chl-a_I was determined to +0,46 ± 0,1 volt. b) The Chl-ai-reaction is stable between p_ʜ = 3 and p_ʜ = 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-a_I^⊕ is reduced by electrons originating from water with the help of hν_II-light. After suppressing water-oxidation (by aging, extraction of plastoquinone, treatment with digitonin, addition of CMU)¹ Chl-a_I^⊕ 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. S_I.