1862 SPECTROSCOPY AND FLUORESCENCE OF PIGMENTS CHAP. 37C 



After addition to a chloroplast suspension, of 10~^ m./I. DPN, a re- 

 duction of 0.05% of the added nucleotide could be observed in the photo- 

 stationary state. This very small shift in the oxidation-reduction equi- 

 librium, whose discovery was made possible by the extreme sensitivity 

 of the compensation apparatus, explains why no reduction of DPN could 

 be observed directly in Hill reaction experiments (chapter 35, section 

 B4(/)), and why DPNH2-mediated reductions of organic substrates could 

 be obtained, by means of illuminated chloroplasts, only with very low 

 yields: the rate of the back reaction (reoxidation of DPNH2) is so high as 

 to make a significant accumulation of reduced DPN in the stationary state 

 of the illuminated system impossible. 



As to the significance of the observations of Duysens, and of Lunde- 

 gardh, concerning photostationary oxidation of cytochrome-like com- 

 pounds in illuminated photosynthesizing cells, they can be interpreted 

 either as evidence of direct participation of these compounds in the photo- 

 chemical hydrogen transfer from water to carbon dioxide (or, rather, to 

 an organic compound into which CO2 had been incorporated, such as PGA), 

 or as evidence of their participation in oxidative processes (back reactions), 

 coupled with the reduction process. The first hypothesis (Hill, Lunde- 

 gardh) suggests photochemical transfer of electrons from reduced cyto- 

 chrome to the organic acceptor (perhaps via DPN or TPN). The transfer 

 of hydrogen (or electrons) from H2O to the oxidized cytochrome would 

 then require another photochemical reaction. To account for the observed 

 shift, the relative probability of the two photochemical reactions would 

 have to be such as to establish a photostationary state with most of the 

 cytochrome in the oxidized state. The quantum requirement of the 

 hydrogen transfer reaction as a whole would be (at least) 8, since two 

 quanta ^vill be needed to transfer each of the four required H atoms (or 

 electrons), first from water to the cytochrome, and then from the cyto- 

 chrome to the final acceptor. 



The other hypothesis (preferred by Duysens) is that the photochemical 

 hydrogen transfer from H2O to TPN (or to another compound of a similar 

 reduction potential), occurs directly, i. e., by a single quantum, but that 

 one part of the reduced photoproduct reacts back \vith the oxidized photo- 

 product, with a cytochrome as final or intermediate H acceptor, and the 

 reoxidation energy may become available to assist in a further reduction 

 step (as first suggested in the "energy dismutation" hypothesis in Vol. 1, 

 chapters 7, p. 164 and 9, p. 233). If the reoxidation energy is stored as 

 phosphate bond energy, the ATP produced in this way may, for example, 

 enable reduced pyridine nucleotide to reduce PGA to a triose (as repeatedly 

 suggested before, e. g., in Chapter 36, p. 1717). 



