ABSORPTION SPECTRUM CHANGES 97 



duced photochemically and is also in chemical equilibrium, in the dark 

 and through enzymatic reactions, with both the photoproduced 

 oxidant and other redox systems in the algae. The 648-m;u band like- 

 wise does not appear to he due to chlorophyll a; it may be a chloro- 

 phyll h derivative but it is also possible that it represents some other 

 type of compoTuid, for example, a cytochrome. Possibly it is the photo- 

 produced oxidant, a precursor of molecular oxygen. 



Fourth, a partial thermal inactivation produces a change in re- 

 sponse characterized by increases in absorption over the entire wave- 

 length region observed. We cannot account for this phenomenon at 

 present. 



One cannot say, presently, with any degree of certainty what the 

 compounds involved are. The results reported here and in the earlier 

 literature are tabulated in Table I. 



To summarize, the simplest interpretation of the present findings 

 consistent with earlier work on luminescence and absorption spec- 

 troscopy is the following : 



1 . The absorption bands at 480 and 520 m/z are due, respectively, to 

 the oxidized (X) and reduced (XH) states of the same compound. 

 The assignment of the 520 band to the reduced state follows (a) from 

 Witt's observations on the effect of added hydrogen acceptors which 

 depress the 520 changes and (6) from the fact that the absorption falls 

 below the steady dark level immediately following illumination and 

 then, in the succeeding dark period, rises to the normal dark level. 

 Since it is unlikely that the increase is due to the accumulation in the 

 dark of precursors of molecular oxygen similar to those produced 

 photochemically and since one would a priori expect the primary re- 

 ducing agent to be in djmamic chemical equilibrium with other 

 metabolic pools of hydrogen in the light as well as the dark, it is 

 probable that the 520-m/i band is due to a reduced compound. 



2. The 648-m/x band may represent the oxidized form of the pre- 

 cursor of Oo, that is, the photochemically produced oxidant (YOH). 



S. During illumination there is always an increase in the concentra- 

 tion of XH over its steady dark value, but in the succeeding dark 

 period it may fall below the dark level at steady state, because XH 

 can be consumed by a concurrently produced oxidant. Such a picture 

 is also adequate to explain the qualitative aspects of the induction 

 effects in absorption spectrum changes and in luminescence. 



Such an inequality between reductant and oxidant could arise be- 



