753 



SUMMARY 



of spectroscopic changes as affected by additions of co- 

 factors, electron donors or acceptors, and inhibitors, clearly 

 associate photosystem II with the high potential side of 

 electron transport--specif ically , with the release of oxygen, 

 Photosystem I, on the other hand, appears to function in 

 inducing electron flow at the negative potential end, provid- 

 ing the reductant for ferredoxin, TPN, the viologens, etc. 



The conversion center for photosystem I is nov/ generally 

 agreed to be the long wavelength chlorophyll "P700" (Kok), 

 probably a part of the oriented chlorophyll complex (Olson). 

 Upon excitation by light energy it donates an electron to an 

 associated, unidentified acceptor, which must then be quite 

 a strong reductant. [The remaining radical might be responsi- 

 ble for one of the light induced EPR signals (Beinert)]. 

 PyOO apparently transfers one electron per mole, behaving as 

 a redox reagent with a potential (+ 0.^3 v) slightly higher 

 than that of cytochrome f_ (+ .36 v) with which it appears to 

 be associated in a charge transfer complex. This reaction 

 is observed even at liquid nitrogen temperature (Witt, ChanceX 

 A third component, the recently discovered copper enzyme 

 plastocyanin (Katoh) was reported to be another part of this 

 trapping center complex. 



Present knowledge or even speculations on the trapping 

 center of photosystem II are meager by comparison. Studies 

 of fluorescence yield (Duysens, Butler, Kok) clearly indicate 

 a chlorophyll as being the final light collector but no un- 

 equivocal evidence has been found for a color change of this 

 trap upon excitation. 



It is usually assumed that photoact II creates an oxidant 

 with a potential higher than + 0.8 volts, so that the oxidant 

 by itself can oxidize water. However, there is no absolute 

 need for such an assumption. An alternative presented at the 

 meeting (Hoch), could involve the electron in a jump over any 

 potential span, just so long as enough energy is provided to 

 bridge the gap of + O.h volts between P700 and water. 



The reductant made by photoact II according to most (but 

 not all) is likely to have a potential of about 0.0 volts. 

 More than one laboratory identifies this compound with plast- 

 oquinone (Witt, Trebst). However, the evidence, still 

 indirect, is based mainly upon the assumption that absorption 

 changes at 520 mu are due to an assumed complex between 

 plastoquinone and another ( carotenoid? ) pigment. Some support 



