* 



Chapter 35 



PHOTOCHEMISTRY OF CHLOROPHYLL IN VITRO AND IN VIVO 

 (ADDENDA TO CHAPTERS 4, 18 AND 19) 



Experiments witli chloroplast suspensions and products obtained by 

 their dispersion and fractionation have narrowed (l)ut not quite bridged) 

 the gap that had separated the photochemistry of chlorophyll in the living 

 cell from the photochemistry of chlorophyll in solution. If this monograph 

 were planned now, the part of it dealing with chloroplast-sensitized reac- 

 tions would not have been tucked away among the unrelated and mostly 

 wasted efforts described in chapter 4 ("Photosynthesis and Related Proc- 

 esses Outside the Living Cell"), but would have found its logical place at the 

 end of chapter 18 ("Photochemistry of Pigments in Vitro"), forming a 

 transition to chapter 19 ("Photochemistry of Pigments in Vivo"). Be- 

 latedly, we have adopted this plan in the present chapter, which thus con- 

 stitutes an addendum to chapters 4, 18 and 19 of Volume I. 



A. Photochemistry of CHLOROPHYLLf in Solution* 



1. Bleaching of Chlorophyll in Methanol 



(First Addendum to Chapter 18, Section A2) 



The reversible bleaching caused by illumination of oxygen-free chloro- 

 phyll solutions in methanol, first observed by Porret and Rabinowitch, was 

 described in Vol. I, p. 486. McBrady and Livingston (1948) have contin- 

 ued the investigation of this phenomenon, which may provide clues to the 

 function of chlorophyll in photosynthesis. Figure 35.1 gives a good 

 illustration of the phenomenon. (The open circles represent the transmis- 

 sion of the solution in light, the black dots its transmission in the dark.) 

 McBrady and Livingston (1948) confirmed Livingston's earlier suspicion 

 that the rate of the back reaction, and with it also the extent of bleaching 

 in the photostationary state, can vary considerably from case to case— 



* Bibliography, page 1625. 



t Very little is known about the photochemistry of other photosynthetic pigments. 

 According to Krasnovsky, Evstigneev et al. (19522), phycoerythrin solutions are much 

 more stable in light than chlorophyll solutions; no reversible photochemical changes 

 could be observed in them. 



1487 



