CHLOROPHYLL-SENSITIZED OXIDATION-REDUCTIONS 1507 



ent from the acid-base interaction, and which is without marked influence 

 on the absorption spectrum- — perhaps a coordinative binding of the base in 

 the center of the molecule, reminiscent (as suggested by Krasnovsky and 

 Brin 1949) of the hemochromogen-protein bond (known to change the oxi- 

 dation potential of the hemochromogen). In chlorophyll the complexing 

 may occur at the magnesium atom; it is not quite clear how a similar 

 binding can occur in pheophytin (cf. table 35. IB). 



Speculations concerning the activating effect of organic bases on the 

 photoreducibility of chlorophyll obviously need to be related not only to 

 the absorption data (chapter 21, pp. 647-649, and chapter 37C, section 

 2), but also to fluorescence measurements (chapter 23, pages 766-771, 

 and chapter 37C, section 4) and chemical evidence. One obviously deals 

 here with events in at least two — if not more — sensitive centers in the 

 chlorophyll molecule which can be affected separately or simultaneously. 

 One of them may be the cyclopentanone ring V with its keto-enol tautom- 

 erism; the other, the central magnesium atom. A correlation of all the 

 pertinent evidence — derived from optical absorption spectra, infrared 

 spectra, fluorescence, "allomerization," phase test, and photochemical 

 activity — will be attempted in chapter 37C. 



Krasnovsky and Brin (1948) compared the "photochemical activity" (capacity to 

 react with ascorbic acid in light) of various types of chlorophyll preparations. They 

 found reactivity in organic solvents, oils, lecithin, and emulsions of lipoid solvents in 

 water; also in colloids obtained by dilution of alcoholic solutions by aqueous detergents 

 (anionic, cationic, or neutral), in similarly prepared chlorophyll-protein colloids, and in 

 chloroplasts and grana suspensions prepared with the same detergents. No photo- 

 chemical activity could be found in colloidal solutions prepared by dilution of alcohol 

 with water (before or after coagulation by electrolytes); or in chlorophyll adsorbates on 

 the proteins, zein or gliadin, and their colloidal solutions (for the position of the absorp- 

 tion peaks in these preparations, see table 37C.IIIA). 



5. Chlorophyll-Sensitized Oxidation-Reductions 

 (First Addendum to Chapter 18, Part C) 



Capacity for reversible photochemical oxidation or reduction is impor- 

 tant if a pigment is to serve as sensitizer for oxidation-reduction reactions — 

 in the same way as capacity for reversible nonphotochemical oxidation or 

 reduction is important for an oxidation-reduction catalyst. Only one 

 chlorophyll-sensitized oxidation-reduction reaction in vitro was described 

 in Volume I (page 513)— the oxidation of phenylhydrazine by methyl red. 

 This reaction was discovered by Bohi in 1929, and studied quantitatively 

 by Ghosh and Sengupta in 1934. They found that the quantum yield of 

 methyl red bleaching can reach, or even exceed, unity. Livingston, Sickle 

 and Uchigama (1947) pointed out that errors could have been caused, in 

 Ghosh's and Bohi's work, by ill-defined nature of the chlorophyll prepara- 



