150G PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



in the presence of oxygen or quinone. Hydrogen sulfide and cysteine also 

 reduced chlorophyll in dioxane irreversibly. 



Similar experiments with other dyestuffs (Mg-phthalocyanine, ribo- 

 flavin, (8-carotene, safranin T, neutral red, and phenol-indophenol) showed 

 that many of them (but not j8-carotene) also can be more or less reversibly 

 reduced in light by ascorbic acid (some also by pyruvic acid) ; the photo- 

 reduction depends on the solvent, being enhanced by the affinity of the lat- 

 ter for protons. 



Krasnovsky and Brin (1953) again discussed the question of why bases 

 promote the photochemical reduction of chlorophyll. Absorption studies 

 (cf. chapter 37C, section 2) showed that some bases cause a new absorp- 

 tion band at 640 m/x to appear in the chlorophyll spectrum (it becomes the 

 dominant long-wave band in piperidine, cf. table 21. VII). However, the 

 appearance of this band (which occurs only in polar solvents and must be 

 ascribed to polarization and ionization of an acid group, such as the enol 

 group in position 10 in chlorophyll) does not parallel photochemical activa- 

 tion. The latter can be produced by a small amount of a base (such as 

 phenylhydrazine) added to methanol solution without noticeable change in 

 the absorption spectrum. Krasnovsky and Brin concluded that activation 

 must be attributed to a reaction between pigment and base which is differ- 



Table 35.IB 



Influence of Bases on Photochemical Reduction (Measured by Decline of 



Absorption in Red) of Chlorophyll a and Pheophytin a (after Krasnovsky and 



Brin 1953) 



Chi a, % Pheo a, % 



Un- Restored Unre- Restored 



Medium reduced in dark duced in dark 



Pyridine 6 87 15 68 



Piperidine 31 39 5 50 



Nicotine 61 81 ■ — 



Pyrrole 81 81 



Quinoline 93 94 72 86 



Phenylhydrazine 4 62 — 



Ethanol 82 85 73 76 



+pyridine (50)* 42 52 43 59 



+piperidine (50)* 19 29 16 36 



+nicotine (95)* 8 58 36 72 



+urotropine (80)* 49 29 — — 



+quinoline (90)* 98 98 61 90 



+aiyinine (satur.) 94 93 — ■ 



+phenylhydrazine (100)* 52 85 — — 



-1-ammonia (satur.) 9 26 — 



+KOH (0.1-0.01 N) 98 99 — — 



* Number in parenthesis means milligrams of base in 7 ml. solution. 



