62 PROBLEMS IN PHOTOSYNTHESIS 



globin could be significantly reduced by the action of light. Warburg and 

 Negelein (23, 32) studied the photodissociation of various important iron 

 carbonyl compounds with the help of manometry and calculated the quantum 

 yields of these reactions. They studied the following substances : 



Carbon monoxide-pyndine-hemochromogen, a compound of heme obtained bv 

 cleaving off globin from hemoglobin, with pyridine and CO. The bound iron 

 is bivalent, as in hemoglobin. If heme iron is designated as [Fe], the photo- 

 chemical cleavage can be represented as 



[Fe]CO + Nhv -^ [Fe] + CO 



Carbon monoxide f err ocysteine, a CO compound with ferrocysteine fa colourless 

 complex of bivalent iron and cysteine*). The photodissociation of this sub- 

 stance can be depicted as 



[Fe](CO)2 + N}n> -^ [Fe] + 2CO 



Iron pentacarbonyl, a compound produced by passing CO over iron filings at 

 about 80 ° C. Light causes the following breakdown 



Fe(CO)5 + Nhv -* Fe(CO)4 + CO 



Even though the chemical and optical characteristics of these three iron 

 carbonyl compounds vary greatly, the photochemical cleavage reactions all 

 have the quantum yield ^ = 1 . 



By contrast, Warburg" (23) found that in cleaving carbon monoxide hemo- 

 globin the quantum yield was 0.25 (quantum requirement 4) 



[Fe]CO + 4 Xhv ^ [Fe] + CO 



the quantum number 4 is related to the degree of polymerization of hemio- 

 globin. According to Anderson (1), salts and acidity cleave 4-fold poly- 

 merized hemoglobin to bi-polymerized hemoglobin. Biicher and Negelein 

 (5) found that increasing the salt concentration and decreasing the pH lowered 

 the quantum requirement from 4 to 2. Myoglobin is a non-polymerized 

 heme compound and, by cleaving its CO compound, Biicher and Negelein 

 (5) and Biicher and Kaspers (4) found that ^p = \ 



[Fe]CO + Xhv ^ [Fe] + CO 



Warburg (23, 32, 33) also studied the photochemistry of the CO compound 

 of iron oxygenase (cytochrome oxidase). This could not be done with the 

 pure enzyme because it has not yet been isolated and its constitution is still 

 unknown. Instead of direct measurements being made, the inhibition by 

 CO of cell respiration was investigated. Torula yeast and vinegar bacteria 

 were used as test objects. The result of these experiments was (p — 1, so that 

 in all probability cytochrome oxidase is not a polymerized heme compound. 



* The addition of a ferric salt to a neutral, pure cysteine solution produces a deep-blue ferric 

 cysteine complex which, after some time, becomes colorless. The cysteine reduces the ferric iron 

 to ferrous iron and is thereby oxidized to cystine. If the solution is shaken in air, it turns blue again, 

 producing trivalent iron. This process can be repeated until the iron oxidizes all the cysteine to 

 cystine. 



