PHOTOXIDATION AS THE CAUSE OF BLEACHING 499 



of chlorophyll in ethanol — corresponding to as much as 20 molecules of 

 oxygen per molecule of chlorophyll. Gaffron (1933) found that fresh 

 ethyl chlorophyllide solutions in acetone absorbed oxygen with a quantum 

 yield of the order of 0.1-0.3; the yield decreased with time, but was 

 still as high as 0.006 even after one month of storage. If this were the 

 quantum yield of bleaching, all chlorophyll would be bleached, in moder- 

 ately intense hght, in less than a minute. Since no such rapid bleaching 

 has ever been observed, practically all oxygen must have been transferred 

 to the solvent or to oxidizable impurities. Thus, the uptake of oxygen 

 during the bleaching period is not a conclusive proof of the oxidation of 

 chlorophyll. Experiments on allomerization (pp. 460 et seq.) showed that 

 chlorophyll can take up one molecule of oxygen without appreciable 

 change in color. (We do not know how this reaction is affected by light.) 

 In allomerization too, the nature of the medium is of paramount im- 

 portance (oxygen being taken up in methanol or ethanol, but not in ether 

 or pyridine). Thus, one cannot be certain whether chlorophyll is the 

 final oxygen acceptor in this case either. In reaction scheme (16.8-16.10) 

 on page 461, chlorophyll and methanol were assumed to share the 

 absorbed oxygen between them. 



Gerland (1871) observed that chlorophyll solutions, which have taken up oxygen 

 in the dark, bleach afterwards in hght, even in the absence of oxygen. Whether this 

 observation indicates that oxygen, taken up in the allomerization process, can later be 

 transferred to other parts of the molecule where it causes bleaching, is difficult to say; 

 Gerland's observations were carried out with crude extracts and have not been repeated 

 with pure chlorophyll preparations. They are supported indirectly by observations on 

 other dyestuffs. In contact with air, these dyes first form peroxides or "moloxides," 

 which are transformed into stable oxidation products by further exposure to Ught (see, 

 for example, Gebhard 1909, 1910) : 



light light? 



(18.20) D + O2 V DO2 > oD (D = dye) 



dark 



Gaffron (1933) opposed mechanism (18.20) for the photoxidation of chlorophyll, because, 

 according to his observations, chlorophyll does not absorb oxygen and does not form 

 peroxides — neither in hght nor in the dark. His experiments were carried out in 

 acetone, and therefore do not conflict with the observations on the oxygen uptake in 

 allomerization, which occurs only in alcohols. 



Because of the low partial pressure of oxygen, which suffices to bring 

 about the maximum rate of bleaching of chlorophyll, the possibility that 

 bleaching may be due to a direct reaction between excited fluorescent 

 chlorophyll molecules and molecular oxygen can be discounted. A 

 reaction between long-lived tautomeric chlorophyll (tChl) and molecular 

 oxygen, on the other hand, can provide a plausible mechanism of 

 bleaching : 



(18.21a) Chi* >tChl 



(18.21b) tChl + Os > HO2 + oChl 



