BLEACHING OF CHLOROPHYLL IN METHANOL 



1493 



ceedingly high absorption peak at 400 mn, and only a weak band in the 

 red.) 



Figure 35.2 shows the rate of irreversible bleaching and the extent of 

 reversible bleaching as functions of oxygen pressure, [Oo]. The antiparal- 

 lelism of the two developments — decrease in reversible bleaching and 

 increase in irreversible bleaching — is easily apparent. In the neighborhood 

 of 0.1 mm. O2, the irreversible reaction is comparatively slow and the re- 

 versible reaction not yet completely hihibited. At 0.5 mm. O2, irreversible 

 bleaching has reached its maximum rate, and reversible bleaching has been 



0.6 - 



o 



Fig. 35.2. Reversible and irreversible photobleaching of chlorophyll as a 

 function of the molality of dissolved oxygen (after Livingston 1949): (O) Rate of 

 irreversible bleaching (right scale); (D) extent of reversible bleaching (left scale). 



reduced to almost zero. The low oxygen concentration sufficient to "satu- 

 rate" the irreversible oxidation and to inhibit reversible bleaching can mean 

 one of two things: either chlorophyll molecules associate with o.xygen 

 molecules, the association being practically complete when the concentra- 

 tion ratio is 1:1 (for evidence of such association, cf. Vol. I, page 460), or 

 oxygen reacts with a long-lived active form of chlorophyll (such as tChl), 

 which lives for a period of the order of 10 ~^ sec. Since, when irreversible 

 bleaching is oxygen-saturated, its quantum yield is still very low, irrevers- 

 ible bleaching must be initiated by a reversible step, with the back reaction 

 of the unstable intermediate competing very successfully with the perma- 

 nently bleaching second step of oxidation. Livingston (1949) suggests the 

 reaction sequence tChl -f O2 -^ tChlOj; tClilO. -^ Chi + O2; tChlOa -}- 

 Chi — > 0(yhl -f- Chi, where OChl stands for permanently oxidized chloro- 



