BLEACHING OF CHLOROPHYLL IN METHANOL 1495 



ible bleaching was only 7.8 X 10"^; in other words, the great enhance- 

 ment of the stationary bleaching by iodine must be due entirely to a slowing 

 down of the back reaction. 



When stationary reversible bleaching is as strong as it can be in the pres- 

 ence of iodine (of the order of 10%), the bleaching effect of the photometric 

 light beam cannot be neglected. Livingston showed that this influence was 

 largely responsible for the observed deviations from the -y/l law. 



In summing up, Livingston pointed out that reversible bleaching ap- 

 parently requires the presence of polar molecules. This parallels results 

 obtained by the same investigator in the study of chlorophyll fluorescence 

 (page 764) ; the latter, too, was found to require the association of chloro- 

 phyll with polar molecules (alcohols or amines) . 



Livingston and Ryan (1953) continued the study of reversible bleaching 

 of chlorophyll in two directions. In the first place, they attempted to 

 identify the spectrum of the "bleached" state (its only previously known 

 feature being reduced absorption in the red). For this purpose, mono- 

 chromatic scanning beams (isolated by interference filters) were sent 

 through a methanolic chlorophyll solution, illuminated by an immediately 

 adjoining 1000-watt incandescent lamp (through a Corning 3-66 filter) in 

 a constant temperature bath. Experiments of this type permit calculation 

 of the product (concentration of changed chlorophyll) X (difference be- 

 tween the absorption coefficients of changed and normal chlorophyll) for 

 each scanning wave length; by assuming that the absorption coefficient, 

 (xP, of the photoproduct is zero at the wave length where the bleaching ef- 

 fect is strongest, a minimum value for the concentration of the bleached 

 form can be calculated, and a consistent set of absorption coefficient ob- 

 tained for all other wave lengths. (They will all be in error, by a constant 

 factor, if the assumption aP = was wrong.) 



The estimated absorption coefficients of the ''phototrope" (as the "re- 



Table 35.IA 



Average Specific Absorption Coefficients of Chlorophyll a and b and Their 



Phototropes, Chi a* and Chi b*, in Methanol, Determined for Bands Centered 



AT X„ (after Livingston and Ryan, 1953) 



