BLEACHING OF CHLOROPHYLL IN METHANOL 1491 



(35.6) tChl + I2 > tChl-Iz (analogous to moloxide formation) 



(35.7) tChM2 > Chi + I2 



These two reactions lead to enhanced stationary bleaching (extended 

 life-time of the complex tChl • I2) and make the back reaction a first-order 

 reaction. 



Similar explanations could be suggested for the effects of oxalic acid on 

 reversible bleaching. 



Further experiments on the reversible bleaching of chlorophyll were 

 made by Knight in Livingston's laboratory (c/. Livingston 1947, 1948, 

 Knight and Livingston 1950). The effects of solvent, temperature, oxygen 

 concentration, chlorophyll concentration and of certain additions were ex- 

 amined. 



The extent of steady-state reversible bleaching of chlorophyll a in 

 oxygen-free methanol was found to be 1.10 rel. units at 13.3° C, 1.00 at 

 27.3° C. and 0.98 at 39.5° C. This small but probably real temperature 

 coefficient indicates a very small activation energy of the back reaction. 



Experiments at three chlorophyll concentrations (1 X 10~^, 2 X 10~^ 

 and 5 X 10~^ mole/hter) at 31° C, showed, quite unexpectedly, a slightly 

 decreasing absolute bleaching: A [Chi] — 1.3, 1.0, and 1.0 X 10 ~^ mole/ 

 liter, respectively. Taking into account the increase in absorption, Living- 

 ston (1949) estimated that the extent of reversible bleaching at constant 

 light absorption is inversely proportional to the square root of chlorophyll 

 concentration. This implies that the rate of the back reactions is acceler- 

 ated by an increase in chlorophyll concentration. 



It was mentioned before that in pure methanol the back reaction is of 

 second order with respect to A [Chi], indicating that it is brought about 

 by the encounter of two tChl-molecules : 



(35.8) tChl + tChl > 2 Chi 



or more generally, of two molecules produced by bleaching, e. g. : 



(35.9a) rChl + oChl > 2 Chi, or 



(35.9b) rChl + oS > Chi + S, or 



(35.9c) oChl + rS > Chi + S 



(where S can stand, as before, for a molecule of the solvent, or an impurity). 

 None of these schemes involves the participation of normal chlorophyll 

 molecules in the back reaction. An apparent acceleration of the back reac- 

 tion by nonexcited chlorophyll molecules thus remains to be made plausible. 

 In Vol. 1 (p. 515), a similar effect of increased pigment concentration 

 ■ — the decrease in quantum yield of chlorophyll-sensitized reactions — was 

 explained by the competition : 



^^^■^^) ^^"^''l+Chl > oChl + rChl 



