1492 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



with the second reaction leading to recombination : 



(35.11) oChl + rChl > 2 Chi 



Livingston (1949) gave a somewhat different interpretation of the inhibiting 

 effect of high pigment concentration on reversible bleaching: 



(35.12) tChl + Chi > 2 Chi 



However, this means that the back reaction ceases to be second order in 

 respect to bleached chlorophyll — an implication not yet confirmed by ex- 

 periment. It should also be taken into consideration that at higher chloro- 

 phyll concentration, absorption becomes less uniform and is concentrated 

 in a thinner layer of solution ; this favors second-order back reactions, such 

 as tChl + tChl -^ Chl2 (in the same way as would an increase in light in- 

 tensity). 



The observations (and speculations) concerning energy transfer between 

 pigment molecules (chapter 32) and its role in the self-quenching of fluores- 

 cence (chapter 23, section A5, and chapter 37C, section 4b) suggest another 

 possibility — that increased chlorophyll concentration could diminish re- 

 versible bleaching by dissipation of excitation energy in the course of its 

 migration, as suggested in explanation of self-quenching. However, con- 

 centration effects were observed by Knight and Livingston in a range 

 (10~^ M) which is far below that where concentration quenching becomes 

 noticeable (10"^ M); they must be therefore attributed to the deactiva- 

 tion of metastable (and not of fluorescent) chlorophyll molecules. The 

 only additional possibility derived from the energy migration concept is, 

 then, that the deactivation reaction (35.12) may be caused by resonance 

 rather than by actual kinetic encounters. (The probability of resonance 

 exchange in the metastable state was mentioned in chapter 23, p. 785, and 

 in chapter 32, p. 1290-1291.) 



That the back reaction is not simple was indicated by the previously 

 noted effect of impuriiies. In a renewed study, the influence of traces of 

 water was investigated. It was found that the presence of 2% water in a 

 2 X 10 ~^ mole/liter solution of chlorophyll a in methanol increases the 

 steady-state bleaching by as much as a factor of three. It also accelerates 

 considerably the rate of irreversible bleaching. In benzene as solvent, no 

 reversible bleaching could be observed at all, and only a very slow irrevers- 

 ible bleaching took place in the absence of oxygen. The presence of 1% 

 methanol in benzene was sufficient, however, to produce as strong a revers- 

 ible bleacliing as that which occurs in pure methanol. In carbon tetrachlo- 

 ride (specially purified to remove the reductible impurity mentioned 

 above) the irreversible bleaching is very strong. (Complete bleaching 

 to a straw-yellow solution can occur within 3 min.; the product has an ex- 



