QUENCHING AND ALLOMERIZATION 787 



dants can be attributed, with a fair degree of certainty, to a (reversible) 

 oxidation of the excited chlorophyll molecule by the quencher — perhaps 

 the same reaction which causes the reversible bleaching of chlorophyll in 

 light (^'ol. I, page 486, and chapter 35). Thus, quenching experiments 

 confirm the repeatedly noted capacity of chlorophyll to serve as a photo- 

 chemical reductant. (In chapter 35, we will present evidence that it can 

 act also as a photochemical oxidant.) 



Some of the effects described by Livingston (1948) clearly belong to a 

 different type — which we may call "pseudo-quenching." These are 

 fluorescence changes caused by chemical reactions between nonexcited 

 chlorophyll molecules and the quencher (or stimulant). The most striking 

 results of this type were obtained with iodine. When traces of iodine were 

 added to a solution of chlorophyll, fluorescence began to change gradually; 

 minutes or even hours were needed to reach a steady state. This points to 

 a slow chemical conversion of chlorophyll to a compound with a different 

 capacity for fluorescence. Probably, the reaction is an irreversible oxida- 

 tion; perhaps, preceded by transient complex formation. As discussed 

 elsewhere (cf. page 613), the product appears to be similar to (or identical 

 with) allomerized chloro-phyll, as obtained by slow oxidation of alcoholic 

 chlorophyll solutions in air. According to Fischer (c/. Vol. I, page 459), this 

 is chlorophyll oxidized at carbon atom 10. Similar to allomerization in air, 

 the reaction with iodine (and a similar one with bromine) occurs only in 

 alcoholic solution (methanol or ethanol) but not in ether or carbon tetra- 

 chloride. In the case of chlorophyll a, the final product has about 55% 

 of the fluorescence intensity of the nonallomerized solution; in the case of 

 chlorophyll 6, it fluoresces twice as strongly as the initial compound. 



Admixture of carbon tetrachloride to methanol lengthens the time 

 needed to complete the allomerization by iodine, from about 5 minutes for 

 chlorophyll a in pure methanol, to over an hour in a mixture of equal parts 

 of methanol and carbon tetrachloride. 



With chlorophyll 6 in methanol, fluorescence declines at first upon the 

 addition of iodine; but later (in about 30 minutes) it begins to increase 

 again and reaches, after 10-15 hours, a steady level about twice as high as 

 the original one. AVith bromine and chlorophyll 6, the minimum is reached 

 faster (in about one minute), and the high steady level is approached in 

 about an hour. The initial dip in fluorescence may be taken as sign of 

 complex formation — the subsequent increase, as indication of the conver- 

 sion of chlorophyll to the allomerized form. 



Higher amounts of iodine, and particularly of bromine, quench the 

 fluorescence of chlorophyll more or less completely, by causing a deeper 

 chemical change in the chlorophyll molecule. 



Effects similar to those caused by small quantities of iodine or bromine 



