490 PHOTOCHEMISTRY OF PIGMENTS IN VITRO CHAP. 18 



only possible explanation of reversible bleaching. This hypothesis will 

 be discussed below (page 491). 



First, however, we shall consider the suggestion of Franck and 

 Livingston (1941), and Livingston (1941), that a combination of tautomer- 

 ization and dismutation may explain the facts which could not be inter- 

 preted by any one of these processes separately: 



(18.11a) Chi* >tChl (tautomerization) 



(18.11b) tChl + Chi > oChl + rChl (dismutation) 



(18.11c) oChl + rChl > 2 Chi (back reaction) 



In this scheme, tautomerization leads to an intermediary state which is 

 long-lived enough to provide an opportunity for dismutation; but the 

 bleached state is represented, not by tautomerized chlorophyll, but by 

 the products of dismutation, oChl + rChl, and therefore requires a 

 bimolecular reaction for its termination. The inhibiting effect of oxygen 

 was attributed by Franck and Livingston to a catalytic acceleration of 

 the back reaction (18.11c), e. g.: 



(18.12a) rChl + O2 > Chi + HO2 



(18.12b) HO2 + oChI > Chi + Oj 



(18.12) oChl + rChl > 2 Chi 



In a similar way, the inhibiting effect of ferrous ions can be attributed 

 to a catalytic acceleration of reaction (18.11c) by the system Fe+++-Fe++. 

 The bleaching of chlorophyll by ferric ions was attributed by Rabinowitch 

 and Weiss (1937) to its reversible oxidation (c/. page 465). Franck and 

 Livingston suggested that in this case, too, tautomerization is a prelimi- 

 nary step: 



(18.13a) Chi* )-tChl 



(18.13b) tChl 4- Fe+++ > oChI + Fe++ 



(18.13c) oChl + Fe++ > Chi + Fe+++ 



The low concentration of Fe+++ ions, which is sufficient to obtain a 

 maximum effect (c/. Fig. 53), supports this point of view. Measure- 

 ments of the effect of ferric chloride on fluorescence should show whether 

 the reaction of Chi* with Fe+++ competes with fluorescence, or whether 

 this competition is eliminated by the intermediate formation of a tau- 

 tomer, as assumed in (18.13a). Reaction (18.13c) can be accelerated 

 by ferrous salts but — ^in contrast to (18.11c) — it cannot be affected by 

 oxygen. This gives a plausible explanation of why the reversible bleach- 

 ing caused by ferric chloride is insensitive to oxygen. 



Although these schemes explain satisfactorily the reversible bleaching 

 of chlorophyll solutions (as well as the quenching phenomena mentioned 

 on page 483 and described in more detail in Vol. II, Chapter 23), 



