556 PHOTOCHEMISTRY OF PIGMENTS IN VIVO CHAP. 19 



the capacity of chlorophyll for reversible reduction. Thus we must 

 make our choice between schemes of the type (a) or {c)—e. g., between 

 the energy dismutation theory (9.10) (in the form 19.12, in which HZ is 

 identified with chlorophyll), and the Franck-Herzfeld mechanism 7.VA 

 (or its less specific prototype, 7.V). 



Reversible oxidation of chlorophijll to a decolorized form (which tends to 

 support theories of type a), as well as the occurrence of chlorophyll in two 

 interconvertible green forms belonging to two different levels of oxidation 

 (which, if definitely established, would provide strong support for theories 

 of type c), were both discussed in chapter 16 (pp. 456 et seq.). We 

 recall that the experiments of Rabino witch and Weiss (1937) have indi- 

 cated that chlorophyll is reversibly oxidizable to a decolorized compound 

 ("oxychlorophyll"), and that its oxidation can be brought about by a 

 photochemical reaction (page 488). However, several objections stand 

 in the way of an identification of the yellow " oxychlorophyll " of Rabino- 

 witch and Weiss with oChl in equation (19.11) : for instance, the instability 

 of " oxy chlorophyll " (which is irreversibly changed by illumination with 

 blue-violet light, by contact with water, and even by standing for more 

 than a few minutes). However, conditions in the living cell may be 

 such as to stabilize the oxidized form, or to make its instability less 

 dangerous, by providing for a rapid return into the reduced colored state 

 of all oChl molecules not used for the evolution of oxygen. (It was sug- 

 gested on page 546 that the reversal of the primary photochemical process, 



e. g., by reaction {oChlHX} > {ChlX}, is very rapid in the living 



cell, because the two products remain finked in a complex.) 



As to the existence of two interconvertible green forms of chlorophyll, 

 our discussion in chapter 16 indicated several possible systems of this 

 type, e. g., chlorophyll and protochlorophyll (7,8-didehydrochlorophyll), 

 chlorophyll and dihydrochlorophyll (2-vinyl-bacteriochlorophyll), chloro- 

 phyll and 10-monodehydrochlorophyll (or 10-hydroxychlorophyll), and 

 chlorophyll a and chlorophyll b. However, a reversible photochemical 

 interconversion has yet to be demonstrated for any of these pairs. 



It may be useful to reflect on the thermodynamic properties which 

 the chlorophyll system should possess in order to fulfill the functions 

 assigned to it in theories of types a and c. In the first case, its oxidized 

 form is called upon to oxidize water by a thermal reaction (directly or 

 indirectly); thus, its normal potential must be exceptionally negative 

 (< - 0.8 volt), which is not impossible for a free radical (c/. page 232). 

 In the second case, the reduced form of chlorophyll is required to reduce 

 carbon dioxide (directly or indirectly) in the dark, which calls for an 

 exceptionally positive potential (> +0.4 volt). In case c, the oxidation- 

 reduction potential of chlorophyll could lie in the middle between these 

 two extremes. 



