554 PHOTOCHEMISTRY OF PIGMENTS IN VIVO CHAP. 19 



(c) Photochemical Oxidation and Photochemical Reduction of Chlorophyll 



The earliest variations of this theory assumed an interconversion of 

 chlorophylls a and 6. When Willstatter and Stoll (1913) found that 

 chlorophyll h contains one oxygen atom more (and two hydrogen atoms 

 less) than chlorophyll a, they conceived the idea that, in the course of 

 photosynthesis, chlorophyll a may be oxidized to h (by reducing carbon 

 dioxide) and then reduced again (by oxidizing water). This prompted 

 them to search for changes in the ratio [a]:[£'] during intense photo- 

 synthesis. No such changes were found; and Willstatter and Stoll 

 considered this as a decisive argument against the theory of an o ^ ' h 



transformation in photosynthesis. This is not necessarily so, since the 

 equilibrium may be rapidly re-established in the dark (c/. page 550). 

 However, no simple way has been found to oxidize chlorophyll a to 

 chlorophyll h in vitro {cf. Stoll and Wiedemann, page 466). Furthermore, 

 as discussed on pages 405 et seq., colored algae {Phaeophyceae, Rhodo- 

 phyceae, Diatomeae and Cyanophyceae) — i. e., the vast majority of photo- 

 synthesizing organisms — contain no chlorophyll h; this certainly speaks 

 against a chemical equihbrium involving the two forms. 



Although abandoned by Willstatter and Stoll (1918), the hypothesis of a reversible 

 a V ^ b transformation has been revived by Dixon and Ball (1922), who postulated 



that chlorophyll a, by photochemically reducing carbon dioxide, is converted into 

 chlorophyll b. 



Ught 



(19.15) RCHs + COj > ECHO + H^CO 



(Chi a) (Chi b) 



Chlorophyll b oxidizes water, also photochemically, and is thus restored to chlorophyll a: 



light 



(19.15) ECHO + H2O > RCH3 + O2 



(Chi 6) (Chi a) 



In a different (and highly implausible) form, the a v ^ b conversion hypothesis 



was again presented by Baly and Morgan (1934) and Baly (1935, 1941). They assumed 

 the following two reactions (RHj = Chi a; RO = CHI b): 



(19.16a) {RH2CO2} "—^ {ROCH2O} "-^ RO + CHjO 



(blue) (red) 



(19.16b) RO + carotene > RHj + xanthophyll 



Reaction (19.16a) was assumed to proceed in two photochemical steps — the first leading 

 to a complex (ChlbCHjO) and the second hberating free CH2O. The first step was 

 assumed to require a quantum of blue light, the second a quantum of red light. Reaction 

 (19.16b) was considered to be a dark reaction (Blackman reaction). 



Baly's scheme ignores the fact that photosynthesis involves not only a reduction 

 of carbon dioxide, but also a Hberation of oxygen from water. (Neglect of this point 

 is conspicuous also in Baly's experiments on artificial photosynthesis, pages 85 et seq.), 

 and stops at the formation of formaldehyde and xanthophyll from carbon dioxide and 

 carotene. Furthermore, Baly ignores the elementary fact that photosynthesis can 

 proceed in pure blue fight as weU as in pure red fight. 



