1528 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



in the complex must be delayed until after the conversion of excited chloro- 

 phjdhde into the metastable form, tChl. Another possible mechanism is 

 analogous to that suggested by Livingston for the sensitized methyl red- 

 phenylhydrazine reaction (cf. section 5 above) ; in this scheme no reaction 

 occurs until the complex of tChl with one reactant encounters the other re- 

 actant (cf. equations 35. 15c and d). 



In any case, the reaction probably involves a one-step oxidation- 

 reduction, followed by dismutation, or another short chain of elementary 

 steps, bringing the quantum yield up to a value close to 1 (which implies the 

 utilization of two oxygen atoms, i. e., the transfer oi four electrons, or hy- 

 drogen atoms, by a single quantum). Because of the likelihood that the 

 over-all reaction is composed of several successive elementary steps, in- 

 volving intermediate free radicals or other unstable products, the quantum 

 yield could be sensitive to factors such as the nature and purity of the sol- 

 vent, concentration of the reactants, light intensity and temperature. 



In some more recent papers, Burk and Warburg (1951) abandoned the 

 quest for a quantum yield of 1.0 in the actinometer, since they found that 

 in pyridine, with ethyl chlorophyllide as sensitizer, a quantum yield of close 

 to 1 required not only in a relatively low light intensity, but also the pres- 

 ence of a certain amount of impurities (such as piperidine) in the solvent. 

 In pure pyridine the newly observed quantum yields were between 0.8G 

 (0.1 )ueinstein per minute per vessel) and 0.69 (3 jueinsteins per minute per 

 vessel) . It was stated that by using crystallized pheophorbide (instead of 

 chlorophyllide) and a large volume actinometric vessel (80 ml. instead of 7 

 ml. liquid), a constant quantum yield of 0.70 could be obtained for light 

 absorptions up to 4 jueinsteins per minute. 



Still more recently, Warbiu'g and co-workers (1953) again assumed a 

 quantum yield of 1.0 for a pheophorbide-thiourea actinometer of 120 ml., 

 up to a light flux of 1 /ieinstein/min. 



Schenck (1953) found that an intermediate in the oxidation of thiourea 

 in the actinometer — formed to an extent up to 81% of the oxidized thio- 

 urea— is the sulfinic acid NH=C(HS02)— NH2. 



Among new qualitative data of the photochemical action of chlorophyll we can 

 mention Pepkowitz's (1943) observations of the photochemical destruction of carotene 

 in the presence of chlorophyll. Because of the dependence of the rate of destruction on 

 the amount of chlorophyll present, it is suggested that chlorophyll takes part in the re- 

 action, and not merely sensitizes it. 



B. Photochemistry of Chloroplast Preparations* 



(Addendum to Chapter 4, Part A) 



In chapter 4, Volume I (page 61) we have described in brief the produc- 

 tion of oxygen by leaf macerates and dry leaf powders in light, first noted 



* Bibliography, page 1G27. 



