1526 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



1927, 1933, cf. Table 18.11). This reaction appeared to have a hmiting 

 quantum yield of about 1, reached when the concentration of the oxidation 

 substrate was sufficiently high and that of the sensitizer sufficiently low. 

 Since one of the reagents— oxygen — is present only in small concentration in 

 the liquid phase, shaking of the reaction vessel is needed to avoid yield 

 deficiency due to local exhaustion of oxygen. 



The autoxidation of thiourea (substituted for allyl thiourea since it 

 shows a less pronounced dark reaction) with ethyl chlorophyllide as sensi- 

 tizer was reinvestigated by Warburg and Schocken (1949), with a view on 

 using this reaction in a manometric actinometer. Several solvents were 

 tried out. Pyridine was found to be the most satisfactory, acetone (used 

 in Gaffron's work) being too volatile, and dioxane producing complica- 

 tions (autoxidation quantum yields >1, with efficiency increased by traces 

 of copper, and decreased by cyanide) conceivably caused by peroxide forma- 

 tion. Attempts to substitute other reductants, such as hydroquinone, 

 for thiourea, gave no positive results. In addition to ethyl chlorophyllide, 

 protoporphyrin could be used as sensitizer. The green pigment is suitable 

 for the work in red and blue, the red one for the work in green and blue; 

 mixed together, they give almost complete absorption throughout the 

 visible spectrum. 



A vessel 14 ml. in volume was used, containing 200 mg. thiourea and 2 

 mg. crystalline ethyl chlorophyllide (or 10 mg. crystalline protoporphyrin) 

 in 5 cc. pyridine. It was illuminated for periods of the order of 15-30 min. 

 through a flat bottom (area, 9 sq. cm.) with light from a large monochroma- 

 tor (beam cross section, 2 sq. cm.). The vessel communicated with a ma- 

 nometer filled with pure oxygen, and was shaken during illumination. 

 Quantum yields of oxygen consumption obtained in this way are tabulated 

 in Table 35.11. 



Table 35.11 



Quantum Yields of Oxygen Consumption in Sensitized Autoxidation of Thiourea 



(after Warburg and Schocken 1949) 



Absorption, O2, 7. molecules O2 

 X, m/x microeinsteins micro moles per quantum 



