474 THE ACCESSORY PIGMENTS CHAP. 17 



hydrogen can be added catalytically to the 11 double bonds of carotene 

 or luteol. Usually, hydrogenation of double bonds by molecular hydro- 

 gen does not take place spontaneously in the absence of a catalyst, but 

 noncatalytical hydrogenation has actually been observed in colloidal 

 solutions of certain carotenoids. 



Formally, the relation between carotene and the carotenols is that of 

 an oxidation-reduction pair. Willstatter and Stoll (1913) were struck by 

 the occurrence of leaf pigments in pairs (chlorophyll a and chlorophyll b; 

 carotene and xanthophyll) . The second compound is in both cases an 

 oxidation product of the first one, so at least it appears on paper. One 

 could thus suggest that the two pairs form an oxidation-reduction system, 

 for example, that illuminated chlorophyll a, converted into chlorophyll h 

 by reducing carbon dioxide, can be restored by carotene — the latter being, 

 in its turn, converted into xanthophyll. Hypotheses of this type will be 

 discussed in chapter 19 (page 554). They were abandoned by Willstatter 

 and Stoll (1918) because all attempts to convert xanthophyll into carotene 

 (or vice versa) have remained unsuccessful. Ewart (1918) said that zinc 

 dust reduces luteol in alcohol to carotene; but Strain (1938) was unable 

 to find any carotene in the reduction products of luteol obtained by this 

 or similar treatments. 



Even if we do not consider carotene and xanthophyll as two forms of 

 an oxidation-reduction catalyst, we can still speculate about the role of 

 these substances in photosynthesis. We may, for example, turn our 

 attention to their unsaturated nature and capacity to bind hydrogen, 

 and consider that they might serve as hydrogen transmitters. Another 

 interesting possibility is that the carotenoids may play an active part 

 in the liberation of oxygen. 



It has been known since Arnaud (1889) that carotenoids are easily autoxidizable. 

 Carotene exposed to air for several weeks takes up about 12 oxygen atoms per molecule. 

 The process probably begins with the formation of peroxides, but it does not stop there. 

 When about 11 atoms of oxygen have been taken up, the carbon chain begins to crack, 

 with the liberation of low-molecular volatile compounds. Glyoxal (HOC=COH) has 

 been identified among these products by Pummerer, Rebmann, and Reindel (1931), and 

 carbon dioxide by Escher (1932). From 0.6 to 0.8 mole of carbon dioxide was Hberated 

 in eight weeks by one mole of carotene. The absorption of oxygen by carotenoids is 

 probably caused by the formation of "double-bond pero.xides": 



0—0 



I I 



— C:=C— + O2 > — C— C— 



II II 



These peroxides might be able to transfer oxygen to other acceptors, thus explaining the 

 carotene-catalyzed oxidation of unsaturated compounds. Olcovich and Mattill (1931), 

 Olcott (1934), Monaghan and Schmitt (1932), and Franke (1932) found that traces of 

 carotene accelerate the oxygen absorption by many autoxidizable substances, e. g., 

 linoleic acid. 



