522 PHOTOCHEMISTRY OF PIGMENTS IN VITRO CHAP. 18 



The elementary photochemical act of vision consists in the conversion 

 of visual purple into a mixture of protein and the yellow carotenoid, 

 retinene, and later into the colorless vitamin A. In a dark process, the 

 visual purple is restored from these decomposition products, by direct 

 reversal of the photochemical reaction in the case of retinene, and by a 

 more complicated circular process in the case of vitamin A, according to 

 the scheme: 



Ught . 



visual purple <. ^ retinene + protein > vitamin A + protein 



t 1 



This cycle proves that carotenoids are capable of reversible photochemical 

 reactions — a property which may serve them in good stead in their 

 participation in photosynthesis. This participation, which has long been 

 denied, was recently confirmed by comparative experiments on the yield 

 of photosynthesis in the light absorbed by chlorophyll alone, and in the 

 light absorbed by both chlorophyll and the carotenoids (c/. Vol. II, 

 Chapter 30). 



However, it is by no means certain that the participation of the 

 carotenoids in the sensitization of photosynthesis is based on a reversible 

 chemical reaction. The carotenoid-sensitized fluorescence of chlorophyll 

 in vivo (cf. Vol. II, Chapter 24) shows that the electronic excitation 

 energy of the carotenoids can be transferred to chlorophyll. (This 

 phenomenon was quoted on page 515 as proof that the "bulk" transfer 

 of electronic excitation energy is not improbable between two molecules 

 with overlapping absorption bands.) It is feasible— indeed probable — 

 that carotenoid-sensitized photosynthesis also is initiated by such a trans- 

 fer of electronic excitation energy to chlorophyll. 



Almost nothing is known about the sensitizing effect of carotenoids 

 in vitro. Perhaps the belief in the rule that nonfiuorescent dyes do not 

 sensitize has prevented many investigators from even attempting to use 

 these dyestuffs in sensitization experiments. In the one case, when 

 carotene was tested for its sensitizing action, the result was positive: 

 Karrer and Strauss (1938) found that colloidal carotene solutions sensitize 

 the autoxidation of benzidine. Addition of gelatin or d,Z-alanine en- 

 hanced the effect. 



The phycohilins are described as "very sensitive to light and oxygen" 

 (see, for instance, Schiitt 1888). According to Lemberg (1930), the 

 protein-free pigments are even less stable than the chromoproteids. 

 (Free phycoerythrobilin is quickly oxidized to phycocyanobilin by 

 oxygen.) Nothing else is known about the photochemistry of these 

 compounds or their sensitizing efficiency. Experiments indicate, how- 

 ever, that they can act as sensitizers in photosynthesis (cf. Vol. II, 

 Chapter 30). 



