CAROTENOIDS AND PHYCOBILINS 521 



complex, ChlA, can be expected to have an intrinsic capacity for fluores- 

 cence different from that of free chlorophyll; but there is no reason for 

 the intensity of its fluorescence to change with an addition of excess A. 

 The hypothesis of complex formation with the acceptor includes the 

 two possibilities, Ba2 and B^2, one based on energy transfer within the 

 complex, and one assuming a chemical reaction between the two compo- 

 nents of the complex. Franck and Wood (1936) considered the first 

 possibility, with the specific suggestion that the transfer of the excitation 

 energy from Chi to A leads to the dissociation (dehydrogenation) of A. 

 This mechanism is similar to that suggested earlier by Franck and Levi 

 (1934) for kinetic encounters, except that now a residue of the dissociated 

 acceptor remains attached to the sensitizer, and in this way some of the 

 dissociation energy may be compensated for by the affinity between the 

 sensitizer and this residue, for example (instead of 18.36) : 



Ught 



(18.43) ChlAH > ChIA + H 



In (18.43), the acceptor is designated by AH and A is a radical whose 

 affinity for chlorophyll is likely to be stronger than that of the saturated 

 molecule, AH. In a complex, energy transfer and chemical reaction are 

 very closely related phenomena. Equation (18.43), for example, en- 

 visages a chemical change not only in the "acceptor part," but also in 

 the "sensitizer part" of the complex. One can make one more step and 

 assume a true intermolecular oxidation-reduction reaction within the 

 complex, as: 



light 



(18.44) {ChlA} ). {oChlrA} (or {rChloAj) 



Mechanisms of this type are of particular interest in connection with the 

 problem of photosynthesis, since there, the reaction substrates have often 

 been assumed to be permanently associated with chlorophyll. We shall 

 return to this problem in chapter 19. 



D. Photochemical Properties of the Carotenoids 



AND PhYCOBILINS* 



Not much is known about the photochemical decomposition of the 

 carotenoids, although they are described as "light sensitive" (c/. Zech- 

 meister 1934), One natural process of great importance is closely related 

 to the photodecomposition of carotenoids: the bleaching of visual purple, 

 which is the basis of vision in the retinal rods. Visual purple is a protein 

 complex containing a derivative of vitamin A as its prosthetic group 

 (of. Wald 1942); the relationship between vitamin A and carotene was 

 mentioned on page 471. 



* Bibliography, page 525. 



