546 



PHOTOCHEMISTRY OF PIGMENTS IN VIVO 



CHAP. 19 



(19.4a) 



(19.4b) 



Chi* 



Chi* 



^<^Chl 



HZ 



X J 



z 



+ h.p or 



*<Chl 



Chi*- 

 Chi* 



+0 





oChl 



+A 



oA + Chl 



V 



Chi 



tChl- 



^C 



X J 1. Hx; 



The state on the right side of equation (19.4b) may be the "long-Hved 

 activated state" in vivo. This state is formed, in the Uving cell, more 

 rapidly than the corresponding state in vitro (as witnessed by the ten 

 times lower yield of fluorescence) ; but it is also more rapidly destroyed 

 by back reactions (as witnessed by the much higher concentration of 



oxygen required for efficient photoxidation in 

 vivo as compared with solutions; cf. page 530). 

 Both these differences can be interpreted as 

 consequences of the association of the pigment 

 in vivo with the photosensitive components be- 

 fore the primary process and with the products 

 of their transformation after this process. They 

 can thus be quoted as arguments in favor of the 

 association hypothesis. 



We may use schemes 19.11 and 19. Ill to for- 

 mulate more precisely the influence of oxygen on 

 fluorescence and sensitization by chlorophyll 

 in vitro and in vivo. 



Scheme 19.11 represents the probable condi- 

 tions in vitro (/ct in this scheme may refer to 

 tautomerization or to a reversible reaction with the solvent). The quan- 

 tum yield of fluorescence, according to scheme 19.11, is (cf. Rabinowitch 

 1941): 



hi kt 



(19.5) 



+0, 



oChl 



+A 



oA + Chi 



Scheme 19.11.— Fluo- 

 rescence and sensitized 

 autoxidation in chloro- 

 phyll solutions. 



<Ptso\. 



kf + (fci + /CcLChl] + kt) + /cJEOj] kt + k+ A:J[02] 

 where ki is the rate of fluorescence and h -f A:c[Chl] that of self-quenching 

 (not shown in scheme 19.11). (The two terms correspond to internal 

 energy dissipation in the solvated chlorophyU molecule and to collisions 

 with other chlorophyll molecules; cf. Weiss and Weil-Malherbe 1944). 



The quantum yield of oxidation of Chi to oChl (which in presence of a 

 "saturating" quantity of acceptor may be considered as the rate-de- 

 termining step in sensitized autoxidation) is: 



T.ol. = 



kf + k + k*l022 



+ 



kllO,:\ + ki ' k! + k + k*l022 

 If, at [O2] = 10-1000 mm., k^lOz] is > A;, the fluorescence yield, 

 (p, must depend on oxygen pressure in this range; and if ko[02] is ^ k^, 



