FLUORESCENCE AND SENSITIZATION YIELDS 547 



the oxidation yield, 7, will be practically independent of the partial 

 pressure of oxygen: 



At the low [Chi] values, kc^kt and since ki<k,y must approach 1 (still 

 assuming that A is present in excess). These are the conditions encoun- 

 tered in the study of the chlorophyll-sensitized autoxidation of amines 

 in vitro {cf. pages 509, 513 and 518-520). 



In vivo, on the other hand, we may postulate the sequence of reactions 

 represented in scheme 19. III. 



{xCh(*HZ} 



{xChl HZ) 



{x Chiz} ♦JiHaO 



4^A 



{xChlHZ} f oA 



Scheme 19.III. — Fluorescence and 

 sensitized autoxidation by chlorophyll 

 in the hving cell. 



Scheme 19. IIP gives, for the quantum yield of fluorescence: 



(19-S) ^-" = k, + l[ + k, 



— independent of [O2] — and for the yield of sensitization: 



^^^•^^ ^"" ~ kt + k,' A:t[02] + ki 



—proportional to [O2], if K » k'olOi']. Equation (19.8) is based on 

 the assumption that the intermediate forms, {HXChlZ} and {XChlZ}, 

 do not accumulate, during the illumination, in quantities commensurate 

 with those of the basic form, {XChlHZ}, and thus do not participate in 

 light absorption. If this condition is not fulfilled, the contribution of 

 the intermediate forms to fluorescence must be taken into consideration 

 (as will be done in volume II, chapter 24). This contribution can be 

 considerable because the intermediate forms are not subject to "chemi- 

 cal" quenching by reaction (19.4b). This explains, among other things, 

 why oxygen sometimes enhances instead of quenching the fluorescence 

 of chlorophyll in vivo — it transforms the short-lived intermediate, 

 * Self-quenching by internal conversion (rate ki) not shown in scheme 19. III. 



