502 PHOTOCHEMISTRY OF PIGMENTS IN VITRO CHAP. 18 



or on a primary interaction of chlorophyll with oxygen: 



(18.27a) Chi* + O, > oChl (compare 18.21) 



(18.27b) oChl + A > oA + Chi 



(18.27) A + O2 > oA 



In mechanism (18.24-18.26), the acceptor prevents chlorophyll from react- 

 ing with oxygen. In mechanism (18.27), the oxidation of chlorophyll 

 actually takes place, but is reversed by a reaction with the acceptor. 



Whatever mechanism we assume for the inhibition of chlorophyll bleaching by 

 autoxidizable compounds, this inhibition obviously contradicts scheme (18.22), according 

 to which the oxidative bleaching of chlorophyll, instead of being inhibited by autoxidiz- 

 able substrates, was supposed to occur only in their presence. 



Not all known cases in which chlorophyll is stabilized in respect to 

 molecular oxygen can be explained by diversionary or catalytic effects. 

 The same result can apparently be achieved also by association of the 

 pigment with certain substances which make it photostable without 

 themselves suffering a permanent or temporary sensitized oxidation. 

 This truly protective action can be explained, for example, by an acceler- 

 ated dissipation of the excitation energy in the pigment-protector 

 complex. If this dissipation competes with fluorescence, the ' ' protected " 

 pigment will be nonfiuorescent; but since we have assumed that photo- 

 chemical transformations often are preceded by tautomerization, dissi- 

 pation may compete only with the latter process and leave fluorescence 

 unaffected. 



The action of protective colloids, investigated by Wurmser (1921), 

 probably is of this type. The bleaching velocity could be reduced by 

 50% by as Httle as 0.05% of gelatin or casein; 0.86% albumen or 1.45% 

 gum arabic were required to produce the same effect, whereas starch 

 had no appreciable influence even in a concentration of 2%. Wurmser 

 noticed a parallelism between the stabilizing action of a colloid on gold 

 colloids and its efficiency in protecting chlorophyll from oxidation in light. 



The protective action of proteins may fall into the same category. 

 It was observed by Noack (1927) in artificial protein-chlorophyll com- 

 plexes, and by Lubimenko (1927) and Smith (1941) in colloidal protein- 

 chlorophyll leaf extracts. Katz and Wassink (1939) noted the consider- 

 able stability to light and oxygen of colloidal extracts from purple 

 bacteria, as contrasted with the extreme sensitivity of molecularly 

 dispersed bacteriochlorophyll. 



The fact that artificial chlorophyll-protein complexes are nonfiuores- 

 cent indicates that association of chlorophyll with these compounds fur- 

 ther shortens the lifetime of the short-lived fluorescent state; the high 

 partial pressure of oxygen required for photoxidations sensitized by 



