492 PHOTOCHEMISTRY OF PIGMENTS IN VITRO CHAP. 18 



This discussion was not intended to show that a reversible photo- 

 chemical reaction with the solvent provides a better explanation of the 

 sensitization by chlorophyll and of its bleaching than does a reversible 

 tautomerization; but merely to suggest that neither explanation can as 

 yet be accepted as final. 



Photochemical reactions between chlorophyll and solvent were made plausible also 

 by experiments of Knorr and Albers on changes in chlorophyll fluorescence with time 

 (described on pages 497 and 501). These investigators found that photodecomposi- 

 tion occurs, in certain chlorophyll solutions, even in an atmosphere of pure nitrogen or 

 carbon dioxide, and that in one case (chlorophyll a in acetone) it is even inhibited by 

 oxygen. This reminds one of the oxygen inhibition of reversible bleaching, and can be 

 explained by the assumption that the primary reaction in hght is a reverjjible reaction 

 with the solvent (acetone), whose reversal is catalyzed by oxygen; in the absence of 

 oxygen, the primary product hves long enough to suffer an irreversible decomposition 

 by violet or ultraviolet light (cf. page 465). 



Another point in need of elucidation is the possible existence of a 

 reversible chlorophyll-oxygen complex. In all the above equations, the 

 effect of oxygen was attributed to encounters between Chi*, tChl, rChl 

 or rS molecules and free oxygen, and the effectiveness of small oxygen 

 concentrations could therefore be taken as a sign of the existence of a 

 long-lived activated state of chlorophyll. However, this efficiency could 

 also be explained by a reversible association of chlorophyll with oxygen, 

 in a complex which is saturated at very low partial pressures of the latter. 

 We remember {cf. page 465) that " allomerization " (which prevents the 

 discoloration of chlorophyll in the phase test) was attributed by Conant 

 and Fischer to the uptake of one molecule of oxygen. 



However, several arguments speak against a similar explanation of 

 the effect of oxygen on reversible bleaching. In the first place, reversible 

 bleaching can be observed even with completely allomerized chlorophyll 

 — which, according to the concept of Conant, Stoll, and Fischer, is 

 already "saturated" with oxygen. In the second place, if chlorophyll 

 molecules were associated with oxygen even at [O2] = 10~® mole per 

 liter, it would be difficult to explain why oxygen pressures of the order 

 of one atmosphere are required to bring about the quenching of chloro- 

 phyll fluorescence. 



Nevertheless, the similarity between the reversible bleaching of 

 chlorophyll in oxygen-free methanol, the reversible discoloration of 

 chlorophyll solutions by ferric salts (in the dark and in light), and the 

 first, reversible stage of the phase test, should not be dismissed as acci- 

 dental. The analogy between the phase test and the thermal reaction 

 with ferric chloride has already been discussed in chapter 16 (page 465) ; 

 a complete theory should include the photochemical effects as well. It 

 could perhaps be attempted along the following lines; 



