LIGHT ABSORPTION BY PIGMENTS 171 VIVO 



1859 



In stronger light, another spectral change becomes superimposed on 

 the one represented in fig. 37C.37, probably correlated with the changes in 

 the infrared represented in fig. 37C.35. While the latter reveals trans- 

 formations in the pigment complex, the difference spectrum in fig. 37C.37 

 clearly indicates— as shown by the dashed line— the oxidation of a cyto- 

 chrome-type pigment. A cytochrome similar to cytochrome c had been 

 extracted from Rhodo spirillum ruhrum by Vernon (1953) and Elsden, 

 Kamen and Vernon (1953). 



Duysens (1954^) made similar studies with Chlorella. No significant 

 effect could be noted in the long-wave region of chlorophyll absorption; 

 characteristic changes were observed, however, in the violet, blue, and green 



SSO 



500 



iSO 



Fig. 37C.38. Changes in absorption spectrum in illu- 

 minated Chlorella (after Duysens 19542). 



(cf. fig. 37C.38); the peak at 420 m/x was tentatively attributed to the 

 oxidation, in light, of cytochrome /—a compound found in chloroplast 

 material by Hill and co-workers (cf. chapter 35, section B4(/)). The two 

 other peaks — a "negative" one at 475 m/x and a "positive" one at 515 

 mix — could not be associated with any known pigment. These two peaks 

 were found also in an illuminated leaf, an algal thallus, and a grass blade. 

 (No measurements were made on these objects at 420 m/x.) 



The temptation is great to associate the 515 m/x band with bands in 

 sunilar positions, which appear in reversibly reduced and reversibly oxi- 

 dized chlorophyll in vitro, as well as in chlorophyll transferred, by intense 

 illumination, into the metastable triplet state. We commented elsewhere 

 in this chapter on the spectroscopic similarity of these products, and quoted 

 Weller's suggestion that this similarity indicates that all of them are radi- 

 cals (or biradicals) with interrupted all-round conjugation in the porphin 

 system. There is, however, one difficulty in the way of attributing the 

 515 m/u band in illuminated Chlorella cells to chlorophyll-derived radicals: 



