1778 



CHEMISTRY OF CHLOROPLAST PIGMENTS 



CHAP. 37b 



3000 



4000 5000 



WAVE LENGTH A 



6000 



7000 



Fig. 37B.5. Absorption spectra of the "brown phase." Chlorophyll a in 10% mono- 



isopropyl amine, 10% isopropyl benzene and 80% 1:1 propane-pro pene: ( ) 190° 



K., ( ) 170° K., ( ) 160° K. (after Freed and Sancier 1953). Curve (...) has 



been derived by Dunicz el al. (1951) for the transient intermediate of chlorophyll a at 

 300° K 



cized). Weller suggested that the brown intermediate is a triplet (diradi- 

 cal) form of the anion formed by enolization and acid dissociation in posi- 

 tion 9. This diradical is stabilized by resonance between several struc- 

 tures, with free valencies in different positions in the — normally conjugated 

 — ring system. The striking similarity of the absorption spectra of the 

 phase test intermediate, the anaerobically bleached chlorophyll (in steady 

 or flashing light), the reversibly reduced chlorophyll and the reversibly 

 oxidized chlorophyll (in rigid solvent) can be understood if all these prod- 

 ucts have a similar radical or biradical structure, with interrupted all-round 

 conjugation. 



