OXIDATION, ALLOMERIZATION AND REDUCTION OF CHLOROPHYLL 1779 



The brown intermediate of the "phase test" was studied by Freed and 

 and Sancier (1953) by low-temperature spectroscopy. They found that if 

 chlorophyll a (or b, or h') is dissolved at low temperature in a base (isopro- 

 pylamine), a brown (or red) solution is formed, which becomes green upon 

 warming and again brown (or red) upon (immediate) cooling. The reac- 

 tion becomes irreversible if the solution in pure amine is allowed to stand 

 after warming; but if the amine is diluted (e. g., 10% amine in 45% pro- 

 pane -I- 45% propene; or 10% amine in 40% propane + 40% propene + 

 10% isopropyl benzene), the reversibility is preserved. The absorption 

 spectrum of the brown (or red) solution is very similar to that given by 

 Dunicz et al. (1951) for the "brown phase" (Fig. 37B.5). The green color 

 of the solution in amine-containing solvent can be "frozen in" by sudden 

 cooling from room temperature to —190° C; upon warming up, the 

 "brown phase" reappears in a certain interval of temperatures. It is thus 

 confirmed that the brown product exists in equilibrium mth green chloro- 

 phyll, and that its formation (by enolization?) requires an activation en- 

 ergy. The suggested identification of the brown (or red) low-temperature 

 products of the reaction ^\^th amines, with the similarly-colored intermedi- 

 ates of the phase test, is supported by the observation that no such products 

 are obtained with allomerized chlorophyll. No red compound could be ob- 

 tained from chlorophyll b in the presence of diisopropylamine, indicating 

 that basic reaction alone is not sufficient for its formation. 



The reversible chemical reduction of chlorophyll to a leuco compound 

 by zinc and organic acid, claimed by Timiriazev, and Kuhn and Winter- 

 stein, but found to result in irreversible changes by Albers, Knorr, and 

 Rothemund {cj. Vol. I, p. 457), was again studied by Kosobutskaya and 

 Krasnovsky (1950). They used chlorophyll (o + b, a, b), pheophytin, 

 the complexes of pheophytin with zinc and copper, and magnesium phthalo- 

 cyanide. The addition, to 3 ml. of 10^^ M pigment solution in pyridine, 

 of 0.3 g. zinc powder in 0.0() ml. glacial acetic acid, under vacuum, caused 

 brownish discoloration of all pigments; phthalocyanin became entirely 

 colorless. Re-admission of air restored the green color in every case; 

 however, only with two compounds — Mg phthalocyanin and Zn pheo- 

 phytin — were both the red and the blue-violet peak restored exactly to their 

 original positions. In compounds of the a-series, the bands of the final 

 product lay at 661 and 431 m^— the positions they have in Zn pheophytin. 

 With chlorophyll b, the bands were at 656 and 470 m^u before, and at 644 

 and 459 mn after the "cycle." When very concentrated solutions of 

 chlorophyll b were used, a weak band in the region 510 530 m/x was 

 noticeable in the reduced state (cf. Chapter 35 for the presence of this 

 band in photochemicaUy reduced chlorophyll). 



With the nonfluorescent Cu pheophytin, the "cycle" leads to the an- 



