4 li. LIVINGSTON 



ISOMERS, TAUTOMERS, AND ADDITION COMPOUNDS OF 



CHLOROPHYLL 



Freed and Saucier (1) have shown that a tautomeric form of chloro- 

 phyll is stable at low temperatures, and they have studied, at inter- 

 mediate temperatures, the tautomeric equilibria. This tautomer- 

 ism is fast even at 150°K. The absorption maxima of the low-tem- 

 perature forms are displaced to the red by about 300 A. The tem- 

 perature at which the low- and high-temperature forms are present 

 in comparable amounts is a sensitive function of the isomeric form 

 of the chlorophyll. For example, the spectra of chlorophylls b and h' 

 are almost identical at room temperature. Reducing the temperature 

 to 75°K. changes both alike, so that the spectra of their low-tem- 

 perature forms are also identical. Plowever, the temperatures at which 

 the tautomers coexist in equal amounts are 180° and 230°K., respec- 

 tively, for chlorophylls h and b'. (More recent work by the same 

 authors (./. Am. Chem.. Soc, 76, 198, 6006 (1954)) indicates that the 

 species that are stable at low temperatures are probably solvates 

 rather than tautomers of the unsolvated molecules.) 



Chlorophylls a and b form addition compounds with water, alcohols, 

 amines, and other "bases." Similar adducts are formed by bases with 

 metal-complexed porphyrins and chlorins, but not with pheophytins 

 or metal-free porphyrins (2). These facts support Krasnovskii's con- 

 tention (3) that the point of addition is the metal atom of the pigment. 

 Unlike the other pigments, the fluorescent yield of chlorophyll is 

 strongly affected by the addition of the base. Solutions of chloro- 

 phyll in pure dry hydrocarbons, or similar solvents, are practically 

 nonfluorescent. The stability constants of compounds of chloro- 

 phylls, porphyrins, and chlorins with a given base do not differ by a 

 factor of more than seven. As measured by these stability constants 

 the oxygen bases are abnormally strong relative to nitrogen bases (4). 



At — 80°C. a solution of chlorophyll in isopropylamine is reddish 

 brown, but when the solution is warmed to ordinary temperatures 

 it becomes green and exhibits its normal spectrum (5). Except for a 

 slow side reaction at the higher temperatures, the process is strictly 

 reversible. The absorption spectrum of the low-temperature solution 

 is identical with that of the Molisch phase test intermediate (6). This 

 spectrum is similar to, but not the same as, the spectrum of chloro- 

 phyll in its triplet state, as formed by flash illumination (7). Since 

 there is good reason for the belief that the phase test intermediate 



