CHLOROPHYLL FORMATION 401 



gen atoms situated in positions 7 and 8 of ring IV. However, on theo- 

 retical grounds this structure has been called in question by Rabinowitch 

 (1951, p. 623). Perhaps the same possibility exists for protochlorophyll 

 which exists for chlorophyll, that the conjugated double bonds may not 

 have fixed positions (cf. Smith, 1937, p. 500; StoU and Wiedemann, 1938, 

 p. 211). 



Although the phytyl group has not been identified directly in proto- 

 chlorophyll, indirect evidence points to its presence. The magnesium 

 content of protochlorophyll from leaves, 2.73 per cent (Koski and Smith, 

 1948), agrees with that calculated from the phytyl-containing structural 

 formula for protochlorophyll proposed by Fischer and Oestreicher 

 (1939-1940). 



The distribution of protochlorophyll and its derivatives between aque- 

 ous hydrochloric acid and ether [the hydrochloric acid number; see Fischer 

 and Orth (1943, p. 601)] has been used to indicate the presence or absence 

 of the phytyl group. By this test, Noack and Kiessling (1929, p. 16) 

 found pumpkin-seed-coat protochlorophyll to color a 12 per cent hydro- 

 chloric acid solution, whereas chlorophyll a did not color a 22 per cent 

 solution. This low value for protochlorophyll suggested the absence of 

 the phytyl group. But Granick (1950) examined protochlorophyll pre- 

 pared from etiolated barley leaves by the procedure of Koski and Smith 

 (1948) and found a hydrochloric acid number of -^ 25. For protochlo- 

 rophyll minus the phytyl group the value was '^ 11. Thus the pres- 

 ence of a phytyl group in undamaged protochlorophyll is indicated. 



Attempts to show the presence of the phytyl group in protochlorophyll 

 by means of the action of chlorophyllase have not been successful (Noack 

 and Kiessling, 1929; Mayer, 1930). That chlorophyllase may act on 

 protochlorophyll, however, is strongly indicated by the observations of 

 Granick (1950). 



Krasnovskii and Voinovskaya (1949)' claim that protochlorophyll is 

 reduced photochemically in pyridine solution by ascorbic acid and that 

 it can act as a photosensitizing agent for the transfer of hydrogen from 

 ascorbic acid to saffranin T. 



3-2. PHYSICAL PROPERTIES OF PROTOCHLOROPHYLL 



Spectral Absorption of Protochlorophyll. Of all the characteristic physi- 

 cal properties possessed by protochlorophyll, its spectral absorption is 

 perhaps the most useful. It has proved to be invaluable for identifying 

 this pigment in various extracts, for determining it quantitatively, and 

 for identifying it as the active light-absorbing agent for its own trans- 

 formation to chlorophyll. 



The absorption bands of protochlorophyll were first measured in 



2 The authors are indebted to Harold W. Milner for his translation of this article 

 from the Russian. 



