SPECTRUM AND STRUCTURE OF PORPHIN DERIVATIVES 623 



prolochlorophyll, a green compound which, according to H. Fischer {cf. 

 Vol. I, page 445), nevertheless is a prophyrin rather than a chlorin or phor- 

 bin. The spectrum of protochlorophyll (cf. fig. 21.8 and Table 21. IV) 

 does not show the predominance of the red band over the bands in yellow 

 and green to the same extent as do the typical spectra of dehydro- or tetra- 

 hydroporphin derivatives; but it resembles these spectra somewhat more 

 than it does the typical porphyrin spectra in figure 21.10. A re-examina- 

 tion of the structure of the protochlorophyll molecule is therefore desirable. 

 It is noteworthy that "protopheophytin," obtained from protochlorophyll 

 by the action of acids, was found to have a typical porphyrin spectrum. 



A similar case is presented by chlorophyll c. As stated on p. 014, 

 this pigment has an arrangement of bands similar to those of the chloro- 

 phylls a and b, but the red ))and is very weak compared to the Soret band ; 

 in fact, the spectra of chlorophyll c (insert in fig. 21. 5C) and protochloro- 

 phyll (fig. 21.8) are extremely similar. It is therefore significant that 

 Granick concluded, from chemical evidence, that chlorophyll c, too, is a 

 porphin rather than a chlorin derivative (cf. chapter 37). 



Another interesting problem of the same character was raised by an 

 investigation of Aronoff and Calvin (1943). They prepared (by condensa- 

 tion of benzaldehyde with pyrrole) several compounds that tliey interpreted 

 as isomeric hexaphenylporphins. Some of these compounds had porphyrin 

 spectra of the "etio type" (fig. 21.10, 1), but others had spectra with a pre- 

 dominant sharp band in the red (fig. 21.13, curve B). The authors 

 thought at first that these green isomers may contain one pyrrole nucleus 

 turned around, placing its N atom on an outside corner. 



Rabinowitch (1944) suggested they could perhaps be interpreted as 

 chlorins: Chlorins isomeric with hexaphenylporphin could be formed, e. g., 

 by attachment of one phenyl group to a pyrrole nucleus, and shifting of the 

 two liberated hydrogen atoms to another pyrrole nucleus, thus: 



Tetrophenylporphin Tetrophenylchlorin 



