64 



III. PORPHYRIN CHEMISTRY 



IdentiUcation of coproporphyrin isomers. For the tetraraethyl ester of 

 coproporphyrin I melting points of 248-258° C. are given in the literature. 

 The tetraraethyl ester of coproporphyrin III melts remarkably lower and 



(III) 



Fig. 9. Coproporphyrins I and III. 



its identification is complicated by the fact that it has a "double melting 

 point," i.e., it coalesces at least partially at temperatures variously reported 

 as 130-153°, resolidifies, and melts again at 150-179° C. Jope and O'Brien 

 have recently studied the melting points of the esters carefully and give a 

 melting points composition curve {lJf2G) (Fig. 10). 



From Figure 10 it is evident that melting point determinations do not 

 permit one to detect less than 10% coproporphyrin III in coproporphyrin I 

 unless the resolidification point is also taken, and likewise allow up to 15% 

 coproporphyrin I in coproporphyrin III to escape detection. The melt- 

 ing points of the copper complex salts of the esters {1914; 2896) may give more 

 reliable results — for I, 28-1°, and for III, 206°; the melting point given by 

 Fischer {861, p. 491) is too low. It is also noteworthy that the melting point 

 of synthetic coproporphyrin III does not tally entirely with that of copro- 

 porphyrin III ester from natural material. The synthetic ester evidently 

 shows the primary melting point less well, but melts at a temperature well 

 below that of the second melting point of the natural ester (158-162° as com- 

 pared with 173-179°) {U26; cf. also 801,827,862). 



The identification of a porphyrin as coproporphyrin III merely on the 

 basis of the melting point of its methyl ester is therefore quite inadequate. 

 It is possible, e.g., that admixture of other porphyrins to coproporphyrin I 

 may considerably depress the melting point of its ester and thus lead to a 

 wrong identification with coproporphyrin III. Whenever possible micro- 

 analyses should be carried out, the uniformity of the porphyrin ester should 

 be demonstrated by chromatography, and melting points of its metal com- 

 plex salts should be studied. 



The separation of the coproporphyrin I and III tetraraethyl esters is 

 usually effected by recrystallization frora methyl alcohol in which the type I 

 ester is much less soluble. From this one would expect the type III ester 

 remaining in the mother liquors to contain a little type I ester; ether has 

 also been used for this separation (2271). A separation by chromatography 

 on alumina with elution of the type III ester by 35% aqueous acetone has 

 been claimed {2999), but could not be confirmed {11^26). The esters can be 



