LIGHT ABSORPTION 



78 



chromogenic nucleus by aliphatic chains, and anion formation exerts 

 therefore only a small influence on light absorption in the visible 

 pa: t of the spectrum (cf. Tables VI and VII). The absorption bands 

 are, however, somewhat altered in position and are less sharp. Par- 



es 

 o 

 1^ 



600 500 



WAVELENGTH, m/x 



400 



Fig. 14. Absorption spectrum of coproporphyrin tetramethyl ester in 0.15 A' 

 hydrochloric acid (after Jope and O'Brien, H26). 



ticularly the band in the ultraviolet is shifted 20-30 lUfx toward 

 shorter wavelengths and is greatly decreased in strength {1310,11^26). 

 These phenomena are interpreted as being due to an aggregation or 

 polymerization of porphyrin molecules and are also observed with 

 porphyrin metal compounds such as hematin. They will therefore 

 be discussed further in Chapters V and VI. 



In contradistinction to anion formation, the formation of porphyrin 

 cations occurring on the nitrogen atoms of the porphin system has 

 a more profound influence on the absorption curve. In the spectro- 

 scope the "acid porphyrin spectrum" shows only two distinct absorp- 

 tion bands with a very weak third band between them. 



