PORPHYRINOGENS 77 



violet light of a wavelength of more than 280 mju) as well as by light 

 absorbed in the visible range {29a). It has been used by many investi- 

 gators for the demonstration of small amounts of porphyrins (322, 

 560,562,564-566,568,1179,1181), even of the infinitesimally small 

 amounts of porphyrin in a single erythrocyte {1505; cf. Chapter XII), 

 and for their estimation {cf. Section 7). 



The fluorescence spectra of porphyrins have been carefully studied by 

 Dhere {57 3, o7 8-580,680) and later by Stern and co-workers {2(}31,:i<;32,2(i37, 

 264-3). As we have pointed out in the case of the absori)tion spectra, the 

 formation of hydrochlorides brings about a more radical change than does 

 the ionization of the carboxyls. The fluorescence spectra behave similarly, 

 the spectra of solutions in mineral acid (Dhere's type II) differing consider- 

 ably from those given by solutions of porphyrins in organic solvents or alkali 

 (Dhere's type I). The latter shows a main emission Ijand which almost 

 coincides with the first absoiption band in the orange of porphyrin solutions 

 in organic solvents, and three further bands toward the infrared. Stern 

 found another weak emission band at wavelengths which coincide with 

 those of the weak absorption band la (at about .)y.5-(j()4 m^u) of the porphy- 

 rins, and further emission bands in the infrared. Dhere's type II fluorescence 

 spectrum, that of the porphyrins in mineral acid also shows an emission band 

 at the position of the first absorption band (about GOO mju) and two to three 

 further bands toward the infrared. The work of Dhere contains excellent 

 photographs of these emission spectra. The influence of the side chains on 

 the position of the emission bands is quite similar to that on the absorption 

 bands {1551). 



The intensity of the fluorescence of porphyrins in aqueous solution 

 shows a minimum at the isoelectric point or zone of the porphyrins 

 {cf. also 2266). By studying the fluorescence-pH curves, even iso- 

 meric porphyrins, e.g., coproporphyrins I and III can be distinguished, 

 but the method requires experience and pure substances. At the acid 

 side of the isoelectric point the fluorescence of coproporphyrins I 

 and III does not differ and has a maximum at p\\ 1-2, but the 

 curves differ between /)H 2 and 6 {1298,1426), evidently due to the 

 electrostatic interaction of two carboxyl groups with one another. 

 Uroporphyrins I and III show similar differences {2910). 



Porphyrins in gelatin phosphoresce when irradiated with visible 

 light of 580-480 mn {132). 



5. PORPHYRINOGENS 



By the addition of six- hydrogen atoms, porphyrins with saturated 

 side chains, e.g., mesoporphyrin, are converted into colorless por- 

 phyrinogens {874). These contain four pyrrole nuclei linked by CH2 



