PORPHYREXIDE AND PORPHYRINDIN 665 



while for porphyrexide only the free radical form is possible: 



H3C 6 



I U 

 H,C— C— N 



I *\ 



C = NH 



HN=C— N 

 H 



Porphyrexide 



The diamagnetic form is more stable than the paramagnetic by about 0.56 

 kcal/mole. Magnetic studies have indicated the free electrons to be fairly 

 well localized and not diffusely distributed over the molecule. Possibly these 

 free electrons contribute to the color of these substances: porphyrexide is 

 red and porphyrindin a deep blue. Upon reduction the color disappears 

 (leucoporphyrindin may be slightly yellow), this being the basis for the col- 

 orimetric titration of SH groups. Porph>Texide has an absorption maximum 

 at 460 m// and porphyrindin at 653 m// (Kuhn and Franke, 1935). 

 Reduction involves the change from 



O- O- OH 



I I I 



— N+= or — N+— to — N — 



and a disappearance of free radicals. The oxidation-reduction potentials at 

 pH 7 and 18° are: 



Porphyrexide: £"„' = + 0.725 v 

 Porphyrindin: E^' = + 0.565 v 



SO that porphyrexide approaches the oxygen potential and both are well 

 above most systems commonly seen in biological work. The oxidation of 

 simple thiols is very rapid, cysteine and glutathione reacting almost instan- 

 taneously. 



Porphyrindin is not very stable and the solid crystalline dye should be 

 stored at low temperatures and desiccated. Greenstein (1938) pointed out 

 that porphyrindin solutions are stable enough for an hour but the activity 

 then decreases. Brand and Kassell (1940) reported that solutions at 0^ show 

 3% deterioration in 1 hr, 5% in 2 hr, and 9% in 4 hr. At 25^ deterioration 

 occurs at a rate of about 0.5% per minute. Reactions of enzyme SH groups 

 can usually be carried out at 0°, but in work with tissues at physiological 

 temperatures this spontaneous decomposition must be borne in mind. Por- 

 phyrindin should be quantitatively determined in all accurate work since 

 it is seldom pure; this may be done by titrating with asorbic acid (Chinard 

 and Hellerman, 1954). 



