162 



David L. Drabkin 



Contribution of Porphyrin to the Over-all Spectroscopic Character of 

 Haemin Chromoproteins. In the early days of the investigation of the struc- 

 ture of cytochrome c, Theorell (1939) questioned whether the thio-ether 

 linkage he had proposed (Theorell, 1938) for the union of the protein with 

 the haemin at positions 2 and 4 was present in the native compound or was 



190 200 



Fig. 14. Absorption spectra showing transition, with change in pH, from pyridine 

 ferriprotoporphyrin to its hydroxide. In all solutions the final concentrations of 

 haemin Fe and pyridine were 0-1 mM and 5700 mM/1. respectively. The pH was 

 modified by the inclusion of HCl in all solutions except that represented by curve 

 9. Curve 1, absorption spectrum of species 1, probably dipyridine ferriproto- 

 porphyrin, pH = 7-26. Curve 9, absorption spectrum of species 2, pH = 11 -38. 

 Curves 2 to 8, absorption spectra of mixtures of species 1 and 2. The pH values 

 of the solutions represented by curves 2 to 8 were 7-62, 8-32, 8-95, 9-45, 9-86, 9-98, 

 and 10-36. The insert in the figure shows the partition of species 1 and 2 against 

 pH (see legend to Fig. 12). The solid circles are from data presented in the figure. 

 The open circles are based upon absorption data not shown. See the text. 



artifactually obtained by the vigorous hydrolysis conditions employed in the 

 isolation of porphyrin c. This led the writer (Drabkin, 1942b) to investigate 

 the spectra of cyanide, pyridine and carbonyl derivatives of proto-, meso-, 

 and coproferrohaem, and corresponding derivatives of haemoglobin and 

 ferrocytochrome c. In protohaemin positions 2 and 4 are occupied by the 

 unsaturated vinyl group, whereas in meso- and coprohaemin the substituents 

 in this position are respectively ethyl and propionic groups. The spectra of 

 the meso- and copro- derivatives were virtually indistinguishable from each 

 other (Figs. 15 to 17 and see Drabkin, 1942b, for the individual spectra of 



