ARRANGEMENT OF THE HEMES 251 



hemoglobin in solutions of urea as indicating that the half molecules 

 are slightly less asymmetric than the original. In view of the uncer- 

 tainty which now attaches to the derivation of molecular dimensions 

 from the frictional ratio if the degree of hydration is uncertain, and 

 in view of the disagreement between these two workers, further 

 experimental work is required. 



4.2.3. Arrangement of the Hemes. The configuration within such 

 a molecule is of great importance. The iron atom, in the center of 

 the large flat plate of the heme molecule is able to coordinate with 

 imidazole situated on either side (Section 3.2.), while the carboxyl 

 groups give one edge of the molecule a hydrophilic character (Chap- 

 ters III and V). The four hemes are indistinguishable from one 

 another in any of the reactions of the pigment; this strongly suggests 

 that they are arranged symmetrically with respect to one another as 

 well as with respect to the protein.* Pauling put forward two alter- 

 natives for such an arrangement, the hemes being arranged at the 

 corners of a square or of a tetrahedron (2123). He considered the 

 former the more likely. Lemberg (1683) suggested the alternative 

 possibilities that the heme lies flat on the surface, or is imbedded in 

 the molecule between two imidazole groups; in the event that the 

 imidazoles are firmly bound, a hemochrome structure would be 

 expected. Theorell's investigations on cytochrome c confirm the 

 latter possibility for this molecule (^755). 



The optical properties of crystals of hemoglobin and oxyhemoglobin 

 provide evidence of a more direct nature. Reichert and Brown (2224.) 

 observed that the crystals of oxyhemoglobin showed strong pleo- 

 chroism, being dark red and almost opaque in two directions of 

 extinction and light red and transparent in the third. Perutz (213^) 

 examined the absorption spectra of the crystal in polarized light. 

 When the electrical vector of the light was parallel to the a-direction 

 of the crystal, the absorption band of hem/globin in the red at 637 

 m/i was weak and diffuse, while the bands in the green were faintly 

 visible. When the vector was parallel to the /3- or 7-directions the 

 band in the red was strong, broad, and sharply defined at the bound- 

 aries, while the absorption in the green was too strong for the bands 

 to be distinguished. The sharpness and intensity of the oxyhemo- 

 globin bands showed similar behavior when the crystal was examined 

 in the appropriate directions. Since the light absorption is greatest 

 when the electrical vector of the polarized light is parallel to the 



* Compare footnote on page 266. 



