252 



VI. HEMOGLOBIN 



plane containing the conjugated double bonds of the heme, and least 

 when the electrical vector is in a direction normal to this plane, it 

 must be concluded that the hemes lie approximately parallel to the a 

 plane of the crj'stal, the plane which contains the optic axes jS and 7. 

 In the light of the x-ray structure of the crystal the hemes must 

 be approximately in the a plane of the molecule. Two such arrange- 

 ments are indicated in Figure 6, A and B, where a cross section in 

 the c plane as seen from the direction of the c axis is shown. The 

 heme plates, seen edgewise, are represented by rectangles. Of these 

 A seems more likely since the hemes are more symmetrically placed 

 with respect to the protein than in B. A third possibility is that all 

 the four hemes lie in the same central a plane of the molecule. This 



Fig. 6. Position of hemes in hemoglobin molecule. 



is represented by C in Figure 6, where a cross section in this a plane 

 is shown, the heme plates being represented by squares. 



If the molecule A dissociates into halves in the c plane (or if C 

 dissociates in the a plane), the configuration of the hemes \^ith respect 

 to the protein in their environment will be radically changed and 

 they will become even more exposed. If, on the other hand the 

 splitting of A occurs in the a plane (or of C in the c plane), the imme- 

 diate environment of the hemes will undergo little change. In the 



