J. C. KENDREW 



the ridges is also the distance between polypeptide chains, viz, 

 9-5 A. 



A further and better resolved b projection of lattice B (not illustrated 

 here) shows that along each ridge is a series of vector peaks spaced 

 about 5 A apart. This must correspond to a repeat distance of 5 A 

 along the chain. 



We are now in a position to interpret the peaks of the a projection, 

 mentioned in (b) above. This projection is made on to a plane which 

 intersects the ac plane along the line marked ' a projection ' in Figure 4c, 

 and it will be seen that the polypeptide chains are almost end-on as 

 viewed in this projection. The peaks of the a projection are, in fact, 

 the interchain vectors of the end-on projection of the chains. Since 

 the two molecules in the cell are related by a screw diad, one is turned 

 through 180° relative to the other, so that the chains in the two molecules 

 are parallel and do not interfere with one another in the b projection. 



THE ORIENTATION OF THE HAEM GROUP 



The strong absorption in the visible region of the spectrum of haem- 

 containing pigments is attributed to a selective absorption by the planar 

 conjugated ring system of the haem group itself, and such absorption 

 is suffered only by that component of the electric vector of the incident 

 radiation which lies in the plane of the ring system. Where the crystal 

 displays powerful pleochroism — that is to say, where the absorption 

 of transmitted plane-polarized light is a function of the direction of 

 polarization relative to the crystal axes — one may deduce that all the 

 haem groups throughout the crystal are more or less parallel, and 

 proceed to relate the plane in which they lie to the crystal axes. 



Myoglobin crystals are strongly pleochroic, and it emerges that the 

 haem groups lie in a plane containing the b axis and approximately 

 bisecting /?. Reference to Figure 4c shows that they must therefore lie 

 roughly perpendicular to the direction of the polypeptide chains, one 

 forming part of each molecule : a similar arrangement exists in haemo- 

 globin. The operation of the screw diad ensures the parallelism of the 

 two haem groups in each unit cell. 



CONCLUSIONS : THE STRUCTURE OF HORSE MYOGLOBIN 

 AND ITS RELATION TO HORSE HAEMOGLOBIN 



I conclude by showing how all these observations may be integrated 

 to give a picture of the myoglobin molecule. This picure may perhaps 

 not be regarded as more than plausible at the present stage ; however 

 it can be correlated reasonably well with all the x-ray data so far 

 obtained, and its close relation to the haemoglobin structure deduced 

 by Perutz may be considered a further point in its favour. 



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