Crystal Structure of Horse Myoglobin 



It will be noted that one of the projections is derived from lattice A„ 

 and the other two from lattice B. These have been chosen because they 

 illustrate most clearly the striking features which will be mentioned 

 below. Comparison between Patterson projections from slightly 

 different lattices of the same crystal will not affect our deductions 

 about the shape of the molecule since studies of other protein crystals 

 have shown that during shrinkage the /w/ramolecular distances, and 

 hence those peaks in the Patterson diagrams which correspond to 

 /n/ramolecular vectors, do not alter ; it is only the /ntermolecular 

 distances which shrink. In the present instance this has been confirmed 

 for the b projection in the transition A — B and for the a projection in 

 the transition B — Dry though not all the relevant projections are 

 illustrated here. 



(a) The c projection (Figure 4a) — The most prominent feature of this 

 projection is the very powerful ridge of vector density through the 

 origin and perpendicular to b. Parallel to it are two further ridges 

 at the top and bottom of the cell, spaced from it by half the b dimension, 

 i.e. 15-4 A. These extremely striking features must be due to parallel 

 concentrations of projected electron density, also 15-4 A apart 

 (see p. 178). Thus the projection fits in well with the scheme of 

 Figure 3b, since the two disks in that scheme would project into two 

 parallel bars of projected electron density separated by b/2 (this 

 separation is obligatory owing to the operation of the screw diad axis). 

 We may also note the prominent peaks at 4 A from the origin in a 

 direction parallel to b ; these must be part of the intramolecular pattern, 

 though their interpretation is not at present clear. 



(b) The a projection (Figure 4b) — This projection is surprisingly 

 similar to the c projection just discussed, exhibiting the same general 

 structure, perhaps even more strikingly than before. This is exactly 

 what the model v/ould lead us to expect, since the two disks would 

 project into two parallel bars of projected electron density on this 

 plane as well — or, indeed, on any plane parallel to b. There is, however, 

 a new phenomenon — each of the ridges of vector density is partly 

 segmented into prominent peaks about 10 A apart. To understand 

 these it is first necessary to discuss the third projection. 



(c) The b projection (Figure 4c) — Here we have an entirely different 

 type of diagram, the most prominent feature of which is an irregular 

 but distinct ridge of vector density through the origin at an angle of 

 about 20° to the a axis. Parallel to this, and about 9-5 A from it, are 

 two further ridges ; beyond these again and spaced from them by a 

 similar distance can be seen traces of a further pair. The interpretation 

 placed on these ridges is that they are the vector equivalent of the 

 parallel polypeptide chains, seen in plan. The distance between 



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