(a) Insulin ultralente, where the N-terminus of chain B can be modelled for group A, but not for group B. (b) On the other hand, the N-terminus of chain D can be modelled for group B, but not for group A. In chain D it is also possible to observe two groups of different rotamers for His5. (c, d) Fo − Fc density contoured at 3σ around one of the zinc ions for (c) data in group A and (d) data in group B. While density showing the presence of the ion is very evident in (c), it could be concluded quite reasonably that the zinc is missing in (d). This is also supported by the different rotamer of His5 in chain D; no bonds to a zinc ion could be coordinated by different dimers with the histidine tilted in this way.