Comparing Crystal Structures

Another approach is to compare polymorphic forms of the same compound. This allows the comparison of molecules with the same constitution and configuration in different crystal fields. Do these different potential environments lead to different conformations There are few comparative crystal structural studies of organic polymorphs. For d-mannitol the molecules had the same conformation in all three polymorphic forms studied with differences in torsion angles for the two that were studied in detail of only a few degrees (see Table I). 

The relative ability of group 14 atoms to increase the coordination number up to five is clearly shown by comparing crystal structures of such compounds as Me3MCN (M = Si, Ge and Sn). The minimal intermolecular contact between M and N atoms for silicon... 

Given such realities, an immediate need in crystal engineering is to be able to compare crystal structures. Many will appreciate that the structure of, say. 

Direct evidence for polymerization in the channels is given by crystal structure analysis. We can compare crystal structures before the polymerization and after the polymerization. Computed tomography works well in simulating whether or not inclusion polymerization will take place. Comparison of cross sections of channels with those of polymers enables us to consider whether or not the resulting polymers can be accommodated in the channels. 

This structure is called close packed because the number of atoms per unit volume is quite large compared with other simple crystal structures. 

Frazao C, C Topham, V Dhanaraj and T L Blundell 1994. Comparative Modelling of Human Rer Retrospective Evaluation of the Model with Respect to the X-ray Crystal Structure, Pure and A Chemistry 66 43-50. 

Polyethylene. The crystal structure of this polymer is essentially the same as those of linear alkanes containing 20-40 carbon atoms, and the values of Tjj and AHf j are what would be expected on the basis of an extrapolation from data on the alkanes. Since there are no chain substituents or intermolecular forces other than London forces in polyethylene, we shall compare other polymers to it as a reference substance. 

The polymers compared all have similar crystal structures but are different from polyethylene, which excludes the possibility for also including the latter in this series. Also note that the isotactic structure of these molecules permits crystallinity in the first place. With less regular microstructure, crystallization would not occur at all. 

By comparing the crystal structures of these complexes with a further complex of the 434 repressor DNA-binding domain and a synthetic DNA containing the operator region OR3, Harrison has been able to resolve at least in part the structural basis for the differential binding affinity of 434 Cro and repressor to the different 434 operator regions. 

How is the binding specificity of the heterodimer achieved compared with the specificity of Mat a2 alone The crystal structure rules out the simple model that the contacts made between the Mat a2 homeodomain and DNA are altered as a result of heterodimerization. The contacts between the Mat o2 homeodomain and DNA in the heterodimer complex are virtually indistinguishable from those seen in the structure of the Mat o2 monomer bound to DNA. However, there are at least two significant factors that may account for the increased specificity of the heterodimer. First, the Mat al homeodomain makes significant contacts with the DNA, and the heterodimeric complex will therefore bind more tightly to sites that provide the contacts required by both partners. Second, site-directed mutagenesis experiments have shown that the protein-protein interactions involving the... 

The prolactin receptor, PER, which regulates milk production in mammals, belongs to the same receptor class as the growth hormone receptor. In addition to binding the hormone prolactin, PER also binds and is activated by growth hormone. The extracellular domain of PER forms a very stable 1 1 complex with growth hormone in solution this complex has been crystallized and its structure determined (Figure 13.21). We shall compare this structure with the 1 2 complex of the same hormone with GHR. 

Compared with most other crystalline polymers the permeability of P4MP1 is rather high. This is no doubt due to the ability of gas molecules to pass through the open crystal structure with the large molecular spacing. 

Crystal structure determination has also been done with -butyllithium. A 4 1 n-BuLi TMEDA complex is a tetramer accommodating two TMEDA molecules, which, rather than chelating a lithium, link the tetrameric units. The 2 2 -BuLi TMEDA complex has a structure similar to that of [PhLi]2 [TMEDA]2. Both 1 1 -BuLi THF and 1 1 -BuLi DME complexes are tetrameric with ether molecules coordinated at each lithium (Fig. 7.2). These and many other organolithium structures have been compared in a review of this topic. ... 

---