Macromolecular crystals problems with

The measurement of diffraction data from crystalline macromolecules presents additional problems. In the first place, the intensity of the diffraction is related to the size of the unit cell. Crystals with large unit cells diffract less strongly than do crystals with small unit cells. This is because there are fewer unit cells per unit volume for a macromolecular crystal. As a result, there is a need for very sensitive detection devices that can measure the intensities of weak Bragg reflections with high precision. A second related complication that arises with large unit cells is that the number of Bragg reflections is increased, and therefore the... 

The Patterson synthesis (Patterson, 1935), or Patterson map as it is more commonly known, will be discussed in detail in the next chapter. It is important in conjunction with all of the methods above, except perhaps direct methods, but in theory it also offers a means of deducing a molecular structure directly from the intensity data alone. In practice, however, Patterson techniques can be used to solve an entire structure only if the structure contains very few atoms, three or four at most, though sometimes more, up to a dozen or so if the atoms are arranged in a unique motif such as a planar ring structure. Direct deconvolution of the Patterson map to solve even a very small macromolecule is impossible, and it provides no useful approach. Substructures within macromolecular crystals, such as heavy atom constellations (in isomorphous replacement) or constellations of anomalous scattered, however, are amenable to direct Patterson interpretation. These substructures may then be used to solve the phase problem by one of the other techniques described below. 

The most demanding element of macromolecular crystallography (except, perhaps, for dealing with macromolecules that resist crystallization) is the so-called phase problem, that of determining the phase angle ahkl for each reflection. In the remainder of this chapter, I will discuss some of the common methods for overcoming this obstacle. These include the heavy-atom method (also called isomorphous replacement), anomalous scattering (also called anomalous dispersion), and molecular replacement. Each of these techniques yield only estimates of phases, which must be improved before an interpretable electron-density map can be obtained. In addition, these techniques usually yield estimates for a limited number of the phases, so phase determination must be extended to include as many reflections as possible. In Chapter 7,1 will discuss methods of phase improvement and phase extension, which ultimately result in accurate phases and an interpretable electron-density map. 

It is often the case in the X-ray crystallographic studies of biological macromolecules that only noisy or insufficient experimental data is available. If an approximation of the expected macromolecular structure is available beforehand, the situation can be remedied without recourse to further more complete or more accurate data collection. However the remedy requires that the independently available rough model be correctly oriented with respect to the crystal axes. In principle, the formulation of this orientation problem involves exhaustive search calculations in vast multi-dimensional spaces. In practice, such enormous calculations cannot be done with present-day computers. However, simulated annealing strategies can overcome such limitations. This article will focus on such strategies. 

The differences between standard thermotropic LCs and macromolecular condis crystals are summarized in Fig. 8. The first three and the last two points make it easy to experimentally identify low molecular mass LCs. For macromolecules, however, the viscosity may be suflBciently large to lose the obvious liquid character the birefringence does not always show the well-known LC texture (55) the small ASj of LCs may be confused with partial crystallinity of the condis crystals and in polymers, some larger main-chain rigid groups are not always easily identifiable as mesogens. This leaves points four and eight for differentiation between the two mesophases. Points five and six are more difficult to establish, and solid state NMR and detailed X-ray structure-determinations may be necessary for full characterization. Furthermore, borderline structures may be possible between thermotropic LCs, amphiphilic LCs, and condis crystals. A few examples and the resolution of their structures are discussed next, to illustrate the resolution of some of these problems. 

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