Diffraction crystal-like structures

Virtually all of what is known about the secondary and tertiary structure of tubulin has been gleaned from a limited number of spectroscopic and chemical modification studies. Failure to obtain crystals of suitable quality for X-ray diffraction studies likely results from both heterogeneity in the subunits and the propensity of tubulin to polymerize into many polymorphs (see Section III). George et al. (1981) reported that as many as 17 distinct protein peaks may be discerned after isoelectric focusing of purified tubulin. 

The elucidation of the crystal structures of polymers from their x-ray diffraction patterns is frequently a difficult and laborious task. The work usually proceeds by trial and error methods in which calculated intensities for likely structures are compared with the observed intensities of diffraction spots. Furthermore, x-ray fibre photographs often contain relatively few reflections and it is always possible that more than one structure may give a reasonable fit with the observed intensity data. Additional information which can be obtained from infrared spectra can often provide considerable help with both these difficulties and in particular many trial structures can be eliminated without recourse to time-consuming calculations of x-ray intensities. 

Montmorillonite-Calcite (me) Mixture. Heated mixtures of montmorillonite and calcite yielded the phases given in Table I. Although the montmorillonite structure persisted through 400 °C, it underwent dehydroxy lation between 400 and 500 °C. Grim and Bradley (16) have shown that the general layered structure is able to survive the elimination of the (OH) water with moderate readjustments. This structure produces an X-ray diffraction pattern like that given in Table III. Table III represents data close to those observed in this study. This phase is called dehydroxylated montmorillonite in Table I. This phase disappeared between 700 and 800 °C as a result of the complete destruction of the montmorillonite crystal structure. Calcite decomposed between 500 and 600 °C to form lime that was present through 900 °C. 

It is worth noting that the structure in Figure 1.10 not only looks ordered, but it is perfectly ordered. Moreover, in recent decades, many crystals with five-fold symmetry have been found and their structures have been determined. These crystals, however, do not have translational symmetry in three directions, which means that they do not have a finite unit cell and, therefore, they are called quasicrystals quasi - because there is no translational symmetry, crystals - because they produce discrete, crystal-like diffraction patterns. 

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