Crystalline structure dichroism measurements

Nuclease behaves like a typical globular protein in aqueous solution when examined by classic hydrodynamic methods (40) or by measurements of rotational relaxation times for the dimethylaminonaphth-alene sulfonyl derivative (48)- Its intrinsic viscosity, approximately 0.025 dl/g is also consistent with such a conformation. Measurements of its optical rotatory properties, either by estimation of the Moffitt parameter b , or the mean residue rotation at 233 nin, indicate that approximately 15-18% of the polypeptide backbone is in the -helical conformation (47, 48). A similar value is calculated from circular dichroism measurements (48). These estimations agree very closely with the amount of helix actually observed in the electron density map of nuclease, which is discussed in Chapter 7 by Cotton and Hazen, this volume, and Arnone et al. (49). One can state with some assurance, therefore, that the structure of the average molecule of nuclease in neutral, aqueous solution is at least grossly similar to that in the crystalline state. As will be discussed below, this similarity extends to the unique sensitivity to tryptic digestion of a region of the sequence in the presence of ligands (47, 48), which can easily be seen in the solid state as a rather anomalous protrusion from the body of the molecule (19, 49). 

X-ray Diffraction any moment can Combines well with other techniques as dichroism or birefringence. Dynamic techniques have been developed Considerable data reduction. Amorphous orientation measurements are not strongly sensitive. Must know crystalline structure 20,000 la, 12, 76, 78,79... 

Rheo-optical techniques (46 8) afford information on the strain dependence not only of stress but also of optical quantities associated directly with the structure or molecular morphology. The techniques were developed extensively for crystalline polymers to investigate the molecular deformation mechanism underlying the tensile elongation. In this part, the chain orientation behavior is characterized by infrared dichroism measured simultaneously with tensile deformation at a constant rate of elongation. 

Another important application is infrared dichroism measurements for the assessment of chain (or group) orientation. This topic is treated in Chapter 9, and the discussion is not repeated here. It should be mentioned that Raman spectroscopy can also be used for the determination of chain orientation. Both IR and Raman spectroscopy are very useful for characterization of the physical structure of crystalline polymers. Assessment of the degree of crystallinity can be made by several methods. The preferred and internally consistent methods are X-ray diffraction, density measurements and calorimetry (DSC/DTA). This topic is described in detail in Chapter 7. However, both IR and Raman spectroscopy provide information about the crystallinity, although it is common for the actual crystallinity values obtained by these methods to deviate from values obtained by the three preferred methods WAXS, density and DSC/DTA. 

In summary, then, it is necessary to measure the fiaction of crystals, the crystalline orientation factor the amorphous orientation factor and possibly the size and size distribution of crystals in order to relate polymer structure to polymer properties. Although the extent of crystallinity is generally measured using density or heat-of-fusion methods, orientation is determined with the help of optical birefringence, dichroism, sonic modulus, or x-ray diffraction [60]. The size of crystals is observed with an optical or electron microscope. 

The orientation distribution functions for the structural units cannot be directly determined from any experimental sources, except for the crystalline units from X-ray diffraction measurements. Usually the orientations of structural units are obtained as several kinds of averages, i.e. the second and/or fourth orders of the orientation factors from optical measurements such as birefringence and absorption/emission dichroism. 

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