Diffraction by proteins and other macromolecules

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 

A Laue photograph -is produced by irradiating a stationary crystal with a beam of X rays that has a wide range of wavelengths ( white radiation). It differs from all of the other methods for collecting diffraction data in that the crystal is stationary throughout the experiment. Diffraction is therefore dependent on the multiwavelength feature of the 

6 Converting measured sin 6 values to unit cell dimensions 

The important preliminary data for a crystal, listed in the introduction to any crystallography article after the compound name, formula, and formula weight, are the unit cell dimensions and space group. These 

FIGURE 7.19. Laue photograph of lysozyme, 100 msec photograph, with synchrotron radiation 0.2 - 2.0 A wavelength, taken at CHESS (Cornell High Energy Synchrotron Source). (Courtesy Edwin Westbrook). 

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