The use of continuous diffraction data as a phase constraint. II. Application to fiber diffraction data

The properties of intensity data in fiber diffraction patterns are considered with the aim of developing strategies for structural studies of helical macro-molecular assemblies. A quantitative calculation is made of the number of independent intensity measurements in a fiber diffraction pattern and the number of degrees of freedom in an image of a helical object. These calculations are based on sampling theory in cylindrical coordinates. Several novel schemes for sampling intensities in a fiber diffraction pattern are demonstrated. A class of Patterson-like functions are introduced that may be useful in determining the helical symmetry of a particle from diffraction data. The use of these results in determining the significance of details of structural models based on fiber diffraction data is discussed.