Classifying and assembling two-dimensional X-ray laser diffraction patterns of a single particle to reconstruct the three-dimensional diffraction intensity function: resolution limit due to the quantum noise

Acta Cryst. (2012). A68, 366-381 (http://doi.org/hxm)

Correlation function of a pair of simulated diffraction patterns for a small protein, lysozyme. When a pair is similar, a high correlation line is observed. The highest k value, up to which the line can be detected against noise, determines the resolution of structural determination.

New X-ray free-electron laser light sources offer the possibility of high-resolution single-particle imaging from repeated measurements of two-dimensional diffraction intensity patterns, each from a target with an unknown random orientation. An algorithm is developed to assemble these patterns into the three-dimensional diffraction intensity function. A key is the calculation of a newly defined correlation function for a pair of diffraction patterns to detect their similarity. The result indicates that, if the current laser light beam can be further focused, a subnanometer resolution structural determination may be possible.