X plus : a comprehensive computationally accelerated structure analysis tool for solution X-ray scattering from supramolecular self-assemblies

X+ is a user-friendly multi-core accelerated program that fully analyses solution X-ray scattering radially integrated images. This software is particularly useful for analysing supramolecular self-assemblies, often found in biology, and for reconstructing the scattering signal in its entirety. The program enables various ways of subtracting background noise. The user selects a geometric model and defines as many layers of that shape as needed. The thickness and electron density of each layer are the fitting parameters. An initial guess is input by the user and the program calculates the form-factor parameters that best fit the data. The polydispersity of one size parameter at a time can be taken into account. The program can then address the assembly of those shapes into different lattice symmetries. This is accounted for by fitting the parameters of the structure factor, using various peak line shapes. The models of the program and selected features are presented. Among them are the model-fitting procedure, which includes both absolute and relative constraints, data smoothing, signal decomposition for separation of form and structure factors, goodness-of-fit verification procedures, error estimation, and automatic feature recognition in the data, such as correlation peaks and baseline. The program's intuitive graphical user interface runs on Windows PCs. Using X+, the exact structure of a microtubule in a crowded environment, and the structure, domain size, and elastic and interaction parameters of lipid bilayers, were obtained.