The following sections describe how to use CHROMIXS to analyse SEC-SAXS data.
If you use CHROMIXS in your work, please cite:
Panjkovich A. and Svergun D.I. (2017) CHROMIXS: automatic and interactive analysis of chromatography-coupled small angle X-ray scattering data.
Bioinformatics, Dec 28
The CHROMIXS program will load and plot a set of SAXS frames (usually thousands) as produced by a SEC-SAXS (size-exclusion-chromatography online SAXS) run.
Frames are represented in respect to elution time, allowing the user to select buffer/sample regions and to generate a SAXS curve.
Once CHROMIXS is running and data has been loaded, the CHROMIXS window shows a plot of frame nr. vs. average intensity, a set of menus on the top and a set of tools on the right.
If molecular weight (MW), radius of gyration (Rg) or other additional data (e.g. UV) are being displayed (with a scale on the y-axis on the right side of the plot), the range of the right y-axis scale can be controlled with the mouse-wheel. Use Shift+mouse-wheel to control zooming on the right axis scale.
This option is available through the graphic interface once CHROMIXS is running (the Load Data Directory button at the top-right opens a dialog that can navigate to the frames directory) or as a command-line argument: --datadir=<PATH>.
Upon loading a data directory, CHROMIXS expects a collection of experimental data files (e.g. ./data/frames/*.dat) in ascii format containing 3 columns: (1) experimental scattering vector, (2) experimental intensity and (3) experimental errors.
Data files should correspond to SAXS frames obtained during elution of the SEC column.
No other *.dat should be present in the directory (e.g. averages, frames from previous runs, etc.).
This format matches what is currently in use at EMBL P12 SAXS beamline (i.e. data/frames directory).
Ordering of the frames is important, the EMBL P12 filename format uses padded zeros (*0001.dat, *0002.dat) which alphabetically match the elution order and is what CHROMIXS uses as default.
The frame nr. currently plotted corresponds to the order in which the frames were read in.
In the case of SOLEIL/Nexus frames, where the order is written in the filename between hash symbols (e.g. XXX#9#.dat, XXX#10#.dat XXX#11#.dat), CHROMIXS will detect this nomenclature and sort the files appropriately.
Again, the frame number displayed on the plot corresponds to the absolut order in which frames were read in, and does not necessarily match the number given in the filenames.
If you use yet another format, please rename your frames so the elution order matches the alphabetical order.
Standard operation after loading data is to select a range of sample frames with the corresponding tool.
After selecting sample frames, a buffer region can be selected manually or predicted using the 'Buffer Automatic Selection' function.
Once sample and buffer regions are chosen, the user can click on 'Process Selection' and CHROMIXS will subtract one buffer frame to each sample frame, scale the subtracted frames and average them.
The output frame will be written to file in the working directory and displayed with the Primus program.
Please see the PRIMUS manual for more details.
The output filename prefix can be set through the command-line option of the same name or through the configuration window.
The File menu allows the user to save a chromixs project file, which contains the data plot and selected regions.
This file can be opened later to continue with the analysis.
Currently, frame data are not saved within the chromixs project file, only their location in the computer's filesystem.