WAXS in Solvent (WAXSiS) computes small- and wide-angle X-ray scattering curves based on explicit-solvent all-atom molecular dynamics simulations.
This section has a few pointers for various aspects of the system. If you need additional help, please contact us.
What are the system requirements for using WAXSiS?
How does the system present results?
WAXSiS will generate both a web page for viewing online and a .tar.gz file for downloading
the results collection if you need it. Example output for a job which includes an uploaded experimental curve
can be viewed here.
The agreement between the structural model (in the PDB file) and the uploaded experimental I(q) curve is quantified by fitting the experimental curve to the calculated curve, using two different metrics:
1) χ2 = N-1 ∑i=1N [Icalc(q) - (f Iexp(q) + c)]2 / σ2(q)
where the fitting parameter f is the overall arbitrary scale, and c is a constant that aims to absorb some uncertainty in the buffer subtraction; or:
2) χ2log = N-1 ∑i=1N [ log Icalc(q) - log (f Iexp(q) + c)]2
Both, χ and χlog are reported in the output notes. Because the first metric uses the inverse experimental errors σ(q) as fitting weights, that fit imposes a rather high weight to small angles. The second, non-weighted fit on a log scale is instead more sensitive to wider angles.
For the general interpretation of SAXS/WAXS curves we refer to a number of excellent reviews [1-4].
 Putnam, Hammel, Hura, and Tainer, 2007. X-ray solution scattering (SAXS) combined with crystallography and computation: defining accurate macromolecular structures, conformations and assemblies in solution. Q. Rev. Biophys. 40:191–285.
 Koch, Vachette, and Svergun, 2003. Small-angle scattering: a view on the properties, structures and structural changes of biological macromolecules in solution. Q. Rev. Biophys. 36:147–227.
 Lipfert and Doniach, Small-Angle X-Ray Scattering from RNA, Proteins, and Protein Complexes. Annu. Rev. Biophys. Biomol. Struct. 36:307–27
 Blanchet and Dmitri I. Svergun, 2013. Small-Angle X-Ray Scattering on Biological Macromolecules and Nanocomposites in Solution. Ann. Rev. of Phys. Chem., 64:37-54
My job was submitted but I don't see any results. What's happened?
If you submitted an email address, the system will email you either on job completion, or when encountering an error. Alternatively, you can keep track of the results progression using the job link that was provided on submission, which will also bring up any errors encountered. Any jobs that are still running or waiting to run will be viewable on the queue page. If all else fails, please contact us.
What are the input limitations to the system?
WAXSiS allows you to optionally upload your own PDB file and your own experimental curve file. The file size limit is 20 MB for each of these. Additionally for PDB files - either uploaded ones or those selected by ID - there must be between 300 and 40000 heavy atoms. Other constraints are highlighted in the submission form options, such as the maximum q scattering vector. Finally, more complex instructions for topology files can be seen below.
How can I cancel my job?
Queued jobs can only be cancelled if you submitted an email address. Visit the queue page, hover over or tap the table row containing your job ID and a cancel icon will appear. Click this and enter your email address to cancel the job. If your job is highlighted green, it means the job has already left the queue and begun processing. A running job can no longer be cancelled.
How is the buffer subtraction option calculated?
Two different conventions are sometimes found for the buffer subtraction:
i) The total buffer intensity is subtracted, that is I(q) = Isample(q) - Ibuffer(q).
ii) The buffer intensity is reduced by the volume fraction v of the solute, I(q) = Isample(q) - (1-v)Ibuffer(q)
The two subtraction schemes only slightly differ at small angles, but highly differ at wide angles around the water scattering peak (~ 2 Å-1). WAXSiS allows you to choose the buffer subtraction scheme.
How is the q scattering vector defined?
The scattering vector q is defined as: q = 4πλ-1sin θ where 2θ is the scattering angle.
What are the units and conventions for the experimental curve?
The experimental curve can be provided in units of Å-1 or nm-1. The momentum transfer may be proved as q = 4πλ-1sin θ where 2θ is the scattering angle, or as s = 2λ-1 sin θ.
Can I fit multiple experimental curves to the calculated curve?
To keep a balance between functionality and simplicity, we do not allow the upload of multiple experimental curves. However, if you wish to fit multiple curves, please contact us and we will be happy to provide the Python script that does the fitting procedure.
How does WAXSiS detect crystallization agents?
Important note: If you use data from WAXSiS for a publication, we
recommend that you download the PDB file from the RCSB, remove molecules
you don't want in an editor, upload the modified PDB to WAXSiS, and
select Keep both ligands and crystallization agents.
PDB files do not contain reliable information on whether a molecule is a crystallization agent or a biologically relevant ligand. Therefore, if selected, WAXSiS tries to remove crystallization agents automatically based on the number of atoms, number of Van-der-Waals contacts, and residue name. At present, ligands are kept if one of these criteria matches:
Check the results notes and PDB file complete.pdb (in the downloadable .tar.gz) to see which molecules were kept and which were removed.
WAXSiS says the PDB ID that I selected lacks a biological unit.
PDB files do not always contain the real conformation in a solution. If the
protein is a multimer (dimer, trimer), the PDB file may contain only the monomer.
Alternatively, if the protein is a monomer, the PDB file could contain multiple
monomers. In such a case, you should make sure that the PDB file contains
what you need.
For these PDB IDs, WAXSiS still allows you to submit your job.
My PDB file could not be processed due to containing exotic elements. Which are these?
PDB files which contain the following elements cannot currently be processed: Arsenic, Silicon.
What is the Convergence option?
The larger the solute, the fewer MD simulation frames are required to achieve a converged
SWAXS curve. As reported in our previous article,
we estimate that the numer of simulation frames required to achieve a statistical uncertainty of
< 2% can be computed as A / N -0.77, where A = 2x105 by default
(Convergence = Normal). Here, N is the number of atoms inside of the envelope.
The Convergence option allows you to modify the prefactor A. With Convergence = Quick, we use A = 0.5x105, and with Convergence = Thorough we use A = 106. For many purposes, Normal convergence is fine. Quick convergence is useful to get results more quickly. If the computed SAXS curve will end up in a paper, we recommend a bit of patience and selecting Convergence = Thorough.
How much slower does the Thorough Convergence option run?
This setting will run slower both in processing time, and time spent in the queue.
While processing time will be several times greater than the default Normal
setting, jobs that are added to the queue with Quick or Normal
settings will always be run ahead of Thorough jobs, even if they were submitted
In short, Thorough jobs will always sit at the end of the queue until they actually begin processing. If the queue is empty, you won't notice this.
What is the "Random Seed" option useful for?
MD simulation require initial velocities for all atoms, which are randomly drawn from a Maxwell-Boltzmann distribution. By default, the same initial random seed is used to generate those initial velocities. Consequently, if you run WAXSiS twice with the same structure, you will generate exactly the same trajectory and the same SWAXS curve. If you want a statistically independent SWAXS calculation, you can tell WAXSiS to use a new random seed instead.
How do I submit a job using trajectory files?
WAXSiS allows you to compute SAXS/WAXS curves form your own molecular dynamics trajectories. To do so, you need to upload three files:
HETATM 1357 MG MG 168 4.669 34.118 19.123 1.00 3.16 MG2+
HETATM 3835 FE HEM 1 17.140 3.115 15.066 1.00 14.14 FE3+
[ Solute ]etc. with all atom numbers of the solute (the first atom has index 1, not 0!)
1 2 3 4 5
[ Solvent ]etc. with all atom numbers of the solvent.
10521 10522 10523 10524
echo Solute | editconf -f system.pdb -n index.ndx -o solute.pdb
echo Solvent | editconf -f system.pdb -n index.ndx -o solvent.pdb
Depending on your connection speed and file uploads, this may take a few minutes.
To cancel, enter the email address you used to submit this job.