Essentially the problem is that there are not yet valid computational tools to fully understand WAXS spectra. SAXS and WAXS are low resolution techniques, yet Wide-angle x-ray scattering (WAXS) can reveal finer details of molecular structures than its counterpart, small-angle x-ray scattering (SAXS). WAXS also reports

small scale changes in the structural features induced by surrounding ions

and ligands. The problem is when you want compare the measured data with existing DNA/RNA structures: now a day they are trying to use MD simulation to fill the gap, but there is still a lot of work to be done. 

Hi Amani,

Reason 1: If by WAXS you refer to diffraction, the first problem you'll run into is that you need a crystal, i.e. at least of volume of your sample in which whatever you're looking at (atoms, molecules, proteins...) exhibits long range order. This is quite painful with many proteins (an art by itself). Then you need to interpret the data (see above answer).

Reason 2: if your nucleic acids are small enough, then you're probably better of using spectroscopy methods such as NMR which has the advantage that you needn't have to crystallize your samples (they should be in solution, so much easier to handle).

Hope this helps

Aurelian, WAXS as well as SAXS are performed in solution, there are examples of WAXS on liquid crystals but the all point is to have an isotropic sample, so that from the diffraction pattern you derive a single curve. Crystals are used for X-ray diffraction but the setting is completely different and so is the outcome (high resolution structures vs low resolution one).  

Perfectly right, I hadn't checked the paper ^^.