Detergent structure in crystals of Ompf porin

In order to purify and crystallise membrane proteins they are usually removed from the cell membrane using detergents which replace the lipid
molecules found in vivo. Information on the binding of these detergent
molecules to the protein therefore gives indirect information on the lipid
binding in the membrane. Due to the fluidity of the detergent these molecules are invisible in high resolution X-ray crystallographic electron density map.
They may however be visualized in low resolution neutron rystallographic
studies on DB21 using the H₂O/D₂O contrast variation method. The figure shows the detergent ring observed around the Ompf porin molecule in single crystals of the protein.

Reference: Pebay-Peyroula E., Garavito R.M., Rosenbusch J.P., Zulauf M., Timmins P.A. - Detergent structure in tetragonal crystals of OmpF porin Structure Vol. 3 pp. 1051-1059 (1995).

Nucleic acid localisation in viruses

The nucleic acid in viruses is almost always less highly ordered than the protein coat which surrounds it. It may be either dynamically disordered (mobile) or statically disordered (different conformations in different virus particles). In either of these cases it will not be visible in high resolution X-ray crystallographic electron density maps.
Neutron crystallography with contrast resolution at relatively low resolution ( ~15 – 25Å) allows the nucleic acid to be seen in a unique way. An example is the case of tomato bushy stunt virus where not only the RNA but ~ 25% of the protein is disordered

References: Timmins, P.A.,Wild, D. and Witz, J. The Three-Dimensional Distribution of RNA and Protein in Tomato Bushy Stunt Virus:A Neutron Low Resolution Single Crystal Diffraction Study. Structure 2, 1191 - 1201 (1994).