Figure 2.

Small-angle scattering from a complex between a deuterated and nondeuterated protein: the contrast variation experiment. The figure illustrates contrast variation using a structure (PDB: 3GMR) of T-cell surface glycoprotein CD1d1 in complex with beta-2 microglobulin. Theoretical scattering profiles were generated for the protein complex in which the beta-2 microglobulin component had its nonexchangeable hydrogens deuterated to 60% in silico.⁶⁶ (A) The proteins each have the same electron density and hence X-ray scattering density and the small-angle X-ray scattering profile therefore yield information on the shape of the entire complex as a uniform contrast (depicted as all black) particle. (B) The neutron scattering contrast for the deuterated (90% gray) and nondeuterated (40% gray) components is distinct, and it is therefore possible to measure scattering from each individual component by solvent matching the scattering density of the other by H/D substitution in the solvent. The nondeuterated protein will be solvent matched around 40% D₂O, whereas the deuterated protein's solvent match point will be 90% D₂O, this value depending upon the deuteration level of the protein (60% in this example). (C) A theoretical set of data acquired for different %D₂O in the solvent (a contrast series) yields information on the shapes and dispositions of the deuterated and nondeuterated proteins. From the contrast series, it is possible to extract scattering profiles corresponding to the deuterated and nondeuterated components (red and blue, respectively) and a cross-term (green) that is related to their relative dispositions. Note that because of the smaller size of the deuterated component, the corresponding P(r) needs to be multiplied by a factor of five to observe the curve clearly. Also, the cross-term contains negative values for I(q) and therefore must be plotted on a linear scale.