SCOMAP-XD: atomistic deuterium contrast matching for small-angle neutron scattering in biology

The contrast-variation method in small-angle neutron scattering (SANS) is a uniquely powerful technique for determining the structure of individual components in biomolecular systems containing regions of different neutron scattering length density ρ. By altering the ρ of the target solute and the solvent through judicious incorporation of deuterium, the scattering of desired solute features can be highlighted. Most contrast-variation methods focus on highlighting specific bulk solute elements, but not on how the scattering at specific scattering vectors q, which are associated with specific structural distances, changes with contrast. Indeed, many systems exhibit q-dependent contrast effects. Here, a method is presented for calculating both bulk contrast-match points and q-dependent contrast using 3D models with explicit solute and solvent atoms and SASSENA, an explicit-atom SANS calculator. The method calculates the bulk contrast-match points within 2.4% solvent D₂O accuracy for test protein-nucleic acid and lipid nanodisc systems. The method incorporates a general model for the incorporation of deuterium at non-exchangeable sites that was derived by performing mass spectrometry on green fluorescent protein. The method also decomposes the scattering profile into its component parts and identifies structural features that change with contrast. The method is readily applicable to a variety of systems, will expand the understanding of q-dependent contrast matching and will aid in the optimization of next-generation neutron scattering experiments.