Extracting water and ion distributions from solution x-ray scattering experiments.
Small-angle X-ray scattering measurements can provide valuable information about the solvent environment around biomolecules, but it can be difficult to extract solvent-specific information from observed intensity profiles. Intensities are proportional to the square of scattering amplitudes, which are complex quantities. Amplitudes in the forward direction are real, and the contribution from a solute of known structure (and from the waters it excludes) can be estimated from theory; hence, the amplitude arising from the solvent environment can be computed by difference. We have found that this "square root subtraction scheme" can be extended to non-zero q values, out to 0.1 Å(-1) for the systems considered here, since the phases arising from the solute and from the water environment are nearly identical in this angle range. This allows us to extract aspects of the water and ion distributions (beyond their total numbers), by combining experimental data for the complete system with calculations for the solutes. We use this approach to test molecular dynamics and integral-equation (3D-RISM (three-dimensional reference interaction site model)) models for solvent structure around myoglobin, lysozyme, and a 25 base-pair duplex DNA. Comparisons can be made both in Fourier space and in terms of the distribution of interatomic distances in real space. Generally, computed solvent distributions arising from the MD simulations fit experimental data better than those from 3D-RISM, even though the total small-angle X-ray scattering patterns are very similar; this illustrates the potential power of this sort of analysis to guide the development of computational models.
BioMaPS Institute for Quantitative Biology, Rutgers University, Piscataway, New Jersey 08854, USA.
Hung T
HT
Nguyen
School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA.
Suzette A
SA
Pabit
School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA.
Lois
L
Pollack
Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey 08854, USA.
David A
DA
Case
D004247:DNA / Q000737:chemistry; D005583:Fourier Analysis; D007477:Ions / Q000737:chemistry; D008956:Models, Chemical; D056004:Molecular Dynamics Simulation; D009113:Muramidase / Q000737:chemistry; D009211:Myoglobin / Q000737:chemistry; D053838:Scattering, Small Angle; D012996:Solutions / Q000737:chemistry*; D014867:Water / Q000737:chemistry*; D014961:X-Ray Diffraction
D007477:Ions; D009211:Myoglobin; D012996:Solutions; D014867:Water; D004247:DNA; D009113:Muramidase