High-Flux X-Ray and Neutron Solution Scattering

Solution scattering is a diffraction technique that is used to study the overall structure of biological macromolecules in the solution state (1 –3 ). Although X-rays are diffracted by electrons and neutrons are diffracted by nuclei, the physical principles are the same. Scattering views structures in random orientations to a nominal structural resolution of about 2–4 nm in a Q * range (Q = 4 π sin /g3/γ; 2/g3 = scattering angle; γ = wavelength) between about 0.05 and 3 nm^-1 (Fig. 1. ). In comparison, the use of diffraction to study single crystals of macromolecules will lead to electron or nuclear density maps at atomic resolution (0.15 nm) as the result of crystalline order. Analyses of the scattering curve I(Q ) measured over a range of Q lead to the mol wt and the degree of oligomerization, the overall radius of gyration R _G (and in certain cases, those of the cross-section and the thickness), and the maximum dimension of the macromolecule. Scattering can be used to monitor conformational changes. The scattering analyses can be quantitatively compared with other physical data in order to check and refine the results. These other methods include electron microscopy (Chapters 1 and 2 ), determinations of sedimentation or diffusioncoefficients (Chapters 5 and 6 ) crystallography (Methods in Molecular Biology: Crystallographic Methods and Techniques [in press]), and molecular graphics modeling.

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