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Figure 1
(a) Small-angle X-ray scattering (SAXS) experiment. The macromolecular solution is exposed to a collimated, monochromatic beam of X-rays and the angular dependence of the scattered radiation is measured. (b) A scattering profile of lysozyme. The lysozyme data (at 6.08 mg ml−1 concentration) were collected with an MLM on the P12 beamline at PETRA III using an EIGER 4M pixel detector (frame rate 750 Hz, exposure time 1.3 ms). (c) The Guinier plot, p(r) function, Kratky plot and ab initio model of lysozyme. The best known model free parameter is the radius of gyration Rg, which is evaluated from the lowest angles using the classical Guinier approximation I(s) ≃ I(0)exp[−(sRg)2/3] and the linear plot ln[I(s)] versus s2 (Guinier, 1939BB30). Rg is sensitive to the overall size and shape of a particle and the zero angle scattering I(0) (also obtained from the Guinier plot) is related to its MW. The electron pair distance distribution function p(r) of a molecule is computed using an indirect Fourier transformation of scattering data and yields the maximum size Dmax of a particle (Glatter, 1977BB26; Svergun, 1992BB81). Integrating the scattering data and calculating the so-called Porod invariant provides an estimate of the particle volume Vp (Porod, 1982BB73). A qualitative indicator of particle flexibility can be obtained by using the Kratky representation where s2I(s) is plotted against ss. Its intensity normalized version, where the momentum transfer is multiplied by the Rg of a particle, facilitates flexibility comparison between different proteins (Kratky & Porod, 1949BB50; Durand et al., 2010BB21). The single-shot exposure lysozyme data can be utilized for standard SAXS analyses including ab initio modelling. The fit of the theoretical scattering based on a lysozyme X-ray crystallographic structure (in red, PDB id: 1lys; Harata, 1994BB110) yields a goodness of the fit χ2) = 1.1.

IUCrJ
Volume 4| Part 5| September 2017| Pages 518-528
ISSN: 2052-2525
https://doi.org/10.1107/S2052252517008740