On Soulé-Porod plots of protein X-ray scattering data

It is shown that inhomogeneous electron density distribution as represented either by a spherical model containing smaller spheres or by a sphere with radially periodic density does not produce positive slope in Soulé–Porod plots (Ih⁴ versus h⁴) of protein small-angle scattering data. It is found that the introduction of small particles into the scattering population can produce the zero or positive slope commonly seen. It is also shown that intersubunit interference is able to account for the low 2θ minimum or dip often observed in Ih⁴ vs h⁴ plots of the scattering from multisubunit proteins.