The isoscattering point in X-ray scattering curves of globular solute particles measured by the contrast-variation method

The isoscattering point is the specific point at which the X-ray scattering intensity of a solute is independent of the electron density of the solvent and has been observed in the X-ray small-angle scattering curves of globular particles such as micelles and viruses. The condition for the appearance of the isoscattering point has been theoretically derived: if all the solutes are spherical particles of radius R₀, the position s* of the isoscattering points are given by 2πs*R₀ = tan (2πs*R₀), where s = 2 sin θ/λ, λ is the X-ray wavelength and 2θ is the scattering angle. If the solutes have a homogeneous structure, the isoscattering points coincide with the zeros of the intensity curves. The appearance of the isoscattering point with a non-zero intensity value is a direct indication that the solutes have an inhomogeneous structure. The isoscattering point is useful for determination of the radius of a globular solute particle to a good accuracy.