The mean radius of structural elongation of the sample.
In a given solute-solvent contrast, the radius of gyration
R_G is a measure of structural elongation if the internal
inhomogeneity of scattering densities has no effect. Guiner
analysis at low Q gives the R_G and the forward scattering at
zero angle I(0).
lnl(Q) = lnl(0) - R_G2Q2/3
Q = 4(pi)sin(theta/lamda)
2theta = scattering angle
lamda = wavelength
The above expression is valid in a QR_G range for extended
rod-like particles. The relative I(0)/c values ( where
c = sample concentration) for sample measurements in a
constant buffer for a single sample data session, gives the
relative masses of the protein(s) studied when referenced
against a standard.
see: O.Glatter & O.Kratky, (1982). Editors of "Small angle
X-ray Scattering, Academic Press, New York.
O.Kratky. (1963). X-ray small angle scattering with
substances of biological interest in diluted solutions.
Prog. Biophys. Chem., 13, 105-173.
G.D.Wignall & F.S.Bates, (1987). The small-angle approximation
of X-ray and neutron scatter from rigid rods of non-uniform
cross section and finite length. J.Appl. Crystallog., 18, 452-460.
If the structure is elongated, the mean radius of gyration
of the cross-sectional structure R_XS and the mean cross sectional
intensity at zero angle [I(Q).Q]_Q->0 is obtained from
ln[I(Q).Q] = ln[l(Q).(Q)]_Q->0 - ((R_XS)2Q2)/2