Scattering curve Debye sphere

Another modelling strategy is based on the use of spherical harmonics, where the excess scattering density p(r) - pg is expanded as a finite series of multipoles in order to approximate the shape of the particle [57]. While the experimental and calculated 1(Q) curves can be compared in this way, it has not yet been shown that the particle shape can be readily visuahzed in the way that is possible by computer displays of Debye spheres. 

With sufficient information on the location of the subunits, more detailed models of the ribosomal quaternary structure can be tested by neutron scattering. Thus an early analysis of the 30S ribosomal particle was performed by comparing experimental neutron contrast variation curves with models based on Debye spheres set at two density levels to correspond to a V-shaped 16S RNA moiety, together with the putative locations of the 21 ribosomal proteins [441]. More recently, the triangula-tion of the 19 30S ribosomal proteins shows that they are not uniformly distributed about the RNA in the 30S subunit, as once believed. The use of deuterated RNA within the 30S ribosomal particle showed an asymmetry in the RNA and protein distribution to confirm this result, where a separation A of 2.5 nm between their centres was calculated [446]. 

Fig. 8.1. Master scattering curves of polydisperse systems of spheres (Debye plot). Each curve is shifted by 1 unit upwards from the previous one with increasing values of a.

For a given structure type we obtain one set of master curves in dependence on the polydispersity. Model calculations were carried out for polydisperse systems of spheres and Gaussian coils, which cover the scattering behavior, observed on PEC particle systems. Figure 1 gives the master representation of the Debye plot for polydisperse spheres, showing significant deviations for different polydispersities. 

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