Interpretation of the X-ray scattering profiles of chromatin at various NaCl concentrations by a simple chain model

In order to interpret the change in the X-ray scattering profiles from rat thymus chromatin, extensive model calculation was carried out. Chromatin is modelled as a string of subunits (nucleosomes) in which disorder is introduced into the positions of adjacent subunits. Disposition parameters characterizing the arrangement of subunits were estimated for various states of chromatin, so that the main feature of the scattering profiles is described. The result indicated that the structure of chromatin changes, as the NaCl concentration increases, from the extended "beads-on-a string" structure to the condensed helical structure. The latter has an outer diameter of about 26 nm with 3-4 nucleosomes per turn. In the intermediate state, it has a loose helical structure. The estimation of disorder suggested that the arrangement of subunits is appreciably disordered even in the condensed helical filament at 50 mM NaCl. Our model for chromatin condensation seems to support models of the "crossed linker" type.