Chromatin fiber dimensions and nucleosome orientation: a neutron scattering investigation

Chromatin fibers were studied in solutions of mM monovalent salt by small angle neutron scattering. The variation of the cross section radius of gyration with H2O/D2O contrast shows that DNA is at much larger average radial distances from the fiber axis than histone. Consequently, the coils of DNA in a core particle must be approximately parallel to the fiber direction. The radii of gyration suggest that the maximum diameter of chromatin and nucleosomes is approximately 14 nm and that the DNA id distributed in two radial layers. The concentration dependence of the scattering maxima near 14 nm spacings furnishes independent support for a 14 nm external diameter and can be interpreted by a double DNA layer configuration.