The superstructure of chromatin and its condensation mechanism. V. Effect of linker length, condensation by multivalent cations, solubility and electric dichroism properties.

Comparison between the internucleosomal distance found by X-ray solution scattering for chicken erythrocyte (23 nm) and sea urchin (30 nm) chromatin indicates that this distance is proportional to the linker length. The diameter of the condensed sea urchin chromatin fibers is about 45 nm which is significantly larger than in chicken erythrocyte chromatin (35 nm). Trivalent cations (Gd, Tb, Cr) and polyamines spermine and spermidine were found to induce compaction at much lower concentrations than the divalent cations but Gd, Tb, Cr induce aggregation before full compaction of the fibers. The influence of hydrogen bonding is illustrated by comparison of the effects of NaCl, ammonium chlorides on condensation. Solubility experiments indicate that there is a nearly linear dependence of the Mg++ concentration at which precipitation occurs on chromatin concentration and confirm the differences between cations observed by X-ray scattering. The chicken erythrocyte chromatin samples were further characterized by their reduced electric dichroism. The values found are consistent with the model derived from X-ray scattering and are compared with those reported in the literature.