Figure 3.

Static energy phase diagrams of different nucleosome geometries for the opening angle ψ and the nucleosome twist angle β. (A) CL geometry with NRL = 169 bp. The step width for ψ and β is 1° and the opening angle ψ results from the condition γ = −α. A broad energy valley exists for [2, 1] conformations (fibers 1–4). More compact [n, 1] fibers with n > 4 exhibit no apparent energy valleys (fibers 7–10). Fiber 5 is our previous model based on the tetranucleosome crystal structure (20). (B) ID geometry with NRL = 197 bp. The angle α was set to 117.5°, and γ was varied to achieve the actual angle ψ. Step widths of 0.5° and 1° were used for ψ and β, respectively. Most of the sterically possible regions in the diagram with large opening angles led to high energies. The [5, 1], [6, 1], and [7, 1] fibers with opening angles between 120° and 140°, and twist angles < 40° yielded the lowest energy (fibers 1–3, 5, and 6). Fiber 4 is the previously derived model for chromatin fibers reconstituted on arrays of strong nucleosome positioning elements (20).