The two angle model—basic notions

Since the linker DNA is assumed straight and the nucleosome non-deformable, the fiber geometry of the two-angle model is completely determined by the entry-exit angle of the linker DNA at each nucleosome and by the rotational angle 

The geometrical structure of the chain in Fig. 2 is determined entirely by 6, (j), and B. The model only describes the linker geometry and does not account for excluded volume effects and other forms of nucleosome-nucleosome interaction it assumes that the core particles are point-like and that they are located at the joints of the linkers, which are straight rods. 

An overview of the possible two-angle fibers is provided in Fig. 3, where 6 and p are varied over the range 0-180° (for a more thorough discussion of the possible structures, see Schiessel et al. [17]. 

Various examples of two-angle fibers were already displayed by Woodcock et al. [12] in their Fig. 2, namely fibers with 0=150° and many different values of (p, corresponding to a vertical trajectory on the right-hand side of Fig. 3. Three different configurations with a fixed value of (p and different values of 6 are displayed in Fig. 3c in another paper by these authors [16]. Schiessel et al. [17] were able to obtain analytical expressions for all the geometrical quantities that characterize 

Beyond pure geometry, the two-angle model is also useful to predict some of the physical properties of the 30-nm fiber, for instance, its response to elastic stress [17]. In an independent study on the two-angle model by Ben-Haim et al. [76] this question has been the major focus, and as demonstrated by Schiessel [72], the elastic properties of the two-angle model as a function of 6 and j are analytically solved completely by combining the results from both papers. 

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