Interhelical parameters

Chain Pair Modeling. In the following analysis, we assume that the chains are regular helices, i.e. that they have screw symmetry, with a repeat distance, t. In a perfect crystal, such chains must either be parallel or antiparallel. Four interhelical parameters are required to define the geometric orientation of chain A relative to chain B (see Figure 2). The parameters and their ranges are ... 

Such a set of interhelical parameters relates directly to the symmetry operations which are found in crystal structures. 

Figure 2. Interhelical parameters required to define the geometric orientation of chain A relative to chain B.

Other relationships between crystallographic elements of symmetry and interhelical parameters can be easily derived. 

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