Screw symmetry, chain pair

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 ... 

In Forms 1 and 2, the hydrogen bonds link the molecules to form infinite chains. For Form 1, the result is a near-planar chain in which adjacent molecules are related by a twofold screw axis. Form 2 contains a three-dimensional chain in which adjacent molecules spiral along a threefold screw axis. In Form 3, pairs of molecules form hydrogen bonded dimers mutually linked across a center of symmetry. Representative views of the three structures are found in Fig. 4. 

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