The Harmony of Helical Macromolecules

The problem of indexing the observed layer lines in fiber diffraction patterns of complex helical structures is illustrated in a comprehensive fashion reviewing the practical aspects of the Cochran, Crick, and Vand theory of helical structures. A practical method that allows indexing the observed layer lines and determining the number of helical residues per turn of the helix is presented. The method is conceptually simple and has the advantage with respect to the literature methods proposed so far (see, for instance, Mitsui in: Acta Crystallogr. 1966 20, 694; Acta Crystallogr., Sect, A 1970, A26, 658) of being more suitable for its implementation in a software work-package aimed at solving helical structures. It may be used also by nonspecialists in diffraction analysis of crystalline polymers or even for didactical purposes.