Helical diffraction. II. The inclusion of a subunit of finite size

In paper I of helical diffraction [Worthington & Elliott (1989). Acta Cryst. A45, 645-654] an expression for the autocorrelation function (a.c.f.) for a helix was obtained. The Fourier transform of this a.c.f. gave a new expression for the diffracted intensity. In paper II the theory is extended to real helices containing subunits of finite size. There are two main approaches to consider. The first approach centers on the calculation of the a.c.f. of the helical array of subunits. In the special case when the subunit contains an assembly of discrete atoms, the expression for the intensity is shown to be equivalent to the classical formula of Franklin & Klug [Acta Cryst. (1955), 8, 777-780]. The second approach bypasses the a.c.f. calculation and expresses the intensity in terms of the Fourier transform of the subunit. The second approach may prove useful in molecular modeling studies of biological helical structures.