InternationalReciprocal spaceTables for Crystallography Volume B Edited by U. Shmueli © International Union of Crystallography 2010 |
International Tables for Crystallography (2010). Vol. B, ch. 4.5, pp. 569-570
## Section 4.5.2.3.2. Diffraction by helical structures R. P. Millane
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Denote by a cylindrical polar coordinate system in reciprocal space (with the *Z* and *z* axes parallel), and by the Fourier transform of . Since is periodic in *z* with period *c*, its Fourier transform is nonzero only on the *layer planes* where *l* is an integer. Denote by ; using the cylindrical form of the Fourier transform shows that

It is convenient to rewrite equation (4.5.2.9) making use of the Fourier decomposition described in Section 4.5.2.3.1, since this allows utilization of the helix selection rule. The *Fourier–Bessel structure factors* (Klug *et al.*, 1958), , are defined as the Hankel transform of the Fourier coefficients , *i.e.* and the inverse transform is Using equations (4.5.2.7) and (4.5.2.11) shows that equation (4.5.2.9) can be written as where, as usual, the sum is over only those values of *n* that satisfy the helix selection rule. Using equations (4.5.2.8) and (4.5.2.10) shows that the Fourier–Bessel structure factors may be written in terms of the atomic coordinates as where is the (spherically symmetric) atomic scattering factor (usually including an isotropic temperature factor) of the *j*th atom and is the spherical radius in reciprocal space. Equations (4.5.2.12) and (4.5.2.13) allow the complex diffracted amplitudes for a helical molecule to be calculated from the atomic coordinates, and are analogous to expressions for the structure factors in conventional crystallography.

The significance of the selection rule is now more apparent. On a particular layer plane *l*, not all Fourier–Bessel structure factors contribute; only those whose Bessel order *n* satisfies the selection rule for that value of *l* contribute. Since any molecule has a maximum radius, denoted here by , and since is small for and diffraction data are measured out to only a finite value of *R*, reference to equation (4.5.2.10) [or equation (4.5.2.13)] shows that there is a maximum Bessel order that contributes significant value to equation (4.5.2.12) (Crowther *et al.*, 1970; Makowski, 1982), so that the infinite sum over *n* in equation (4.5.2.12) can be replaced by a finite sum. On each layer plane there is also a minimum value of , denoted by , that satisfies the helix selection rule, so that the region is devoid of diffracted amplitude where

The selection rule therefore results in a region around the *Z* axis of reciprocal space that is devoid of diffraction, the shape of the region depending on the helix symmetry.

### References

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