Scattering from biopolymers with helical symmetry in solution

Small angle X-ray fibre diffraction on aligned elongated macromolecules has been a routine tool for the structure of materials that either can not be crystallized or in cases in which one wants to study the structure of the m olecule as close as possible to those occurring in solution. One of the pro blems with this approach is that quite often it is not possible to obtain c ompletely aligned samples but only samples in which there is an angular dis tribution around the main orientation axis. This has as a consequence that at larger scattering angles the contributions from different layer lines st art to overlap. A possible method to overcome this problem can be found in combining results from small angle solution scattering with small angle fib re diffraction from molecules aligned with their long axis parallel with re spect to the X-ray beam. In the latter case one only observes the projectio n of the molecule on the basal plane. We show how this method can be applie d to hydrated microtubules.