A high-angle neutron fibre diffraction study of the hydration of deuterated A-DNA

A high-angle neutron fibre diffraction study of the hydration of A-DNA has been performed using the single-crystal diffractometer D19 at the Institut Laue-Langevin (Grenoble, France). The sample was prepared using deuterated DNA extracted from E. coli cells cultured on deuterated nutrients. In common with our previous neutron fibre diffraction studies of DNA, this work exploits the ability to isotopically replace H₂O around the DNA by D₂O. However this study benefitted additionally from the fact that the hydrogen atoms which are covalently bonded to carbon atoms in the DNA sugars and bases were replaced by deuterium so that incoherent scattering and absorption effects were minimised. Successive cycles of Fourier synthesis and Fourier difference synthesis allowed water peaks to be identified and their positional and occupancy parameters to be refined against the observed diffraction data. The results confirm the main hydration features noted in our earlier studies with a clear network of water running along the inside edge of the major groove linking successive O1 phosphate oxygen atoms. The central core of water running along the axis of the double helix is very much clearer in this work. Additionally this study shows chains of ordered water lying in the centre of the major groove.