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Neutron Fibre Diffraction Studies of Hydration Patterns in DNA. V.T. Forsyth, M.W. Shotton, L.H. Pope, H. Ye, C. Boote, P. Langan^[dagger], R.C.Denny^[daggerdbl], H. Grimm^*, W. Fuller, Physics Department, Keele University, Staffordshire, ST5 5BG, England, ^[dagger]Institut Laue Langevin, Avenue des Martyrs, Grenoble, France, ^[daggerdbl]Blackett Laboratory, Imperial College, London University, England, ^*Institit fur Festkorperforschung, Julich

Neutron high-angle fibre diffraction techniques offer opportunities for structural studies that are typically difficult or impossible to address using x-ray methods. Such techniques can be applied to a wide variety of natural and synthetic polymer systems and are particularly powerful in the study of biological systems in which ordered water plays a central role. These methods have been used very successfully to study the location of ordered water around a number of conformations of the DNA double helix. In the D conformation, there is a clear network of water running along the minor groove of the molecule. This network appears to form a "double ribbon" in the minor groove of the B conformation. In the A conformation, ordered water appears to be located almost exclusively in a number of distinct sites in the major groove.

One of the major problems encountered in neutron fibre diffraction work of this type arises from the presence of hydrogen in the samples used. This results in a large amount of incoherent scattering which limits the accuracy with which data can be measured and results in absorption effects that can be difficult to correct. Significant advances have recently been obtained by recording neutron fibre diffraction data from samples made using perdeuterated DNA in which all of the "non-exchangeable" hydrogens (i.e. those attached to carbon atoms) are replaced by deuterium. The DNA was extracted from E.Coli cells cultured specifically for this work at the EMBL outstation in Grenoble. In studies of the A conformation, there was a large decrease in the background level, and the quality of the density maps used to identify structured water was greatly improved. Four sites of ordered water are clear in the major groove of the A conformation of DNA. The position and occupancies of these sites have been refined and compared with the results of previous work on hydrogenated DNA.