HIGH RESOLUTION NEUTRON SPECTROSCOPY OF BIOLOGICAL MATERIALS A. Deriu - Dipartimento di Fisica, and Unit[[daggerdbl]] INFM, Universit[[daggerdbl]] di Parma, 43100 Parma, Italy

Biological materials at a molecular level are heterogeneous systems: they are made up from a heteropolymeric skeleton (polynucleotides, polypeptides, polysaccharides) or from complex multilayered sheets (lipid membranes) in interaction with an aqueous buffer. Water plays a major role in stabilising the large scale arrangement (ternary and quaternary structures) of biomolecular assemblies and controls the activation of most biological processes. It is precisely this `composite' nature combining the high degree of mechanical stability of the solid biopolymer scaffolding with the liquid-like behaviour of the buffer, which accounts for the enormous structural and functional diversity of biomolecules.

Not only structure but also motion is of great importance at the molecular level of biology. The marked temperature dependence of the activity of biomolecules reflects their thermal mobility. A characterization at a microscopic scale of processes and interactions responsible for molecular flexibility and dynamics is a necessary starting point for a deeper understanding of the mechanisms which control the highly specific functions of most biological materials. Dynamical events in biomolecular systems occur on a very large time-scale ranging from femtoseconds to almost seconds. Within this interval, motions occurring in the picosecond to nanosecond time-scale are of particular interest and relevance since they cover the transition region from `discrete' local excitations of small molecular subunits to slower processes involving co-operative motions of massive parts of the macromolecular assembly. This time window is well covered by inelastic and quasielastic neutron scattering, these techniques can therefore play a relevant role in improving the understanding of molecular motions which affect the functionality of biomolecules.

Neutron high resolution spectroscopic studies of biomolecular dynamics have been pursued actively in the past fifteen years. As a result of instrumental advances and progresses in molecular dynamics simulations it is nowadays possible to describe quantitatively the complex low frequency motions exhibited by biologically active systems. Some selected examples referring to different biopolymers and membrane model systems will be illustrated.