COMBINATION OF NEUTRON-SCATTERING AND MOLECULAR-DYNAMICS TO DETERMINEINTERNAL MOTIONS IN BIOMOLECULES

The forms and frequencies of atomic dynamics on the pico- and nanoseco nd timescales are accessible experimentally using incoherent neutron s cattering. Molecular dynamics simulations cover the same space and tim e domains and neutron scattering intensities can be calculated from th e simulations for direct comparison with experiment. To illustrate the complementarity of neutron scattering and molecular dynamics we exami ne measured and simulation-derived elastic incoherent scattering profi les from myoglobin and from the crystalline alanine dipeptide. Elastic incoherent scattering gives information on the geometry of the volume accessible to the atoms in the samples. The simulation-derived dipept ide elastic scattering profiles are in reasonable accord with experime nt, deviations being due to the sampling limitations in the simulation s and experimental detector normalisation procedures. The simulated dy namics is decomposed, revealing characteristic profiles due to rotatio nal diffusional and translational vibrational motions of the methyl gr oups. In myoglobin, for which the timescale of the simulation matches more closely that accessible to the experiment, good agreement is seen for the elastic incoherent structure factor. This indicates that the space sampled by the hydrogen atoms in the protein on the timescale <1 00ps is well represented by the simulation. Part of the helix atom flu ctuations can be described in terms of rigid helix motions.