Low frequency dynamics of proteins studied by neutron time-of-flight spectroscopy

Recent theoretical calculations predict the existence of low-frequency collective motions in globular proteins in the range 1-200 cm-1 which may be of biological significance. We have therefore made neutron time-of-flight measurements of the inelastic scattering in this frequency range from concentrated D2O solutions of two proteins, bovine pancreatic trypsin inhibitor (BPTI), and cytochrome C (both in the oxidised and reduced state). The aim of these experiments is to characterise the low-frequency dynamics of proteins by extraction of the hydrogen-weighted density of vibrational states and to assess the sensitivity of low-frequency protein dynamics to the functional state of the protein. In the case of BPTI we can compare the measured spectra with predicted spectra calculated from a recent normal analysis. Major differences show that considerable work remains to be done to understand the inelastic scattering by proteins. (orig.)