Dynamics of haemoglobin through the 200 K glass-like transition

Abstract

We have measured quasielastic neutron spectra (0.3< Q<1.9 Å −1) with a resolution of 7.5 μeV (HWHM) for two slightly H 2O-hydrated powder samples of haemoglobin, a multimeric protein with many soft degrees of freedom. Window-integrated structure factors S( Q; T) have been determined at 190, 210, 235 and 260 K relative to the values at 100 K, and differential broadenings in S( Q, ω; T) due to the excitation of low-frequency modes around 200 K have been characterised. Questions relating to the glass-like nature of proteins have been examined by studying two samples: One quenched rapidly in liquid N 2, to record spectra from 100 K upwards, and one initially “warm” (260 K) for which spectra are taken at the same temperature points but in the opposite direction, i.e. down to 100 K. Both integrated intensities and line widths reveal small differences between “up” and “down” spectra at temperatures ⩾235 K relative to the 100 K spectra. The normalised S( Q; T) differences, in particular, show a bimodal behaviour with a knee at Q=1.5−1.6 Å −1. EISF values determined for the 260 K difference spectra agree well with estimates of the mobile proton fraction.