Anharmonic Behavior in the Multisubunit Protein Apoferritin as Revealed by Quasi-Elastic Neutron Scattering

Quasi-elastic neutron scattering (QENS) has been used to study the deviation from Debye-law harmonic
behavior in lyophilized and hydrated apoferritin, a naturally occurring, multisubunit protein. Whereas analysis
of the measured mean squared displacement (msd) parameter reveals a hydration-dependent inflection above
240 K, characteristic of diffusive motion, a hydration-independent inflection is observed at 100 K. The
mechanism responsible for this low-temperature anharmonic response is further investigated, via analysis of
the elastic incoherent neutron scattering intensity, by applying models developed to describe side-group motion
in glassy polymers. Our results suggest that the deviation from harmonic behavior is due to the onset of
methyl group rotations which exhibit a broad distribution of activated processes (Ea,ave ) 12.2 kJ · mol-1, σ
) 5.0 kJ · mol-1). Our results are likened to those reported for other proteins.