Probing Structure and Mobility of Proteins in the Amorphous State at Low Hydration

The characterization of proteins in the dry state has implications for the pharmaceutical industry, since it provides deeper understanding of the effect of lyophilisation on the stability and biological activity of bio-macromolecular drugs. We have performed structural and dynamical analyses on a series of lyophilised and hydrated bio-macromolecules with varying degrees of structural complexity by means of Molecular Dynamics (MD) simulations; the simulated dynamical results being compared to experimental findings obtained from neutron scattering. Atomistic simulation of lyophilised proteins is still a challenge since the available force fields, and water molecule topology, used for the modelling have to be carefully correlated with experiment. Fortunately, the outputs from MD simulations, and the time and length scales probed, align directly with those accessed by neutron scattering. In particular, the method of Quasi-Elastic Neutron Scattering (QENS) can explore picosecond to nanosecond dynamics of macromolecular species and thus help validate the efficacy of the MD protocols applied. Here we report on the simulated effect of temperature and hydration on the structural features of the proteins, focusing particularly on the predicted changes in secondary structure and radial distribution. We also present a comparison of the temperature dependence of the mean squared displacement parameter, obtained by analysing the MD trajectories, with those resulting from QENS measurements.