Differences between calcium rich and depleted alpha-lactalbumin investigated by molecular dynamics simulations and incoherent neutron scattering.

We present a study comparing atomic motional amplitudes in calcium rich and depleted alpha-lactalbumin. The investigations were performed by elastic incoherent neutron scattering (EINS) and molecular dynamics (MD) simulations. As the variations were expected to be very small, three different hydration levels and timescales (instrumental resolutions) were measured. In addition, we used two models to extract the mean square displacements (MSDs) from the EINS data, one taking into account the motional heterogeneity of the MSD. At a timescale of several nanoseconds, small differences in the amplitudes between the calcium enriched and depleted alpha-lactalbumin are visible, whereas at lower timescales no changes can be concluded within the statistics. The results are compared to MD simulations at 280 and 300 K by extracting the MSDs of the trajectories in two separate ways: first by direct calculation, and second by a virtual neutron experiment using the same models as for the experimental data. We show that the simulated data give qualitatively similar results as the experimental data but quantitatively there are differences. Furthermore, the distribution of the MSDs in the simulations suggests that the inclusion of heterogeneity is reasonable for alpha-lactalbumin, but a bi-or trimodal approach may be sufficient.