Data Analysis of Dynamics in Protein Solutions Using Quasi-Elastic Neutron Scattering─Important Insights from Polarized Neutrons

Protein dynamics play a vital role in biology. Quasi elastic neutron scattering (QENS) is an ideal method to access these dynamics. To isolate protein dynamics, it is important to separate the signal of the buffer and the protein. Normally data analysis is performed based on the assumption that the scattering spectrum is incoherent. To observe the full range of protein dynamics, it is necessary to perform the experiments in solution. This solution is usually a fully deuterated buffer, while the protein remains protonated. It is generally assumed that subtracting the buffer contribution removes all coherent signal from the measured spectrum, and the rest can be considered as purely incoherent. Up until recently, there was no way to experimentally verify this assumption. Polarized QENS experiments allow for the coherent and incoherent contributions to be separated. By comparing the results from the polarized QENS experiment and the standard analysis method from unpolarized QENS, we are thus able to check this assumption experimentally. We show that the pure incoherent spectrum obtained from polarization analysis does not match the results for unpolarized QENS. We discuss the implications of this for data analysis and possible solutions to the problem, as well as mitigation techniques for standard QENS.