Intrinsic motional displacements and function in proteins Scientific Achievement A method for determining the intrinsic mean square displacement (MSD) of hydrogen (H) in proteins from resolution dependent neutron scattering data has been developed. Significance and Impact The MSD of H in proteins is used as an indicator of whether proteins will function. A large MSD is associated with function. Obtaining the intrinsic, instrument independent values of the MSD in proteins is critical to accurate use of this function indicator. Research Details A simple model is fitted to instrument dependent data, either new or already in the literature, to obtain the intrinsic MSD in biological molecules. The Intrinsic MSD is always larger than instrument broaded, observed MSD. The intrinsic dynamical transition (DT) temperature is always lower than the observed DT temperature. The onset of large motional displacements of hydrogen (H) in proteins is associated with the onset of protein function. The H in proteins is typically attached to molecules and large motions means the molecules can move and interact and initiate chemical processes. The mean-square displacement (MSD) of H is particularly sensitive to temperature. The MSD often increases markedly over a narrow temperature range, denoted the dynamical transition (DT), which signals onset of function at or near this temperature. The MSD of H is widely measured in proteins using quasielastic neutron scattering techniques. A problem is that the observed MSD depends on the energy resolution of the instrument used. Different values for the MSD and different DT temperatures for the same protein appear in the literature. We have developed a method to obtain the intrinsic, resolution independent MSD from resolution dependent data. The method consists of fitting an analytically correct model to the data that has parameters (3) representing the intrinsic MSD, the decay rates in the protein and the resolution width (W) of the instrument to resolution dependent data. From this fit, the intrinsic MSD is obtained, an MSD which is independent of the instrument resolution width. The mean square displacement (MSD) in Angstroms of hydrogen in the protein HS-0.4 vs temperature. The squares are the MSD observed on instruments IN16, IN13 and IN6 which depend on the instrument resolution W. The red dots are the intrinsic MSD obtained from a fit to the same data. The red solid line, the intrinsic MSD drawn through the red dots, is independent of the instrument resolution width W. D Vural, HR Glyde, Physical Review B 86, 011926 (2012). Research conducted at the University of Delaware 1