using THz-TDS In the literature, the transition temperature at which internal protein mobility occurs upon heating from low temperatures has been referred to as the protein dynamical transition temperature . However, there is some controversy regarding the specific motions and relaxation processes that are associated with this dynamical transition. For example, using THz-TDS, protein systems of oxidised cytochrome c solutions and lyophilised horse heart cytochrome c, showed a sharp increase in the linear temperature dependence of the terahertz dielectric response at 200 K . This was attributed to activated side-chain motions which are involved in the dynamical transition. It is important to highlight in this context that there is some evidence to suggest that the temperature of the dynamics transition is frequency dependent and also conditional on the presence of water, although both aspects are still relatively poorly understood and contrasting observations appear to have been reported. We have previously shown in our own work that local dipole mobility is associated with the onset of T g, , regardless whether these motions originate from side groups or larger domains purely depending on the height of the respective energy barriers ; Shmool Table : Gradient, m, of the linear fit (y = mx + c) for the respective temperature regions as outlined by Shmool and Zeitler as well as the respective transition temperatures determined based on the terahertz analysis. For all samples three regions were identified using the data analysis routine. and Zeitler, 2019). We also showed that this appears to be universally so for all organic molecular materials that we have investigated to date including polyalcohols , pharmaceutical drugs, proteins or polymers . Ngai et al. recently highlighted the same phenomenon for a further set of protein samples by carefully re-analysing data previously published in the literature using a range of techniques. Furthermore, the experiments of Mizuno and Pikal reported the observation of an endothermic pre-T g event in the DSC thermogram when heating lyophilised proteins around 310 330 K and attributed this event also to the onset of protein internal dynamics. However the authors linked these motions to the ↵-relaxation process rather than the -relaxation . Frontzek et al. also report a glass-like transition process in dry protein at temperatures similar to T g, . The significance of understanding the onset of the mobility of protein molecules has been highlighted repeatedly as the key to understanding protein stability. Specifically, Mizuno and Pikal suggested a link of the activation of protein motions associated with the ↵-relaxation with initiating protein degradation . Furthermore, the so-called 'T g 50 K rule' has been proposed more widely in the pharmaceutical field as a general rule that a given material should be stored at least at 50 K below its T g .