Overall, global and local mobility both have respective impacts on the physicochemical stability of protein in a dry state and the strength of impact depends on which length scale of molecular mobility is the determinant factor for protein stability. For example, when the short-time scale of molecular mobility is more dominant, analysis of the amplitude of β relaxation processes is necessary for the design and prediction of protein stability. To further investigate the correlation between protein mobility and stability, there are a few spectroscopic and diffraction techniques that have been extensively utilized to characterize protein dynamics such as slow and fast motion in terms of α-relaxation and β-relaxation including DSC, DES, NS, ssNMR, FTIR, and solid-state hydrogen-deuterium exchange (HDX). The pros and cons of each characterization technique are summarized in Table . In this session, we will discuss the application of these various techniques in protein dynamic characterization and correlation with protein stability in the solid state. Literature has suggested that α-relaxation primarily involves segmental relaxations of the main chain, which is typically associated with the material's structural relaxation and is inherently intermolecular. As the temperature approaches the glass transition temperature (Tg), the relaxation time for α-relaxation experiences a significant increase, contributing to the formation of the glassy state at lower temperatures. As such, DSC, which measures the glass transition temperature (Tg) of protein in the solid state, is the most frequently used technique to estimate global mobility in a glassy state. It has been reported that substantial changes in protein stability were observed for temperatures below and above Tg and a common stabilization strategy is to increase the temperature of Tg to slow down the α-relaxation. One empirical way to estimate the degree of α-relaxation dynamics is based on the postulation that log𝜏α scales with the difference between the storage temperature and Tg (Tg-T) approximately, as suggested by the Williams-Landel-Ferry (WLF) equation. Thus, in general, if a glassy matrix has higher Tg, the slower α dynamics and the protein degradation processes will be suppressed to a larger extent. The correlation between protein stability and (Tg-T) has been demonstrated in a variety of sugars and polyols. For example, in a study by Prestrelski and coworkers, the stability of interleukin-2 was compared in formulations with the presence of different carbohydrates, such as sucrose, trehalose, raffinose, and stachyose. It was found that excipients with higher Tg, e.g., raffinose and stachyose, exhibited higher levels of long-term stabilization effect on interleukin Nevertheless, failures of correlating protein stability with Tg were also extensively reported in the literature. The universal correlation between the temperature difference of storage temperature and Tg and protein stability is mostly observed when the system near to or above Tg. When the storage temperature is well below Tg, there is little correlation between protein formulation stability and temperature difference between storage temperature and Tg. In addition, protein formulations of a lower Tg were found more stable than those of a higher Tg, especially when sugar matrices are compared to polymers. For instance, a higher stabilization effect was seen for sucrose and trehalose than maltodextrin and polyvinylpyrrolidone (PVP) on vacuum-dried restriction enzyme EcoRI, even though the sugars possess much lower Tg values than polymers. Moreover, Chang et al. found that the stability of lyophilized Interleukin-1 Receptor Antagonist formulation was significantly improved although the Tg of the formulations gradually decreased by increasing the concentration of sucrose from 1 to 10%. Similarly, as shown in Figure , negative correlation between the Tg -T and stability of the IgG1 with different weight ratios of sucrose was observed. In addition, the mobility of IgG1 τ β was not monotonically correlated with the level of sucrose where there is a maximum reached at a 1:1 ratio. These contradictory results indicate that Tg alone is not sufficient to predict protein stability. Tg is a critical temperature of molecular mobility of amorphous materials but may not be a direct predictor for protein mobility. It has been demonstrated that the scaling between log𝜏α and (T-Tg) can be quite different for various materials, which depends on the fragility, Kauzmann temperature (Tk), and thermal history of the glassy matrix. Moreover, several reports have suggested that Tk, sometimes defined as critical molecular mobility temperature, might be more accurate indicating the molecular mobility in protein formulations.