Change of caged dynamics at Tg in hydrated proteins found after suppressing the methyl-group rotation contribution
In conventional glassformers at sufficiently short times and low enough temperatures, molecules are mutually caged by the intermolecular potential. The fluctuation and dissipation from motion of caged molecules when observed by elastic incoherent neutron scattering exhibit a change in temperature dependence of the mean square displacement (MSD) at the glass transition temperature Tg. This is a general and fundamental property of caged dynamics in glassformers, which is observed always near Tg independent of the energy resolution of the spectrometer. Recently we showed the same change of T-dependence at Tg is present in proteins solvated with bioprotectants, coexisting with the dynamic transition at a higher temperature Td. In these solvated proteins, all having Tg and Td higher than the proteins hydrated by water alone, the observation of the change of T-dependence of the MSD at Tg is unobstructed by the methyl-group rotation contribution at lower temperatures. On the other hand, proteins hydrated by water alone have lower Tg and Td, and hence unambiguous evidence of the transition of MSD at Tg is hard to find. Notwithstanding, evidence on the break of the MSD at Tg can be found by deuterating the protein to suppress the methyl-group contribution. An alternative strategy is the use of a spectrometer that senses motions faster than 15 ps, which confers the benefit of shifting both the onset of methyl-group rotation contribution as well as the dynamic transition to higher temperatures, and again the change of MSD at Tg becomes evident. The break of the elastic intensity or the MSD at Tg coexists with the dynamics transition at Td in hydrated and solvated proteins. Recognition of this fact helps to remove inconsistency and conundrum encountered in interpreting the data that thwart progress in understanding the origin of the dynamic transition and its connection to biological function.