Uncovering a novel transition in the dynamics of proteins in the dry state

The research of dynamics of protein is predominantly focused on the hydrated or solvated state, which is close to the physiological condition of biological relevance. Eclipsed by the hydrated and solvated counterparts, the dynamics of dry proteins are seldom studied in earnest. Studied occasionally, the data of dry proteins were not addressed, possibly due to the preconceived notion that dry proteins are bio inactive, and thus are uninteresting and unimportant. Actually lyophilized dry proteins are important for the pharmaceutical industry as the therapeutic proteins are often stored in lyophilized conditions to achieve long-term stability, which is of direct connection to the dynamics of protein in dry state. In order to uncover the intrinsic dynamics of dry proteins that previous studies overlooked or ignored, we analysed quasielastic neutron scattering, Mössbauer, and Brillouin light scattering data of various dry proteins. We demonstrate the existence of a novel transition of the dry protein dynamics at temperatures around 200 K, with the exact value dependent on the kind of protein. The transition temperature of dry proteins is found here independent of the time/frequency of the measurement, in stark contrast to the well-documented dynamical transition observed in hydrated proteins, the temperature of which is strongly depending on the time window probed experimentally. In addition we point out that this transition in dry proteins is analogous to those observed before in the glassy state of various glass-forming materials, the origin of which is the change of the caged molecule dynamics in response to the change of the temperature dependence of density when crossing the secondary glass transition temperature Tgβ for the Johari-Goldstein β-relaxation. We show that the transition of various dry proteins also occurs around Tgβ, thus suggesting that the underlying microscopic mechanism is the same as the one identified and explained before for many glass-formers of different types. The presence of the transition is independent of the trace amount of water in supposedly dry proteins, although the value of Tgβ can vary somewhat. The novel transition in dry proteins has impact on bio-pharmaceutical research and applications because of the connection to the Johari-Goldstein β-relaxation, which can affect long-term stability of lyophilized dry proteins.