The role of water-polymer coupling in the low temperature dynamical transition of microgels

Hydrated protein suspensions are known to undergo a dynamical transition at low temperatures, typically between 220 and 240 K. This transition consists in a steep enhancement of the protein atoms mobility which is related to the onset of anharmonic motions. While the dynamical transition always occurs in aqueous environments, with the protein confinement preventing the onset of ice crystallization, the role played by water is still a controversial issue. Recently a protein-like dynamical transition was observed also in non-biological macromolecules, such as polymeric microgels [1]. In this contribution, we report a description of the microscopic origin of the low temperature dynamical transition in microgels as obtained from atomistic molecular dynamics simulations [2]. This study is based on a nanoscale model of a microgel network in water, which quantitatively reproduces neutron scattering experiments. By correlating the information extracted from the analysis of the polymer relaxations times, water self-diffusion coefficients and hydrogen bonding interactions we show which molecular processes control the dynamics of both the macromolecule and water below the dynamical transition temperature. Through a further comparison between the low temperature behavior of water in microgels suspensions and that of bulk water, we recognize that it is primarily the macromolecule-water hydrogen bonding interaction that determines water dynamics below the dynamical transition temperature. This study supports the idea that the macromolecule-water coupling is the driving ingredient of the dynamical transition in microgels, implying that it should be a general feature of hydrated macromolecular systems which can form hydrogen bonding interactions. REFERENCES [1] M. Zanatta et al., Sci. Adv., 4 eaat5895 (2018). [2] L. Tavagnacco et al., J. Phys. Chem. Lett. 10, 870-876 (2019).