Protein-like dynamical transition in microgels: the role of water-macromolecule coupling

Hydrated proteins become biologically active above a temperature of about 220 K at which also a sudden increase of the protein mobility occurs. This molecular process, known as the protein dynamical transition, consists in the onset of anharmonic motions which allow proteins to explore conformational states of functional relevance. Thus it has been the subject of intensive research, but a deep understanding of its nature still remains elusive. Evidence of a protein-like dynamical transition was recently reported also for a non-biological system, such as poly(N-isopropylacrylamide), PNIPAM, microgels. [1] These micrometer particles, made of a cross-linked polymer network, are widely investigated as smart materials due to their stimuli-responsive behavior. However, PNIPAM microgels share many features with proteins, because of their extended covalent connectivity and their amphipilic character. In this contribution we describe the molecular origin of the low temperature dynamical transition in microgels, as obtained from atomistic molecular dynamics simulations. [2] The study is based on a nanoscale model of microgel network in water (Figure 1) which has been validated with a direct comparison with elastic incoherent neutron scattering experiments. [1] We show which molecular processes control the dynamics of both the macromolecule and water below the dynamical transition temperature. Through a comparison with the low temperature behavior of bulk water, we demonstrate that below the dynamical transition temperature water dynamics is mainly determined by the macromolecule-water hydrogen bonding interaction. Our findings suggest that the macromolecule-water coupling plays a driving role in the dynamical transition. Therefore this phenomenology should be a general feature of hydrated macromolecular systems able to form hydrogen bonding interactions with water. References [1] M. Zanatta et al., Sci. Adv., 4 2018, eaat5895. [2] L. Tavagnacco et al., J. Phys. Chem. Lett., 10 2019, 870-876.