  The long debated protein dynamical transition was recently found also in
non-biological macromolecules, such as poly-N-isopropylacrylamide (PNIPAM)
microgels. Here, by using atomistic molecular dynamics simulations, we report a
description of the molecular origin of the dynamical transition in these
systems. We show that PNIPAM and water dynamics below the dynamical transition
temperature Td are dominated by methyl group rotations and hydrogen bonding,
respectively. By comparing with bulk water, we unambiguously identify
PNIPAM-water hydrogen bonding as the main responsible for the occurrence of the
transition. The observed phenomenology thus crucially depends on the
water-macromolecule coupling, being relevant to a wide class of hydrated
systems, independently from the biological function.
