  In this work we use a multi-technique approach to gain new insights into the
effects of trehalose on the volume phase transition of
poly(N-isopropylacrylamide) (PNIPAM) microgels, a model synthetic biomimetic
system. By dynamic light scattering the temperature-dependent microgel
hydrodynamic volume is monitored, while Raman spectroscopy and molecular
dynamics simulations explore at the molecular scale changes of solvation and
dynamics across the transition. At 0.72 M trehalose concentration the phase
transition temperature is lowered by $\approx 10$ K, with a concomitant
decrease of transition enthalpy not compensated by a corresponding reduction in
transition entropy. In water-trehalose solution the microgel particles are
$\approx 20\%$ more expanded than in pure water, keeping unchanged their
thermal contraction, swelling capacity, and the amount of absorbed water.
Strongly hydrated trehalose molecules mainly develop water-mediated
interactions with PNIPAM, thus preserving the polymer hydration state both
before and after the transition. However, the presence of trehalose deeply
impacts on the dynamics of the system, with a drastic slowing down of PNIPAM
motions and of its hydration shell. Our investigation reveals a coherent
picture in which trehalose sustains hydration and favors, both
thermodynamically and kinetically, the collapsed conformation of the microgel,
by inhibiting local motions of polymer residues and water.
