  Urea at sufficiently high concentration unfolds the secondary structure of
proteins leading to denaturation. In contrast, Choline Chloride (ChCl) and
urea, in 1:2 molar ratio form a deep eutectic mixture, a liquid at room
temperature and protect proteins from denaturation. In order to get a
microscopic picture of this phenomenon, we perform extensive all-atom molecular
dynamics simulations on a model protein HP-36. Based on our calculation of
Kirkwood-Buff integrals, we analyze the relative accumulation of these
osmolytes around the protein. Additional insights are drawn from the analyses
of the translational and rotational dynamics of solvent molecules and also from
the hydrogen bond auto-correlation functions. In the presence of urea, water
shows slow subdiffusive dynamics around the protein backbone as a consequence
of stronger interaction of water molecules with the backbone atoms. Urea also
shows subdiffusive motion. Addition of choline further slows down the dynamics
of urea restricting its inclusion around the protein backbone. Adding to this,
choline molecules in the first solvation shell of the protein shows the
strongest subdiffusive behavior. In other words, ChCl behaves as a nano-crowder
by excluding urea from the protein backbone and thereby slowing down the
dynamics of the water layer around the protein. This prevents the protein from
denaturation and makes it structurally rigid which in turn is supported by the
smaller radius of gyration and root mean square deviation values of HP-36 when
ChCl is present in the system.
