In this work, we
investigated the influence of stabilizing (
N
,
N
,
N
-trimethylglycine)
and destabilizing (urea) osmolytes on the hydration spheres of biomacromolecules
in folded forms (
trpzip
-1 peptide and hen egg white
lysozyme—
hewl
) and unfolded protein models
(glycine—GLY and
N
-methylglycine—NMG)
by means of infrared spectroscopy. GLY and NMG were clearly limited
as minimal models for unfolded proteins and should be treated with
caution. We isolated the spectral share of water changed simultaneously
by the biomacromolecule/model molecule and the osmolyte, which allowed
us to provide unambiguous experimental arguments for the mechanism
of stabilization/destabilization of proteins by osmolytes. In the
case of both types of osmolytes, the decisive factor determining the
equilibrium folded/unfolded state of protein was the enthalpy effect
exerted on the hydration spheres of proteins in both forms. In the
case of stabilizing osmolytes, enthalpy was also favored by entropy,
as the unfolded state of a protein was more entropically destabilized
than the folded state.