Protein Stability in TMAO and Mixed Urea–TMAO Solutions

Osmolytes
are essential for cellular function under ubiquitous
osmotic stress. Trimethylamine N-oxide (TMAO) is
one such osmolyte that has gained remarkable attention due to its
protein-protective ability against urea. This Review aims at providing
a detailed account of recent theoretical and experimental developments
in characterizing the structural changes and thermodynamic stability
of proteins in the presence of TMAO and urea. New vapor pressure osmometry
and molecular dynamics simulation results on urea–TMAO solutions
are presented, and a unified molecular mechanism of TMAO counteraction
of urea-induced protein denaturation is introduced. In addition, a
detailed technical assessment of molecular dynamics force fields for
TMAO and for urea–TMAO solutions is presented. The force field
analysis highlights how many of the commonly used force field models
are in fact incompatible with solvation thermodynamics and can lead
to misleading conclusions. A new optimized force field for TMAO (Shea­(m))
is presented, and a recently optimized force field for TMAO–urea
(Netz­(m)) that best reproduces experimental data is highlighted.