Direct osmolyte-macromolecule interactions confer entropic stability to folded states

Area of Science:

• Biochemistry
• Physical Chemistry
• Computational Biology

Background:

• Protective osmolytes stabilize protein structures against osmotic stress.
• Traditional models suggest osmolytes are excluded from protein solvation shells.
• Emerging evidence points to direct osmolyte-macromolecule interactions.

Purpose of the Study:

• To investigate the microscopic mechanism of osmolyte-induced protein stabilization.
• To explore direct interactions between protective osmolytes and macromolecules.
• To elucidate the role of van der Waals forces and solvent nonideality.

Main Methods:

• Molecular dynamics simulations of poly(N-isopropylacrylamide) (PNiPAM) in aqueous solutions.
• Analysis of osmolyte accumulation and interactions within the protein's solvation shell.
• Simulations of PNiPAM in solutions with urea and dimethylurea.

Main Results:

• Urea molecules preferentially accumulate around PNiPAM's hydrophobic isopropyl groups via attractive van der Waals forces.
• Formation of low-entropy urea clouds around the protein provides an entropic driving force for folding.
• Preferential urea binding to the folded state decreases the lower folding temperature, aligning with experimental data.
• Solvent thermodynamic nonideality can oppose osmolyte stabilization, potentially leading to denaturation.

Conclusions:

• Direct osmolyte-macromolecule interactions, driven by attractive forces, are a key mechanism for protein stabilization.
• The proposed model reconciles protective and denaturing osmolyte behaviors and solvent effects.
• This work provides a new perspective on the fundamental principles governing protein folding and stability in solution.