@article {2015|1755, title = {How osmolytes influence hydrophobic polymer conformations: A unified view from experiment and theory.}, journal = {Proc. Natl. Acad. Sci. Usa}, volume = {112}, year = {2015}, pages = {9270{\textendash}5}, abstract = {

It is currently the consensus belief that protective osmolytes such as trimethylamine N-oxide (TMAO) favor protein folding by being excluded from the vicinity of a protein, whereas denaturing osmolytes such as urea lead to protein unfolding by strongly binding to the surface. Despite there being consensus on how TMAO and urea affect proteins as a whole, very little is known as to their effects on the individual mechanisms responsible for protein structure formation, especially hydrophobic association. In the present study, we use single-molecule atomic force microscopy and molecular dynamics simulations to investigate the effects of TMAO and urea on the unfolding of the hydrophobic homopolymer polystyrene. Incorporated with interfacial energy measurements, our results show that TMAO and urea act on polystyrene as a protectant and a denaturant, respectively, while complying with Tanford-Wyman preferential binding theory. We provide a molecular explanation suggesting that TMAO molecules have a greater thermodynamic binding affinity with the collapsed conformation of polystyrene than with the extended conformation, while the reverse is true for urea molecules. Results presented here from both experiment and simulation are in line with earlier predictions on a model Lennard-Jones polymer while also demonstrating the distinction in the mechanism of osmolyte action between protein and hydrophobic polymer. This marks, to our knowledge, the first experimental observation of TMAO-induced hydrophobic collapse in a ternary aqueous system.

}, keywords = {Atomic Force, Computer Simulation, Hydrophobic and Hydrophilic Interactions, Mechanical, Methylamines, Methylamines: chemistry, Microscopy, Molecular Dynamics Simulation, Normal Distribution, Polymers, Polymers: chemistry, Polystyrenes, Polystyrenes: chemistry, Protein Binding, Protein Conformation, Protein Folding, Proteins, Proteins: chemistry, Software, Solvents, Solvents: chemistry, Stress, Thermodynamics, Urea, Urea: chemistry, Water, Water: chemistry}, isbn = {1215421109}, issn = {1091-6490}, doi = {10.1073/pnas.1511780112}, url = {http://www.pnas.org/content/112/30/9270}, author = {Mondal, Jagannath and Halverson, Duncan and Li, Isaac T S and Guillaume Stirnemann and Walker, Gilbert C and Berne, Bruce J} } @article {2013|1752, title = {Elasticity, structure, and relaxation of extended proteins under force.}, journal = {Proc. Natl. Acad. Sci. U.s.a}, volume = {110}, year = {2013}, pages = {3847{\textendash}52}, abstract = {

Force spectroscopies have emerged as a powerful and unprecedented tool to study and manipulate biomolecules directly at a molecular level. Usually, protein and DNA behavior under force is described within the framework of the worm-like chain (WLC) model for polymer elasticity. Although it has been surprisingly successful for the interpretation of experimental data, especially at high forces, the WLC model lacks structural and dynamical molecular details associated with protein relaxation under force that are key to the understanding of how force affects protein flexibility and reactivity. We use molecular dynamics simulations of ubiquitin to provide a deeper understanding of protein relaxation under force. We find that the WLC model successfully describes the simulations of ubiquitin, especially at higher forces, and we show how protein flexibility and persistence length, probed in the force regime of the experiments, are related to how specific classes of backbone dihedral angles respond to applied force. Although the WLC model is an average, backbone model, we show how the protein side chains affect the persistence length. Finally, we find that the diffusion coefficient of the protein{\textquoteright}s end-to-end distance is on the order of 10(8) nm(2)/s, is position and side-chain dependent, but is independent of the length and independent of the applied force, in contrast with other descriptions.

}, keywords = {Atomic Force, Biophysical Phenomena, Computer Simulation, Elasticity, Mechanical, Microscopy, Models, Molecular, Molecular Dynamics Simulation, Proteins, Proteins: chemistry, Stress, Ubiquitin, Ubiquitin: chemistry}, issn = {1091-6490}, url = {http://www.pnas.org/content/early/2013/02/13/1300596110.abstract}, author = {Guillaume Stirnemann and Giganti, David and Fernandez, Julio M and Berne, B J} }