Entropy Rules: Molecular Dynamics Simulations of Model Oligomers for Thermoresponsive Polymers

Abstract

We attempted to attain atomic-scale insights into the mechanism of the heat-induced phase transition of two thermoresponsive polymers containing amide groups, poly(N-isopropylacrylamide) (PNIPAM) and poly(2-isopropyl-2-oxazoline) (PIPOZ), and we succeeded in reproducing the existence of lower critical solution temperature (LCST). The simulation data are in accord with experimental findings. We found out that the entropy has an important contribution to the thermodynamics of the phase separation transition. Moreover, after decomposing further the entropy change to contributions from the solutes and from the solvent, it appeared out that the entropy of the solvent has the decisive share for the lowering of the free energy of the system when increasing the temperature above the LCST. Our conclusion is that the thermoresponsive behavior is driven by the entropy of the solvent. The water molecules structured around the functional groups of the polymer that are exposed to contact with the solvent in the extended conformation lower the enthalpy of the system, but at certain temperature the extended conformation of the polymer collapses as a result of dominating entropy gain from “released” water molecules. We stress also on the importance of using more than one reference molecule in the simulation box at the setup of the simulation.

Implementation of the Freely Jointed Chain Model to Assess Kinetics and Thermodynamics of Thermosensitive Coil–Globule Transition by Markov States

“…In prior publications, we found that thermodynamic quantities, such as the enthalpy, may vary significantly between repeated simulations of the CGT. , Conclusively, we presumed the conformational diversity in both states to be of particular importance. Besides, we were convinced that the thermosensitive character of the CGT originates from an entropic effect. ,− Specifically, we assumed that a large variety of globular structures exist, whose properties show significant deviations in terms of thermodynamics and kinetics. However, as of now, distinguishing between conformational substates has been challenging due to the lack of suitable descriptors.…”

“…Besides, we were convinced that the thermosensitive character of the CGT originates from an entropic effect. 18 , 22 − 24 Specifically, we assumed that a large variety of globular structures exist, whose properties show significant deviations in terms of thermodynamics and kinetics. However, as of now, distinguishing between conformational substates has been challenging due to the lack of suitable descriptors.…”

Characterizing Ion-Polymer Interactions in Aqueous Environment with Electric Fields

“…Synthetic metal-chelating polymers are an important technology to neutralize and remove metal contaminants from solution. − Several polymers have been synthesized and tested for metal sequestration applications, − using isothermal titration calorimetry (ITC) − to measure the binding thermodynamics of polymers to metal ions. While ITC enables the direct measurement of heat associated with metal ion binding in solution, it is a global technique that determines the total entropy and cannot separate the competing effects of coupled events (e.g., ion binding versus (de)solvation versus polymer conformation changes). , Computational approaches can enable greater insight into polymer-metal ion interactions by focusing on time and length scales that are inaccessible to ITC or other experimental techniques. , Atomistic simulations such as molecular dynamics (MD) are well-suited to the simulation of entropic effects, − but the quantification of functional properties from MD is nontrivial. .25ex2ex F⃗ false( r⃗ , t false) = q E⃗ total false( r⃗ , t false) = q false( E⃗ water ( r⃗ , t ) + E⃗ polymer ( r⃗ , t ) + ... false) = q ∑ k E⃗ k false( r⃗ , t false) …”

In silico Assessment of Dipeptidyl Peptidase 4 (DPP-4) Inhibiting Potential of a few Bioactive Compounds Present in Black Rice Bran for Effective Management of Postprandial Hyperglycemia

“…bonds formed between the protein and ligand that results in suitable complex formation between the biomolecules [49,51]. It also provides supports for the small range of uctuations observed in RMSD plot.…”