Structural Dynamics of Neighboring Water Molecules of N-Isopropylacrylamide Pentamer

Abstract

Poly(N-isopropylacrylamide) (PNIPAM) is a popular polymer widely used in smart hydrogel synthesis due to its thermo-responsive behavior in aqueous medium. Aqueous PNIPAM hydrogels can reversibly swell and collapse below and above their lower critical solution temperature (LCST), respectively. The present work used molecular dynamics simulations to explore the behavior of water molecules surrounding the side chains of a NIPAM pentamer in response to temperature changes (273−353 K range) near its experimental LCST (305 K). Results suggest a strong inverse correlation of temperature with water density and hydrophobic hydration character of the first coordination shell around the isopropyl groups. Integrity of the first and second coordination shells is further characterized by polygon ring analysis. Predominant occurrence of pentagons suggests clathrate-like behavior of both shells at lower temperatures. This predominance is eventually overtaken by 4membered rings as temperature is increased beyond 303 and 293 K for the first and second coordination shells, respectively, losing their clathrate-like property. It is surmised that this temperature-dependent stability of the coordination shells is one of the important factors that controls the reversible swell-collapse mechanism of PNIPAM hydrogels.

Thermosensitive Hydration of Four Acrylamide-Based Polymers in Coil and Globule Conformations

“…Moreover, we hypothesize that large rearrangements in the solvation shell are necessary for the CGT to occur, which agrees with recent studies. 53 , 75 Therefore, the deceleration of the water dynamics at low temperatures significantly increases transition times.…”

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

“… 38 For all simulations, we used the OPLS2005 force field, 39 , 40 which has been established for simulations of NIPAAM in past publications. 17 , 18 , 41 − 45 In our production runs, we applied the Parrinello−Rahman barostat, 46 , 47 with a reference pressure of 1 bar and the velocity-rescaling thermostat 48 at respective simulation temperatures between 250 and 360 K. We used the LINCS algorithm to constrain the bonds involving hydrogen atoms and used a timestep of 2 fs for our MD integration. 49 , 50 Throughout all simulations, we applied periodic boundary conditions.…”

“…Except for the preparation of the initial polymer configuration, we used the GROMACS MD-Simulation software package throughout this process . For all simulations, we used the OPLS2005 force field, , which has been established for simulations of NIPAAM in past publications. ,,− In our production runs, we applied the Parrinello−Rahman barostat, , with a reference pressure of 1 bar and the velocity-rescaling thermostat at respective simulation temperatures between 250 and 360 K. We used the LINCS algorithm to constrain the bonds involving hydrogen atoms and used a timestep of 2 fs for our MD integration. , Throughout all simulations, we applied periodic boundary conditions. Furthermore, we used a cutoff of 8.85 Å for the evaluation of long-range interactions and applied the particle mesh Ewald method to this end …”

Thermo-Responsive Hydrogels: From Recent Progress to Biomedical Applications

“…The thermodynamics of NIPAM monomer was investigated by Heyda et al [ 36 ], which could allow for better control of PNIPAM phase behavior. Nellas and coworkers performed a molecular dynamics simulation of pentamer PNIPAM [ 37 ], and they suggest a clathrate-like behavior in the coordinate shell might be responsible for the LCST behavior of PNIPAM. Tang et al directly observed the hydrophilicity–hydrophobicity transformation of PNIPAM by the utilization of aggregation-induced emission luminogens [ 38 ].…”