Effects of Salt on the Lower Critical Solution Temperature of Poly (N-Isopropylacrylamide)

Abstract

Classical molecular dynamics simulations were performed to investigate the effects of salt on the lower critical solution temperature (LCST) of Poly (N-isopropylacrylamide) (PNIPAM). PNIPAM is often studied as a protein proxy due to the presence of a peptide bond in its monomer unit. PNIPAM is a temperature sensitive polymer which exhibits hydrophobic-hydrophilic phase transition at its LCST. The presence of salt in the solution will shift its LCST, typically to a lower temperature. This LCST shift follows the so-called Hofmeister series. Molecular dynamics (MD) simulations of PNIPAM in 1 M of NaCl, NaBr, NaI, and KCl were carried out to elucidate the effects of different salt on LCST and protein stability. Our results suggest that direct interactions between the salt cations and the polymer play a critical role in the shift of LCST and subsequently on protein stability. Further, cations have a much stronger affinity with the polymer, whereas anions bind weakly with the polymer. Moreover, the cation-polymer binding affinity is inversely correlated with the cation-anion contact pair association constant in solution.

A new class of nanoengines based on thermoresponsive polymers: Conceptual design and behavior study

“…In other words, above the LCST, the PNIPAM molecules have smaller average extent [19]. For the endgrafted chains in our system, we observe the LCST around or slightly below 305 K; a value which coincides reasonably well with available experimental data [18,[31][32][33][34][35] and other simulation results of free PNIPAM chains [19,24,[36][37][38][39].…”

“…All molecular dynamics (MD) simulations have been performed with the LAMMPS software [22] developed at Sandia National Laboratories using the AMBER 94 force field [23,24] for PNIPAM and the TIP3P model for water [25]. The force field applied for the graphene has been used before for the thermal conductivity calculations of carbon nanotubes and graphene layers by one of the authors [26,27].…”

Role of Solvation Dynamics and Local Ordering of Water in Inducing Conformational Transitions in Poly(N-isopropylacrylamide) Oligomers through the LCST

“…Gangemi et al further extended the study carried out by Longhi et al by increasing the simulation length and by studying PNIPAM oligomer as well as two oligomers of PNIPAM having N-methacryloyl-L-valine and N-methacryloyl-L-leucine comonomers. 36 Gangemi 29 In their study, they carried out simulations of PNIPAM in 1 M of NaCl, NaBr, NaI, and KCl to elucidate the effects of different salt ions on the LCST of PNIPAM. Du et al found that cations have a much stronger affinity with the polymer, whereas anions bind weakly with the polymer.…”

The effects of anionic electrolytes and human serum albumin on the LCST of poly( N -isopropylacrylamide)-based temperature-responsive copolymers

“…The effects of salts on the LCST of PNIPAm have been studied with a variety of measurement methods, such as cloud-point measurement [12], differential-scanning calorimetry [13], and a temperature-gradient microfluidic apparatus together with dark-field microscopy [14]. Furthermore, calculations with computational molecular dynamics simulations have been carried out [15]. LCST of PNIPAm as a function of molecular weight [16] and on that of PNIPAm microgel particles [17] have also been studied.…”