The effects of salt on the lower critical solution temperatures of poly (N-isopropylacrylamide) and its copolymer studied from molecular dynamics simulations

Classical molecular dynamics (MD) 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. MD simulations of PNIPAM in 1 M NaCl, NaBr, NaI and KCl solutions were carried out to elucidate the effects of different salts on the LCST and protein stability. The simulation 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 pairassociation constant in solution. MD simulations were also carried out for PNIPAM in 1 M mixed salt solution containing 0.5 M Na+, K+, Cl- and Br - each. The simulation results further confirmed the conclusions. Additional MD simulations were conducted for PNIPAM-co-PEGMA copolymer in 1 M NaCl solution. Interestingly, Na+ was found to form a complex with multiple O atoms on the PNIPAM-co-PEGMA chain thus greatly enhancing the cationic binding with the copolymer. These results provide significant insight into the effects of salt on protein stability.