Specificity in cationic interaction with poly(N-isopropylacrylamide)

Area of Science:

• Polymer Science
• Computational Chemistry
• Solution Chemistry

Background:

• Poly(N-isopropylacrylamide) (PNIPAM) is a thermoresponsive polymer with applications in drug delivery and biomaterials.
• Understanding polymer-electrolyte interactions is crucial for tuning PNIPAM's properties.

Purpose of the Study:

• To investigate the binding interactions between alkali metal cations (monovalent and divalent) and the PNIPAM polymer chain.
• To elucidate the role of ionic radius, charge, and hydration in these interactions.

Main Methods:

• Classical molecular dynamics (MD) simulations were employed.
• Simulations were performed for PNIPAM in various salt solutions (1 M monovalent alkali chlorides, 0.5 M MgCl2, 0.5 M CaCl2).

Main Results:

• Direct cation-amide O binding strength correlates inversely with ionic radius for alkali ions; Li+ binds strongest, Cs+ weakest.
• Divalent Mg(2+) and Ca(2+) ions exhibit weak binding to the amide group, primarily mediated by water due to strong hydration.
• Direct binding is energetically unfavorable for Mg(2+) and Ca(2+) due to unfavorable dehydration energetics, despite favorable electrostatic interactions.

Conclusions:

• The competition between favorable electrostatics and unfavorable dehydration energetics governs cation-amide binding.
• Monovalent alkali ions preferentially bind directly to the amide O.
• Li+ shows a strong association with PNIPAM's hydrophobic residues.