Effect of Single-Stranded DNA on the Lower Critical Solution Temperature of Poly( N -isopropylacrylamide): Implications for Thermoresponsive Polymer–Nucleic Acid Systems

Abstract

Poly(N-isopropylacrylamide) (PNIPAM) exhibits a coil-to-globule transition at its lower critical solution temperature (LCST), which is sensitive to cosolutes in aqueous solution. Although ions, alcohols, and urea have been extensively characterized, the effect of nucleic acids remains largely unexplored. This study systematically investigates the LCST behavior of PNIPAM in the presence of single-stranded DNA (ssDNA) with varying base compositions (A, T, G, C), chain lengths (10–60 residues), and concentrations (0–25,600 μMres) using differential scanning calorimetry. ssDNAs reduce the LCST, with base-type effects following A > T > G ≈ C and weaker effects observed for shorter chains at equivalent residue concentrations. These trends indicate a mechanism driven by indirect water-mediated interactions. By decomposing the transition energy into solvent-excluded volume, intramolecular and intermolecular attractions, and conformational entropy, the chaotropic behavior of ssDNA is observed. The intermolecular attraction term shows a second-order dependence on ssDNA concentration, consistent with preferential partitioning into the polymer hydration shell. Moreover, this attraction term for a mixed-base sequence can be estimated from single-base coefficients using composition-weighted averaging. The approach accurately predicts the LCST of PNIPAM in the presence of let-7a ssDNA, a 22-nucleotide ssDNA analog of microRNA let-7a, demonstrating good agreement (RMSD = 1.5 × 10–2 °C for LCST, 6.2 × 10–3 kJ/mol for ΔEa). This study establishes a quantitative framework for predicting the effects of nucleic acid cosolutes on PNIPAM LCST.