On urea's ability to stabilize the globule state of poly(N-isopropylacrylamide).

ABSTRACT

Experimental measurements have shown that urea decreases the temperature of the coil-to-globule collapse transition of poly(N-isopropylacrylamide), whereas tetramethylurea increases the collapse temperature [J. Am. Chem. Soc., 2009, 131, 9304]. These data indicate that urea is a stabilizing agent of the globule state in contrast to its denaturing action towards globular proteins. The effect of urea and tetramethylurea on the conformational stability of poly(N-isopropylacrylamide) is investigated by means of the theoretical approach developed to explain the existence of cold denaturation [Phys. Chem. Chem. Phys., 2010, 12, 14245; Phys. Chem. Chem. Phys., 2014, 16, 21755], and already used to rationalize the effect of sodium salts on the collapse temperature [Phys. Chem. Chem. Phys., 2015, 17, 27750]. It is necessary to take into account the delicate trade-off between the increase in the magnitude of the solvent-excluded volume effect caused by urea and tetramethylurea addition to water, and the increase in the magnitude of the energetic interactions among the poly(N-isopropylacrylamide) surface and water and co-solvent molecules.