Insight into the amplification by methylated urea of the anion specificity of macromolecules

Methylated urea and sugar are chaotropic and kosmotropic osmolytes, respectively. In the present work, we have investigated the specific anion effect on the lower critical solution temperature (LCST) behavior of poly(N-isopropylacrylamide) (PNIPAM) in the presence of methylated urea or sugars. Differential scanning calorimetry studies revealed that tetramethylurea can adsorb onto the PNIPAM surface, but glucose is excluded from the PNIPAM surface. The specific anion effect on the LCST behavior of PNIPAM is amplified by methylated urea but not by sugars. The amplification of the anion specificity by methylated urea is attributed to an increased difference in the anion-specific polarization of hydrogen bonds, induced by the formation of PNIPAM/methylated urea complexes via hydrophobic interactions. As the number of methyl groups on the methylated urea increases, the extent of amplification of the anion specificity increases due to increasing hydrophobic interactions between the PNIPAM and methylated urea. Additionally, no amplification of the anion specificity is observed in the presence of urea because a PNIPAM/urea complex cannot be formed via hydrophobic interactions.