Dynamical Properties of N-Isopropylacrylamide Molecule in Water Revealed by (1)H NMR Studies

The hydration of alkyl-amide molecules in water is very important for understanding their solubility in water, which related to the phase behavior of polyamides including protein molecules in water. The (1)H NMR signals of N-isopropylacrylamide (NIPA) in water were investigated for the purpose to reveal the hydration properties of NIPA in the aqueous HCl solutions, which change the phase behavior of aqueous NIPA solutions with HCl concentration as reported previously (J. Phys. Chem. B 2011, 115, 12905-12910). The following observations were revealed by the present studies. (1) The spin-spin coupling of the amide proton with the methyne proton in the HCl solution was observed when a half width of amide peak (∼Δν) was less than about 10 Hz, but it was not when Δν was greater than 10 Hz. (2) The Δν value of amide proton increased with an increase in a mole ratio of H2O to NIPA (rH2O) or with enlargement of the protonated hydration shell, which reduced the energy gap of the cis- and trans-conformations. (3) The Δν value of H2O and its rH2O dependence in the 3 M HCl solution changed drastically between rH2O = 4.0 and 4.7, in which the hydrophilic hydration shell neighboring the amide extended to form the hydrophobic hydration shell. (4) The Δν of methyl proton decreased monotonously with rH2O. This indicates an increase of the rotational mobility of methyl groups in the hydration shell. These results indicate that the elongating lifetime of cis-conformation of amide coupled with the shortened lifetime of double bonding C-N in amide induces the amide-methyne spin-spin-coupling transition and that the cationic state of amide stabilizes the hydrophobic hydration to induce the rotational free motion of amide C-N and the methyl groups.