Component-resolved dynamics of glass-forming dipeptide–water-mixtures

Abstract

We combine differential scanning calorimetry, broadband dielectric spectroscopy, and 1H and 2H nuclear magnetic resonance (NMR) for component-selective studies of molecular reorientation and diffusion in mixtures comprising the dipeptide N-acetyl-glycine-methylamide (NAGMA), which is commonly considered as a protein-backbone model, and deuterated water in a broad temperature range of 140–340 K. For a 7.5 m NAGMA–D2O mixture, crystallization is largely avoided, revealing a separation of the dipeptide-dominated α process, which describes the glassy slowdown, from the water-caused ν process. The latter shows a dynamical crossover at Tg = 175 K and thermally activated motion governed by a temperature-independent Gaussian-like distribution of activation energies with a mean value of Em = 0.49 eV and a standard deviation of σE = 0.035 eV in the glassy state. Detailed NMR analyses show that, despite the time-scale separation, rotational-translational coupling is found for water dynamics at least down into the weakly supercooled regime. Moreover, NMR reveals that the ν process involves a quasi-isotropic reorientation of basically all water molecules even below Tg, while slow or restricted water reorientation does not occur. Based on our findings, we discuss the temperature-dependent coupling of the dipeptide and water motions. For a 2 m NAGMA–D2O mixture, partial crystallization leads to an enhanced temperature dependence. Disentangling the rotational motions of the liquid and crystalline water fractions, we find that the liquid fraction exhibits Arrhenius behavior with Ea = 0.89 eV until a dynamical crossover again occurs upon cooling, while the reorientation of the ice fraction highly resembles that in hexagonal bulk ice.