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

Area of Science:

• Physical Chemistry
• Biophysics
• Materials Science

Background:

• Understanding molecular dynamics in peptide-water mixtures is crucial for protein folding and biomaterial design.
• N-acetyl-glycine-methylamide (NAGMA) serves as a model for protein backbones, and its interactions with water influence biological processes.

Purpose of the Study:

• To investigate the molecular reorientation and diffusion in NAGMA-water mixtures using a combination of spectroscopic techniques.
• To elucidate the coupling between dipeptide and water dynamics across a wide temperature range.
• To analyze the impact of varying NAGMA concentration on mixture dynamics and phase behavior.

Main Methods:

• Differential scanning calorimetry (DSC) for thermal analysis.
• Broadband dielectric spectroscopy (BDS) to study molecular relaxations.
• 1H and 2H nuclear magnetic resonance (NMR) for component-selective dynamics and diffusion studies.
• Component-selective studies of molecular reorientation and diffusion.

Main Results:

• In a 7.5 m NAGMA-D2O mixture, distinct α (dipeptide) and ν (water) processes were observed, with the water dynamics showing a dynamical crossover at Tg = 175 K.
• NMR revealed rotational-translational coupling in water dynamics down to the weakly supercooled regime, with quasi-isotropic reorientation occurring below Tg.
• A 2 m NAGMA-D2O mixture exhibited partial crystallization, leading to different dynamics for liquid water (Arrhenius behavior) and ice fractions.

Conclusions:

• Molecular motions in NAGMA-water mixtures are separable, but coupling exists between dipeptide and water dynamics.
• Water molecules in the glassy state exhibit thermally activated motion with a Gaussian-like distribution of activation energies.
• The study provides insights into the complex interplay of molecular dynamics in biologically relevant peptide-water systems.