Hydration behavior of asparagine: an approach using time domain reflectometry at low temperatures

The dielectric behavior of Asparagine (C₄H₈N₂O₃) in water over the frequency range of 10 MHz to 30 GHz in the temperature region of 278.15-303.15 K in a step of 5 K has been carried out using time domain reflectometry (TDR) at various concentrations of asparagine. The obtained dielectric spectra reveal two relaxation peaks. The low frequency relaxation is attributed to the interaction between solute-solute molecules, while the high frequency relaxation is due to the reorientation of solvent molecules. The various dielectric and thermodynamic parameters were calculated such as the dielectric constant (ε_j), relaxation time (τ_j), effective dipole moment (μ_eff), Kirkwood correlation factor (g₁), hydration number or the number of solvent molecules effectively bounded to solute molecule (Z_ib), effective volume of rotation (V_eff), free energy of activation (ΔF_j), entropy of activation (ΔS_j) and enthalpy of activation (ΔH_j). The static dielectric constant (ε₁) shows increasing trend towards the higher concentration of asparagine, where as the high frequency dielectric constant (ε₂) decreases with the concentration of asparagine. The relaxation time of low frequency (τ₁) and high frequency (τ₂) processes increases towards higher concentration of solute molecule and also towards lower temperature. As the concentration of asparagine increases, the value of effective dipole moment (μ_eff) decreases. With increasing amino acid concentrations hydration dynamics get affected and indicated by decreasing the hydration number (Z_ib) but the hydration dynamics of aqueous asparagine was found least temperature dependent.