Molecular association and hydration dynamics in aqueous alanine: an ultrasonic and thermophysical approach

Studying aqueous amino acids at low temperatures is crucial for gaining insights into protein behavior, their interactions with water, and their potential applications across various fields. Temperature variations can influence amino acid interactions and conformations, thereby affecting protein stability and functionality. In this study, we aim to investigate the thermoacoustic and thermophysical properties of aqueous alanine by measuring acoustic and related parameters over the temperature range of 303.15 to 278.15 K at atmospheric pressure. The study focuses mainly on the molecular interactions between alanine and water molecules below room temperatures. Using the pulse-echo technique, ultrasonic velocity (u) was measured at a fixed frequency of 5 MHz. The measured ultrasonic velocity (u) of an aqueous alanine towards higher concentrations suggests an increased induced intermolecular interaction between alanine-water molecules. The study also concludes the intermolecular interaction between alanine-alanine molecules towards low temperature via isolated hydrated monomers of alanine. Adiabatic compressibility (β), intermolecular free length (L_f), absorption coefficient (α/f²) and relaxation time (τ) and thermodynamical parameters i.e. the free energy of activation (ΔF), entropy of activation (ΔS) and enthalpy of activation (ΔH) of aqueous alanine solution also supported for the dynamics of aqueous alanine in the icy environment. The significant achievement of the study is that there found a formation of water network (higher values of hydration number) or hydration shell around alanine molecules and induced interaction between nearby alanine molecules in the low temperature region.