High-intensity terahertz time-domain spectroscopy (HI-THz-TDS) reveals hydration behavior of lactose in the dilute...

The interaction between sugar and water molecules strongly influences the structural and functional behavior of saccharides in biochemical environments. This study investigated the hydration behavior of lactose, the primary sugar in milk, at millimolar concentrations representative of physiological conditions. High-intensity terahertz time-domain spectroscopy (HI-THz-TDS) was employed to measure the optical properties and dielectric functions of dilute lactose solutions. The spectrometer demonstrated high sensitivity to lactose, achieving a detection limit of 13 mM and resolving subtle hydration effects in the tens of millimolar range. The THz absorption response exhibited a negative deviation from two-component model predictions, indicating the presence of dynamically retarded hydration water distinct from the bulk water. Analysis of the dielectric response yielded a hydration number of 14 to 17, consistent with values obtained from complementary techniques. These results indicate that in dilute solutions, saccharide-induced perturbations to water dynamics are primarily governed by retardation within the first hydration shell, with no clear dominant contribution from long-range effects under the investigated conditions. This work demonstrates the capability of HI-THz-TDS to probe hydration at physiologically relevant mM concentrations and highlights the important role of concentration in modulating saccharide-water interactions.