Specific ion effects are often unpredictable in their manifestation within collective behavior, as the underlying principles of how they mediate water–solute interactions remain unclear. In this study, we investigate the influence of simple ions on amino acids in electrolyte solutions, focusing on arginine, lysine, and serine in potassium iodide, potassium bromide, potassium chloride, sodium chloride, and lithium chloride solutions. We combine experimental techniques and molecular dynamics simulations to examine how monovalent ions affect the dielectric spectra of these complex solutions. Our results reveal three key effects of salt addition: (I) A decrease in the permittivity of the solution, a phenomenon known as the dielectric decrement, which is phenomenologically described by the Langevin function. The extent of the dielectric decrement is dependent on the specific ion. (II) An increase in the relaxation time of amino acids. Furthermore, the relaxation time of water is either increased or decreased depending on the specific salt–amino acid combination. The extent of the change in relaxation time compared to binary mixtures is dependent on the charge density of the added ions. (III) Long-range restructuring of amino acids and water leads to changes in their relative orientation over large distances. These findings provide new insights into how ions affect water–amino acid interactions and their impact on the dielectric properties of electrolyte solutions.

We present a detailed examination of the absorption coefficients in the THz region for different water models using different types of potentials: the non-polarizable SPC/E, the Drude-polarizable SWM4-NDP and OPC3-pol, IPOL-0.13 and the multipole AMOEBA14 water. The primary focus is on understanding the interplay between permanent and induced dipole moments and their influence on the THz spectrum. Although the induced dipoles strongly contribute to the peak at 200 cm⁻¹, merely increasing the induced dipole moments does not improve the agreement with experiments. We aim to investigate the behavior of the intensity at 200 cm⁻¹ depending on the water model. Furthermore, we dissect the THz spectra of the water models into distinct contributions to gain more insight into the inter- and intramolecular interactions. Intermolecular interactions significantly contribute to the low-frequency peak, while the peak observed at 600 cm⁻¹ can be adequately attributed to intramolecular dipole–dipole interactions.