The amino acid proline is accumulated in a variety of plant species in response to environmental stresses, such as high salinity and extreme temperatures. Although the colligative role of proline as an osmoprotectant is well known, its influence on molecular interactions within the cell has received less attention. Here, we investigate the effects of proline on interaction free energies in aqueous environments, and we find that the presence of proline significantly enhances the repulsive force between charged surfaces relative to pure water. At elevated concentrations, proline alters the short-range, structural interaction, forming layers at the surfaces. In the presence of proline and salt, the near-surface hydration structure is disrupted compared to salt solutions without proline. Overall, we observe that the far-field component of the interaction is relatively insensitive to proline concentration above a low threshold, and the results show that proline contributes to maintaining repulsive colloidal interactions while allowing for tuning of osmotic pressure over a wide spectrum of osmolarity.
