The Kosmotropic (Structure-Forming) Effect of Compensatory Solutes

Abstract

Abstract The production and/or accumulation of organic osmolytes, which serve to compensate for osmotic pressure and low cytoplasmic water activity, are the typical properties of many halophilic microorganisms. These so-called compatible/compensatory solutes not only maintain osmotic equilibrium but also protect and stabilize cytoplasmic components against a variety of stress factors. A molecular basis for this is seen in the kosmotropic nature of these solutes, referring to the structure-forming ability in water. Using a gel filtration method and near-infrared spectroscopy, we were able to demonstrate that nature's prime compensatory solutes (betaine, ectoines, proline, N-acetylated diamino acids) strongly influence surrounding water molecules. The hydration numbers observed (three to five molecules of water per molecule of solute) are comparable with those of the “unfreezable water” recently reported for trehalose and are markedly higher than those of disturbing (chaotropic) salts. In addition, a Gaussian analysis of hydration spectra revealed vibration bands similar to those observed in frozen water, indicating that strong hydrogen bonds are induced by the presence of compensatory solutes.