Modulation of lipid phase behavior by kosmotropic and chaotropic solutes - Experiment and thermodynamic theory

By means of differential scanning calorimetry and from a review of published data we demonstrate in this work that low-molecular weight kosmotropic substances (water-structure makers) of different chemical structure such as disaccharides, proline, and glycerol have identical effects on the phase behavior of several kinds of phospholipids and glycolipids. These substances favor formation of the high-temperature inverted hexagonal phase (HII) and the low-temperature lamellar crystalline (L(c)) and gel (L(beta)) phases at the expense of the intermediate lamellar liquid-crystalline phase (L(alpha)). The latter phase may completely disappear from the phase diagram at high enough solute concentration. By contrast, chaotropic substances (water-structure breakers) such as sodium thiocyanate and guanidine hydrochloride expand the existence range of L(alpha) at the expense of the adjacent L(beta) and H-II phases. Moreover, chaotropes are able to induce the appearance of missing intermediate liquid-crystalline phases in lipids displaying direct L(beta) --> H-II transitions in pure water. In previous publications we have considered the influence of chaotropic and kosmotropic substances on the lipid phase behavior as a manifestation of their indirect (Hofmeister) interactions with the lipid aggregates. For a quantitative characterization of this effect, here we derive a general thermodynamic equation between lipid phase transition temperature and solute concentration, analogous to the Clapeyron-Clausius equation between transition temperature and pressure. It provides a clear description in physical quantities of the disparate effects of kosmotropic and chaotropic substances on the relative stability of the lipid-water phases. According to this equation, the magnitude of the solute effect is proportional to the hydration difference of the adjacent lipid phases and inversely proportional to the transition latent heat. The sign and magnitude of the transition shifts depend also on the degree of solute depletion (for kosmotropes) or enrichment (for chaotropes) at the interfaces, in comparison to the solute concentration in bulk water.