Hydration of biological molecules: lipids versus nucleic acids

We used FTIR spectroscopy to comparatively study the hydration of films prepared from nucleic acids (DNA and double-stranded RNA) and lipids (phosphatidylcholines and phosphatidylethanolamines chosen as the most abundant ones) at room temperature by varying the ambient relative humidity in terms of solvent-induced structural changes. The nucleic acids and phospholipids both display examples of polymorphism on the one hand and structural conservatism on the other; even closely related representatives behave differently in this respect. DNA undergoes a hydration-driven A-B conformational transition, but RNA maintains an A-like structure independently of the water activity. Similarly, a main transition between the solid and liquid-crystalline phases can be induced lyotropically in certain phosphatidylcholines, while their phosphatidylethanolamine counterparts do not exhibit chain melting under the same conditions. A principal difference concerning the structural changes that occur in the studied biomolecules is given by the relevant water-substrate stoichiometries. These are rather high in DNA and often low in phospholipids, suggesting different mechanisms of action of the hydration water that appears to induce structural changes on global- and local-mode levels, respectively.