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. © 2002 Wiley Periodicals, Inc. Biopolymers (Biospectroscopy) 67: 499–503, 2002