RNA folding: thermodynamic and molecular descriptions of the roles of ions

The stability of a compact RNA tertiary structure is exquisitely sensitive to the concentrations and types of ions that are present. This review discusses the progress that has been made in developing a quantitative understanding of the thermodynamic parameters and molecular detail that underlie this sensitivity, including the nature of the ion atmosphere, the occurrence of specific ion binding sites, and the importance of the ensemble of partially unfolded states from which folding to the native structure occurs.