The dynamics of unfolded versus folded tRNA: the role of electrostatic interactions

The dynamics of RNA contributes to its biological functions such as ligand recognition and catalysis. Using quasielastic neutron scattering spectroscopy, we show that Mg(2+) greatly increases the picosecond to nanosecond dynamics of hydrated tRNA while stabilizing its folded structure. Analyses of the atomic mean-squared displacement, relaxation time, persistence length, and fraction of mobile atoms showed that unfolded tRNA is more rigid than folded tRNA. This same result was found for a sulfonated polystyrene, indicating that the increased dynamics in Mg(2+) arises from improved charge screening of the polyelectrolyte rather than specific interactions with the folded tRNA. These results are opposite to the relationship between structural compactness and internal dynamics for proteins in which the folded state is more rigid than the denatured state. We conclude that RNA dynamics are strongly influenced by the electrostatic environment, in addition to the motions of local waters.