  The effect of polyvalent molecular cations, such as spermine, on the
condensation of DNA into very well-defined toroidal shapes have been well
studied and understood. However, a great effort has been made trying to obtain
similar condensed structures from either ssRNA or dsRNA, which the latter
carries similar negative charge density as dsDNA, although it adopts a
different helical form. But the analogous condensation of RNA molecules into
well-defined structures has so far been elusive. In this work, we show that
ssRNA molecules can easily be condensed into nanoring structures on a mice
surface, where each nanoring structure is formed mostly by a single RNA
molecule. The condensation occurs in a concentration range of different atomic
cations, from monovalent to trivalent. The structures of the RNA nanorings on
mica surfaces were oberved by atomic force microscopy (AFM). The samples were
observed in tapping mode and were prepared by drop evaporation of a solution of
RNA in the presence of one type of the different cations used. As far as we
know, this is the first time that nanorings or any other well-defined condensed
RNA structures have been reported. The RNA nanorings formation can be
understood by an energy competition between the hydrogen bonding forming
hairpin stems, weakened by the salts, and hairpin loops. This results may have
an important biological relevance, since it has been proposed that RNA is the
oldest genome coding molecule and the formation of these structures could have
given it stability against degradation in primeval times. Even more, the
nanoring structures could have the potential to be used as biosensors and
functionalized nanodevices.
