  We consider a simple model for the unfolding of RNA tertiary structure under
dynamic loading. The opening of such a structure is regarded as a two step
process, each corresponding to the overcoming of a single energy barrier. The
resulting two-barrier energy landscape accounts for the dependence of the
unfolding kinetics on the pulling rate. Furthermore at intermediate force, the
two barriers cannot be distinguished by the analysis of the opening kinetic,
which turns out to be dominated by a single macro-barrier, whose properties
depend non-trivially on the two single barriers. Our results suggest that in
pulling experiments on RNA molecule containing tertiary structures, the details
of the single kinetic barriers can only be obtained using a low pulling rate
value, or in the high force regime.
