  Recent experiments on unzipping of RNA helix-loop structures by force have
shown that about 40-base molecules can undergo kinetic transitions between two
well-defined `open' and `closed' states, on a timescale = 1 sec [Liphardt et
al., Science 297, 733-737 (2001)]. Using a simple dynamical model, we show that
these phenomena result from the slow kinetics of crossing large free energy
barriers which separate the open and closed conformations. The dependence of
barriers on sequence along the helix, and on the size of the loop(s) is
analyzed. Some DNAs and RNAs sequences that could show dynamics on different
time scales, or three(or more)-state unzipping, are proposed.
