Phases of the secondary structures of RNA sequences

Formation of RNA secondary structures is an example of the sequence-structure problem omnipresent in molecular biophysics. A basic theoretical issue concerns the phase behaviour of self-attracting random RNA sequences. By studying a simplified toy model of RNA folding, we show that there are two distinct possible phases for the random RNA below its melting transition a molten phase in which an exponentially large number of allowed secondary structures have comparable free energies and coexist in thermal equilibrium, and a glass phase in which the equilibrium ensemble is dominated by one or a few structures with much lower free energies.