In many processes biomolecules unfold and refold under the effect of a mechanical force, e.g the mRNA
going through the ribosome or the DNA being unzipped by helicases. The understanding of the mechanical
unfolding/refolding process in biomolecules is therefore of great interest. The recent development of 
single molecule experimental techniques offers new ways to investigate the folding of proteins and RNAs 
by manipulating one molecule at a time.

Our work has been focused on modelling the force unfolding/refolding process in RNA molecules and the 
interpretation of the results obtained in experiments using optical tweezers.  In my talk I will explain 
how to use Kramers theory to investigate the dissociation of multiple bonds under mechanical force in 
order interpret experimental results obtained by pulling RNA molecules at different rates. I will 
consider the case of simple RNA structures such as hairpins and show how the fragility of these molecules 
can strongly depend on the value of the applied force. Finally, I will also address the case of more 
complex RNA structures, such as the three-way junction S15 RNA, that show force-induced misfolding. In 
particular, I will show that the misfolding mechanism observed in single molecule force experiments for 
that molecule can be understood as the result of a competition between the formation of two different 
structures.