  We investigate the thermodynamics and kinetics of DNA hairpins that
fold/unfold under the action of applied mechanical force. We introduce the
concept of the molecular free energy landscape and derive simplified
expressions for the force dependent Kramers-Bell rates. To test the theory we
have designed a specific DNA hairpin sequence that shows two-state cooperative
folding under mechanical tension and carried out pulling experiments using
optical tweezers. We show how we can determine the parameters that characterize
the molecular free energy landscape of such sequence from rupture force kinetic
studies. Finally we combine such kinetic studies with experimental
investigations of the Crooks fluctuation relation to derive the free energy of
formation of the hairpin at zero force.
