  Stochastic resonance (SR) is a well known phenomenon in dynamical systems. It
consists of the amplification and optimization of the response of a system
assisted by stochastic noise. Here we carry out the first experimental study of
SR in single DNA hairpins which exhibit cooperatively folding/unfolding
transitions under the action of an applied oscillating mechanical force with
optical tweezers. By varying the frequency of the force oscillation, we
investigated the folding/unfolding kinetics of DNA hairpins in a periodically
driven bistable free-energy potential. We measured several SR quantifiers under
varied conditions of the experimental setup such as trap stiffness and length
of the molecular handles used for single-molecule manipulation. We find that
the signal-to-noise ratio (SNR) of the spectral density of measured
fluctuations in molecular extension of the DNA hairpins is a good quantifier of
the SR. The frequency dependence of the SNR exhibits a peak at a frequency
value given by the resonance matching condition. Finally, we carried out
experiments in short hairpins that show how SR might be useful to enhance the
detection of conformational molecular transitions of low SNR.
