Probing the physicochemical properties of single molecules while varying their temperature is usually carried out with indirect methods, because the laser irradiation that is typically used to induce a temperature jump can destroy the sample. These methods cannot provide in situ temperature information, which leads to uncertainties in the measurements. Hanbin Mao and colleagues at Kent State University have now developed a technique that allows mechanochemical experiments with single DNA molecules to be carried out with precise temperature control.
The researchers use a carbon microparticle that is placed at the tip of a capillary. The microparticle can efficiently absorb light and through a photothermal effect can increase the temperature of its immediate environment by about 30 °C within milliseconds. The microparticle is placed in close proximity to a DNA hairpin, which is tethered between two optically trapped beads that can register the force exerted by the DNA as it unfolds; this mechanical unfolding is temperature dependent and can be used to determine the in situ temperature of the experiment.
With the set-up, Mao and colleagues are able to extract the enthalpy and the entropy of the unfolding of the DNA hairpin, and reconstruct the energy landscape of the transformation.